Should Every Mechanically Ventilated Patient Be Monitored with Capnography from Intubation to Extubation?

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Should Every Mechanically Ventilated Patient Be Monitored With Capnography From Intubation to Extubation?

Ira M Cheifetz MD FAARC and Timothy R Myers RRT-NPS

Introduction Pro: Every Mechanically Ventilated Patient Should Be Monitored With Capnography From Intubation to Extubation Endotracheal Intubation Preventing Mishaps Operating Room Setting

Physiology of the Arterial-Versus-End-Tidal CO2 Difference Carbon Dioxide Elimination Management of Positive End-Expiratory Pressure ˙ Alveolar VE Minimizing the Duration of Mechanical Ventilation Acute Respiratory Distress Syndrome: Prognosis Cardiovascular Physiology Extubation Pro Summary Con: Not Every Mechanically Ventilated Patient Should Be Monitored With Capnography From Intubation to Extubation Patient Position Cyanotic Congenital Heart Disease Transcutaneous Monitoring Trauma Obesity Surgery Patients Specialty Gases Transport Pediatric Respiratory Disease Con Summary Summary

One of the most important aspects of caring for a critically ill patient is monitoring. Few would disagree that the most essential aspect of monitoring is frequent physical assessments. Comple- menting the physical examination is continuous monitoring of heart rate, respiratory rate, and blood oxygen saturation measured via pulse-oximetry, which have become the standard of care in intensive care units. Over the past decade one of the most controversial aspects of monitoring critically ill patients has been capnography. Although most clinicians use capnography to confirm endotracheal intubation, few clinicians use continuous capnography in the intensive care unit. This article reviews the medical literature on whether every mechanically ventilated patient should be monitored with capnography from intubation to extubation. There are numerous articles on capnography, but no definitive, randomized study has even attempted to address this specific question. Based on the available literature, it seems reasonable to use continuous capnography, for at least a

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subset of critically ill patients, to ensure integrity of the endotracheal tube and other ventilatory apparatus. However, at this point definitive data are not yet available to clearly support continuous capnography for optimizing mechanical ventilatory support. We hope that as new data become available, the answer to this capnography question will become clear. Key words: capnography, mechanical ventilation, endotracheal tube, intubation, extubation, monitoring, carbon dioxide, ventilation. [Respir Care 2007;52(4):423–438. © 2007 Daedalus Enterprises]

Introduction labor-intensive and expensive, and generally offers only intermittent measures of P . Although continuous mon- aCO2 Capnography and pulse oximetry in combination is the itoring of arterial blood gases is possible, it is expensive monitoring standard of care in the operating room setting, and continues to have technical limitations. and this combination is becoming more routinely used in Valuable clinical information can be gained from non- several in-patient situations, including moderate sedation invasive CO2 monitoring. Capnometry (digital display of and patient-controlled analgesia. Furthermore, capnogra- data) and capnography (graphical display of data) can be phy has become the standard of care to confirm endotra- either time-based or volume-based (ie, volumetric). Cap- cheal intubation in all hospital settings (operating room, nography refers to the depiction of exhaled CO2 during the emergency department, and intensive care unit). Thus, the entire respiratory cycle and provides a visual display of the “simple” question for this paper is whether capnography waveform. Capnography is a better indicator of dynamic should be recommended for all patients for the duration of changes in gas exchange than is capnometry alone.1,2 It mechanical ventilation. The controversy results from the must be stressed that if capnography is used, the proper fact that this question has not been formally studied in a clinical interpretation of the waveform is essential to the prospective, randomized fashion. Additionally, there have ideal management of the mechanical ventilator and to pro- been no published studies of any design that even attempt vide safe patient care. Deviations from characteristic wave- to address this topic. forms3,4 (Figs. 1 and 2) suggest an abnormality (Fig. 3) The focus of this paper is becoming more pertinent to that requires recognition and possibly correction. bedside clinicians, as important technological advances in Time-based capnography is best known as end-tidal car- capnography have occurred over the past decade. Previ- bon dioxide (P ) monitoring. When used without qual- ETCO2 ously, technical limitations often precluded the continuous ification, the term “capnography” refers to time-based val- use of capnography with critically ill patients. Clinicians ues. A time-based capnogram provides qualitative can now continuously, noninvasively, and accurately mea- information on the waveforms associated with mechanical sure CO2 elimination (both as a partial pressure and a ventilation and a quantitative estimation of the partial pres- volume) and dead-space ventilation, with few technical sure of expired CO2. Volumetric capnography uses a CO2 hurdles. sensor and a pneumotachometer in combination. This per- The accepted standard for measuring CO production is 2 mits calculation of the net volume of CO2 expired by the still arterial blood analysis. The appeal of this technique is patient, which is expressed as a volume of gas (generally blunted clinically because it is invasive, can be somewhat in mL/min) rather than a partial pressure or gas fraction. By analyzing the 3 phases of the volumetric capnogram, clinicians can potentially assess clinical issues of concern Ira M Cheifetz MD FAARC is affiliated with the Department of Pediatric (Fig. 4). Phase 1 of the single-breath CO2 waveform rep- Critical Care Medicine, Duke Children’s Hospital, Durham, North Caro- resents gas exhaled from the upper airways (ie, from an- 1 lina. Timothy R Myers RRT-NPS is affiliated with the Asthma Center, atomical dead space), which generally is void of CO2. An Division of Pediatric Pulmonology, Rainbow Babies and Children’s Hos- increase in phase 1 indicates an increase in anatomical pital, Case Western Reserve University, Cleveland, Ohio. dead space. Phase 2 is the transitional phase from upper to Ira M Cheifetz MD FAARC and Timothy R Myers RRT-NPS presented lower airway ventilation, and it tends to depict changes in a version of this paper at the 38th RESPIRATORY CARE Journal Confer- perfusion. Phase 3 represents alveolar gas exchange, which ence, “Respiratory Controversies in the Critical Care Setting,” held Oc- indicates changes in gas distribution. An increase in the tober 6–8, 2006, in Banff, Alberta, Canada. slope of phase 3 indicates increased maldistribution of gas The authors report no conflicts of interest related to the content of this delivery. paper. A brief review of the pertinent physiology is essential to this debate. The “normal” difference between P and Correspondence: Ira M Cheifetz MD FAARC, Department of Pediatric aCO2 P is approximately 4–5 mm Hg, which represents the Critical Care Medicine, Duke Children’s Hospital, Box 3046, Durham ETCO2 NC 27710. E-mail: [email protected]. normal dead-space ventilation. In a healthy upright person,

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Fig. 3. Time-based capnographic waveform representing severe bronchospasm.

dioxide difference signifies the combination of the anatomical and alveolar dead spaces. Fig. 1. This single-breath carbon dioxide waveform depicts carbon Capnography is a vital monitoring system for critically dioxide elimination as a function of the volume of gas exhaled. ill patients; however, the data obtained from capnography Note that time is not a variable in this graph. P ϭ partial ETCO2 must be integrated with all of the available patient data, pressure of end-tidal carbon dioxide. especially physical assessment. It must be stressed that capnography is not a measurement solely of respiratory function; capnograms must be interpreted in conjunction with other clinical findings. It should be noted that changes in end-tidal CO and CO elimination (V˙ ) almost al- 2 2 CO2 ways precede changes in oxygen saturation, heart rate, and blood pressure. Although most clinicians would agree that capnography Fig. 2. Normal features of a capnogram. A: Baseline represents the offers a unique vantage point on cardiorespiratory physi- beginning of expiration and should start at zero. B: The transitional ology, the role of capnography for mechanically ventilated part of the curve represents mixing of dead space and alveolar patients outside the operating room remains highly de- gas. C: The alpha angle represents the change to alveolar gas. D: The alveolar part of the curve represents the plateau average bated. Thus, the goal of this article is to highlight the alveolar gas concentration. E: The end-tidal carbon dioxide value. potential advantages and disadvantages of continuous cap- F: The beta angle represents the change to the inspiratory part of nography from intubation to extubation for every mechan- the cycle. G: The inspiration part of the curve shows a rapid de- ically ventilated patient. crease in carbon dioxide concentration. (From Reference 4, with permission.) Pro: Every Mechanically Ventilated Patient Should Be Monitored With Capnography gravity causes a vertical distribution difference of venti- From Intubation to Extubation lation and perfusion that increases from the bases to the apices of the lung (Fig. 5). In the lung bases, alveoli are Continuous capnography from intubation to extuba- perfused but not ventilated (ventilation/perfusion ratio tion offers several benefits, including confirming tra- [V˙ /Q˙ ] ϭ zero), whereas in the apical regions, alveoli are cheal intubation, monitoring the integrity of the endo- ventilated but not perfused (V˙ /Q˙ ϭ infinity). On average, tracheal tube (ETT) and ventilatory circuit, assisting the typical V˙ /Q˙ is 0.8, and the alveolar carbon dioxide with the titration of mechanical ventilatory support, as- concentration (P ) is approximately 40 mm Hg. This sessing pulmonary capillary blood flow, and monitoring ACO2 results in a “regionalization” of alveolar CO2. Thus, the for extubation readiness. The technology required to relationship between P and P must be considered perform capnography on expired gas is not new, al- aCO2 ETCO2 in relation to a patient’s V˙ /Q˙ relationship, disease process, though recent advances have greatly improved the reli- and changes in dead-space ventilation. ability and clinical applicability. From the start it must Because of the properties of shunt and venous admix- be noted that capnography has been considered a basic ture, there is normally a small difference between P standard of care in anesthetic monitoring by the Amer- aCO2 and P (P ). Shunt increases CO and decreases ican Society for Anesthesiologists since 1986.6 ACO2 (A-a)CO2 2 O2 in blood returning to the left side of the heart from the The pro stance on this topic is based on the following pulmonary capillaries. P decreases by 2 decrements basic underlying principles: (1) the potential clinical ben- ACO2 during exhalation, through (1) diluted CO2 from very poorly efits of continuous capnography clearly outweigh any po- perfused alveoli (alveolar dead space) and (2) further by tential risks, (2) life-threatening airway disasters can be

CO2 from the conducting airways (anatomical or func- averted with continuous capnography, (3) capnography re- tional dead space). The alveolar versus end-tidal carbon veals changes in circulatory and respiratory status sooner

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Endotracheal Intubation

Capnography as an adjunct to determine that the ETT is in the trachea rather than esophagus is clearly well supported by the medical literature from the intensive care unit, emergency department, operating room, and in the field, by emergency medical personnel.7–16 Capnography is the standard of care to confirm tracheal intubation. Roberts et al10 found in a neonatal population that capnography can more quickly and accurately determine tracheal intubation than can other clinical assessments. Cap- nography correctly identified errant tube placements in 98% of instances, in 1.6 Ϯ 2.4 s. Birmingham et al8 reported that clinical evaluation alone is not reliable for confirming endotracheal intubation. They concluded that, other than direct visualization with laryngoscopy, only capnography is consistently more reliable than any other Fig. 4. Volumetric capnogram. Phase 1 represents the quantity of method. Knapp et al9 reported that the reliability of cap- carbon dioxide eliminated from the upper airways. Phase 2 is the nography, unlike other methods, including auscultation, is transitional zone that represents ventilation from both large and independent of clinician experience (Fig. 6). Kannan and small airways. Phase 3 represents carbon dioxide elimination from Manji14 concluded that capnography “as a mandatory mon- the alveoli and, thus, the quantity of gas involved with alveolar ventilation. (Courtesy of Respironics Inc and its affiliates, Walling- itor during tracheal intubation in the intensive care unit in ford, Connecticut.) conjunction with other methods might improve safety.” Recently, the American Heart Association strongly supported the use of capnography to confirm tracheal intubation of infants and children with a perfusing rhythm. The American Heart Association recommends using a colori-

metric detector or capnography to detect exhaled CO2 to confirm ETT position in the pre-hospital and hospital settings.15–17 Puntervoll et al18 reported limitations of the colorimetric techniques. It should be noted that, during

cardiac arrest, if exhaled CO2 is not detected, tube position must be confirmed by direct laryngoscopy, because the

absence of exhaled CO2 may reflect decreased pulmonary capillary blood flow (ie, poor cardiac output).16 Although time-based capnography (ie, P monitor- ETCO2 ing) is an effective tool for validating tracheal placement of the ETT,7–16 volumetric capnography may be even more effective.19 Time-based (end-tidal) capnography may oc- Fig. 5. Ventilation/perfusion relationship and the carbon dioxide casionally provide a false-positive reading (ie, the ETT is cascade. Left: The vertical difference of ventilation and perfusion not in the trachea but the monitor displays a PETCO value). results in increasing ventilation/perfusion ratio from lung base to 2 lung apex, whereas the alveolar partial pressure of carbon dioxide This is most likely with patients who (1) have recently (P ) increases down the lung. Right: The P cascade. V˙ /Q˙ ϭ ingested carbonated beverages or antacids, (2) have re- ACO2 CO2 A ratio of alveolar ventilation to perfusion. (Adapted from Reference 5, ceived prolonged bag-valve-mask ventilation prior to in- with permission.) tubation, and (3) have the tip of the ETT located in the pharynx. A false-negative time-based result (ie, the ETT is in the trachea but the monitor does not display a P ETCO2 than does pulse oximetry, and (4) mechanical ventilation value) can also occur. This is most likely in patients with can be optimized and the duration of mechanical ventila- severe airway obstruction, depressed cardiac output, pul- tion potentially minimized with continuous volumetric cap- monary emboli, or pulmonary hypertension. Thus, Li20 nography. The bottom line is that capnography is an im- concluded that ETT placement should be confirmed by portant tool for airway monitoring, ventilator management, multiple techniques, including time-based or volume-based and overall cardiorespiratory assessment. capnography.

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Fig. 6. Error rates of experienced and inexperienced clinicians in determining tracheal positioning of the endotracheal tube, with 4 methods: auscultation, capnography (end-tidal carbon dioxide [ETCO2]), self-inflating bulb, and trachea light. (From Reference 9, with permission.)

Preventing Mishaps an ETT or ventilator-circuit disconnect, and 26% were related to a dislodged ETT. It is important to note that Once endotracheal intubation has been confirmed, the none of these adverse events were related to ventilator medical literature supports the use of continuous capnog- malfunction. Based on that Sentinel Event Alert, it seems raphy in the operating room to monitor the integrity of the reasonable to use capnography as an additional tool to ventilatory circuit, including the artificial airway.21–23 Cap- promptly alert caregivers to an inadvertent extubation nography is consistently the most sensitive indicator that or a ventilator-circuit disconnect. Capnography can help an ETT has moved or dislodged. Based on the risk of an avert airway disasters. airway disaster, it seems reasonable to employ capnogra- Recently, the American Heart Association recom- phy in conjunction with the ventilator-disconnect alarm, as mended that during intra-hospital and inter-hospital an important “double-check” for patient safety. transport, exhaled CO2 (qualitative colorimetric detec- Continuous P monitoring detects acute airway ob- tor or capnography) should be continuously monitored ETCO2 struction and hypopharyngeal extubation more rapidly than with intubated patients to assure the continued integrity does vital-sign monitoring or continuous pulse oximetry.24 of the airway.15 Ahrens and Sona25 reported that capnography in intensive care units, emergency departments, recovery rooms, and Operating Room Setting long-term ventilator facilities is an invaluable tool for the early identification of a dislodged ETT.25 They stated that As early as 1986, the use of capnography as a routine “capnography offers the most rapid alarm for a ventilator monitor in the operating room setting was strongly rec- disconnection, by immediately showing loss of expired ommended. Eichhorn et al,22 as part of a major patient- CO .”25 However, it should be noted that a P reading safety/risk-management effort, devised specific, detailed, 2 ETCO2 may still be displayed if the disconnect occurs between the and mandatory standards for minimal patient monitoring

CO2 detector and the ventilator circuit in a patient with during anesthesia. They concluded that the early detection spontaneous respiratory effort. of untoward trends or events during anesthesia would pre- A report by the Joint Commission (formerly the Joint vent or mitigate patient injury, which, they speculated, Commission on Accreditation of Health Care Organi- might halt the substantial upward trend in anesthesia-re- zations) clearly supports the need for additional moni- lated malpractice suits, settlements, judgments, and insur- toring of the integrity of the ventilatory apparatus. A ance premiums occurring at that time. Joint Commission Sentinel Event Alert in 2002 reported Tinker et al26 reviewed 1,175 anesthetic-related closed 19 ventilator-associated deaths and 4 ventilator-associ- malpractice claims from 17 insurance companies over 14 ated devastating neurologic injuries,23 of which 65% years, and found that 31.5% of negative anesthetic out- were at least partly related to the malfunction or misuse comes could have been prevented by additional monitor- of an alarm or an inadequate alarm, 52% were related to ing. Furthermore, the settlements for the incidents that

RESPIRATORY CARE • APRIL 2007 VOL 52 NO 4 427 CAPNOGRAPHY FOR MECHANICALLY VENTILATED PATIENTS were believed to be preventable by additional monitoring were 11 times more expensive than those mishaps that were deemed not preventable. Tinker et al concluded that capnography and pulse oximetry, when used in combination, could have prevented 93% of the preventable mishaps. Williamson et al27 echoed those findings; they concluded that capnography is critical for the detection of general anesthesia adverse events. It is important to em- phasize that these studies that are supportive of capnography from intubation to extubation in the operating room setting were performed with technology that is several Fig. 7. Relationship between P , alveolar partial pressure of aCO2 generations older than what is available today. CO (P ), CO clearance from the capillaries into the alveoli 2 ACO2 2 (P ), and partial pressure of end-tidal carbon dioxide (P ). The medical literature clearly supports the use of con- VCO2 ETCO2 The difference between P and P is directly proportional tinuous capnography to prevent mishaps in the operating aCO2 ETCO2 to the quantity of dead space, because CO -free gas from the room. Unfortunately, data are lacking from the intensive 2 dead space dilutes the CO2-containing gas from the gas- care unit. However, one might speculate that if continuous exchanging alveoli. (From Reference 35, with permission.) CO2 monitoring is required in the operating room, where the physician-to-patient ratio is at least 1:1, then continuous capnography should be required in the intensive care the alveolar dead space dilutes CO2-containing gas from unit, where the physician-to-patient ratio is far less than the gas-exchanging alveoli (alveolar ventilation, P ) ACO2 1:1. and, thus, influences the P difference. (a-ET)CO2 The alveolar concentration of CO (P ) is generally 2 ACO2 Physiology of the Arterial-Versus-End-Tidal CO2 slightly less than that of mixed venous blood, but slightly Difference greater than that of arterial blood. Alveolar and anatomical

dead-space gas, which is void of CO2, dilutes the alveolar Time-based and volume-based (ie, volumetric) cap- CO concentration. Thus, P is normally positive 2 (a-ET)CO2 nography both display immediate responses to changes in (approximately 2–5 mm Hg), because the anatomical dead ventilatory strategy, and they monitor a patient’s overall space is large enough to decrease P below P .28,29 ETCO2 aCO2 cardiorespiratory function. Volumetric capnography simul- However, if the lungs are homogeneous (ie, normal V˙ /Q˙ taneously measures expired CO2 and tidal volume to de- relationship with no, or very minimal, alveolar dead space), termine V˙ from 3 compartments: (1) artificial airway functional residual capacity is reduced, and anatomical CO2 and anatomical dead space, (2) transition from airway to dead space is small (eg, during exercise and in supine alveolar ventilation, and (3) alveolar gas. This information pregnant women),30–32 then the P is only minimally ACO2 is obtained from the single-breath CO waveform (see diluted by CO -free gas, which results in a P that 2 2 ETCO2 Figs. 1 and 4). Both P and V˙ can be used to may be greater than P . A negative P value is ETCO2 CO2 aCO2 (a-ET)CO2 determine alterations in gas exchange in response to changes physiologically possible. It should also be stressed that in mechanical ventilatory support. P represents the temporal and spatial mean alveolar aCO2 Capnography in the critical care setting has been erro- concentration of CO , whereas P represents the peak 2 ETCO2 neously judged as unreliable because of its variability rel- alveolar concentration of CO2 eliminated, which generally ˙ ˙ 28,30–32 ative to arterial CO2 measurements. The evidence avail- reflects slow CO2 elimination from low V/Q areas. able in the medical literature on P as a surrogate for The P value is a function of the rate of alveolar ETO2 (a-ET)CO2 P in mechanically ventilated patients is varied, which emptying and the total dead space of the lungs.28,33 aCO2 has resulted in skepticism in many clinicians. Thus, cap- As alveolar dead space increases, P increases.28 (a-ET)CO2 nography has been underutilized, potentially to the detri- In patients with substantial pulmonary disease, P (a-ET)CO2 ment of patient care. Factors such as patient position, air may increase unpredictably, such that P is no longer ETCO2 leak, changes in airway and alveolar dead space, and al- a reliable reflection of the effectiveness of ventilation and, terations in cardiac output should be expected to affect thus, no longer accurately represents P .34 Conversely, aCO2 P and its relationship to P . Thus, a review of as pulmonary disease improves, P narrows due to ETCO2 aCO2 (a-ET)CO2 physiology is essential to addressing the relationship be- improvement in V˙ /Q˙ matching. In patients with lung dis- tween P and P (P ) and to correctly inter- ease, P is usually an excellent indicator of the aCO2 ETCO2 (a-ET)CO2 (a-ET)CO2 preting the available medical literature. P(a-ET)CO is an efficiency of ventilation (ie, ratio of dead-space volume to ˙ ˙ 2 indicator of V/Q mismatching that results from cardiopul- tidal volume [VD/VT]) (Fig. 7). A patient’s progress dur- monary alterations, and P is directly proportional ing weaning and the consequences of changes in the ven- (A-ET)CO2 36 to the degree of alveolar dead space. Gas void of CO2 from tilator support can be determined by capnography.

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Carbon Dioxide Elimination Decreased V˙ , with a decrease in the phase-2 slope of CO2 the waveform (ie, decreased pulmonary perfusion), indi- 40–42 Capnography allows continuous monitoring of the vol- cates excessive PEEP. Decreased pulmonary perfusion secondary to excessive PEEP is generally caused by ume of CO2 eliminated through the lungs per unit time (V˙ in mL/min). Since V˙ is affected by ventilation, increased intrathoracic pressure, which decreases systemic CO2 CO2 circulation/perfusion, and, to a much lesser degree, diffu- venous return (ie, decreases right-ventricular preload) and sion, it is a sensitive marker for changes in a ventilated increases pulmonary vascular resistance (ie, increased right- 43 patient’s cardiorespiratory status. V˙ can signal changes ventricular afterload). Decreased pulmonary blood flow CO2 in P . Volumetric capnography has been used success- reduces CO2 transport from the tissues to the pulmonary aCO2 1 fully to measure anatomical dead space, pulmonary cap- vasculature, which reduces CO2 elimination. illary perfusion, and effective ventilation, to provide a Phase 3 of the waveform represents gas distribution at breath-to-breath indicator of changes in gas exchange in the alveolar level. An increase in the phase-3 slope depicts response to ventilator settings and to monitor changes in a maldistribution of gas, which can be caused by an inap- cardiorespiratory interactions.37,38 propriately low PEEP setting.

As with oxygen consumption, CO2 production and elim- ˙ ination (V˙ ) is a continuous process. Therefore, V˙ Alveolar VE CO2 CO2 rapidly reflects changes in ventilation and perfusion, regardless of etiology. Additionally, V˙ reflects the phys- ˙ CO2 The VE value displayed on the ventilator represents the iologic response to changes in mechanical ventilator sup- total quantity of gas moving in and out of the lungs per port. Thus, capnography is a useful and sensitive clinical minute (ie, respiratory rate times tidal volume), which rep- tool to assess a patient’s cardiorespiratory and metabolic resents the sum of alveolar and dead-space ventilation. On status.2,39 ˙ the other hand, alveolar VE is respiratory rate times the volume of air that reaches the alveoli and participates in ˙ Management of Positive End-Expiratory Pressure gas exchange at the capillary level (ie, VE minus dead- ˙ space ventilation). Alveolar VE is determined from the volumetric capnogram (see Fig. 4). Phase 3 of the wave- Determining the appropriate positive end-expiratory form represents the quantity of gas exhaled from the al- pressure (PEEP) setting is essential for optimal manage- veoli. Thus, alveolar V˙ is the volume of alveolar gas per ment of a mechanically ventilated patient with acute lung E breath summated over 1 min. injury, but there is controversy concerning the best method Traditional determination of V˙ may not accurately rep- for determining the appropriate PEEP for an individual E resent the volume of gas involved in gas exchange at the patient. The ideal PEEP provides the optimal lung volume alveolar level. Volumetric capnography provides continu- and, thus, the highest oxygenation for the lowest fraction ˙ ous determination of alveolar VE, which might help opti- of inspired oxygen (FIO ), the best pulmonary compliance, 2 mize ventilator management. As dead-space ventilation and the greatest cardiac output. In reality, there is no PEEP approaches zero, then alveolar V˙ approaches total V˙ . setting that achieves all of these goals for a specific patient E E Unfortunately, there are no data that clearly support a at a given time. Thus, it is the clinician’s responsibility to clinical benefit of monitoring alveolar V˙ . determine the PEEP setting that most optimally balances E these cardiorespiratory goals, keeping in mind that this value is likely to change. Minimizing the Duration of Mechanical Ventilation PEEP can be titrated by monitoring V˙ and the volu- CO2 metric capnogram. Changes in lung volume and its asso- Minimizing the duration of mechanical ventilation is ciated effects on pulmonary blood flow alter CO2 elimi- crucial in the management of critically ill patients. Shorter nation. Theoretically, the optimal lung volume (associated ventilation should decrease the risk of ventilator-associ- with the optimal PEEP) should be associated with a tran- ated lung injury, overall morbidity, and hospital costs. At sient increase in CO elimination. V˙ is more informa- our institution we investigated the clinical impact of con- 2 CO2 tive than P during PEEP titration.1,2 tinuous volumetric capnography on shortening the dura- ETCO2 Volumetric capnography can provide valuable information of mechanical ventilation in a heterogeneous group of tion for PEEP management. An increase in anatomical pediatric intensive-care patients, in a randomized controlled dead space is often present when high PEEP is applied.39,40 study.44 Patients managed with continuous volumetric cap- In this situation, an increase in anatomical dead space can nography had significantly shorter ventilation than did the be quickly recognized by an increase in phase 1 of the control patients. In contrast to routine clinical management capnogram (see Figs. 1 and 4) and reducing PEEP may alone, continuous volumetric capnography combined with ˙ improve alveolar minute ventilation (VE). routine clinical management significantly reduced the du-

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Fig. 9. Changes in the ratio of pulmonary dead space to tidal

volume (VD/VT) in adult patients with acute respiratory distress syndrome (ARDS). The boxes represent the 25–75% data interval.

The horizontal lines in the boxes represent the mean VD/VT values. The error bars represent the 95% confidence intervals. * p Ͻ 0.05 Fig. 8. Mortality relative to the quintile of dead-space fraction in between survivors and nonsurvivors on the same day. † p Ͻ 0.001 adult patients with acute respiratory distress syndrome. (From Ref- between survivors and nonsurvivors on the same day. (From Ref- erence 45, with permission.) erence 46, with permission.) ration of ventilation in a heterogeneous sample of infants diac output produces a high V˙ /Q˙ ratio, which decreases and children. P and increases P . Thus, P is a function ETCO2 (a-ET)CO2 ETCO2 of cardiac output for a given V˙ .48 Hence, P and E ETCO2 Acute Respiratory Distress Syndrome: Prognosis V˙ are noninvasive indicators of pulmonary capillary CO2 blood flow. Utilizing this principle, V˙ and P can CO2 ETCO2 Capnography can also be a valuable tool in the progno- be used to monitor pulmonary capillary blood flow in sis for adult patients with acute lung injury. Nuckton et al45 many common clinical scenarios, and the effectiveness of reported that during the first day of onset of acute respi- cardiorespiratory resuscitation. ratory distress syndrome (ARDS), an increased physiologic dead-space fraction was an independent and power- Extubation ful predictor of mortality (Fig. 8). The relative risk of death increased by 45% for every 0.05 increase in dead- Predicting successful extubation remains a challenge, space fraction. This increased risk was greater than any especially in the neonatal and pediatric populations. Though other predictive factor, including severity of illness or re- many measures have been suggested as reliable predictors spiratory compliance. Nuckton et al were the first to report of successful extubation, few have been definitively proven, a lung-specific predictor of mortality for ARDS. In a fol- and of those a large percentage require multiple tests over 46 49–52 low-up study, Kallet et al found that a sustained VD/VT a potentially lengthy period. In adults, spontaneous elevation in adult ARDS patients was characteristic of breathing trials are helpful. nonsurvivors (Fig. 9). In a univariate analysis, a VD/VT With extubation failure rates reported to be as high as Ͼ 0.55 during the first 6 days of ARDS was associated 20%, accurately predicting extubation readiness remains with a significantly higher risk of mortality. Kallet et al an important clinical focus.53,54 Because of the greater concluded that dead-space measurements made beyond the risks with prolonged ventilation49,50,55–59 and the risks of first 24 hours of ARDS might be prognostic.46 reintubation,55,60–67 the decision to extubate must be timed carefully. Though many extubation failure predictors ex- Cardiovascular Physiology ist,68–70 a limited number of studies have shown effective success indicators.

Time-based capnography can be very useful in assess- The physiologic VD/VT is a reliable marker of lung ing changes in a patient’s cardiovascular status. In the disease in adult critical care patients.71 With the advances extreme, increases in P and V˙ during cardiopul- in technology over the past decade, physiologic V /V ETCO2 CO2 D T monary resuscitation signify an increase in cardiac output can be obtained quickly, accurately, and noninvasively (ie, pulmonary capillary blood flow) as spontaneous cir- at the bedside. Using a modified Bohr equation, volu- culation returns.47 On the other hand, a reduction in car- metric capnography can rapidly calculate and display

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5 VD/VT values. Though there have been no studies on the must be carefully considered. More specifically, the dis- effectiveness of physiologic VD/VT as a predictor of ex- ease states and conditions described below create impor- Ͻ tubation success in adults, VD/VT 0.5 is predictive of tant problems for reliable patient monitoring with capnog- extubation success in infants and children.72 An increased raphy. The bottom line is that the monitoring of exhaled physiologic VD/VT warrants investigation prior to an ex- CO2 does not serve as an important tool for airway mon- tubation trial. itoring, ventilator management, or overall cardiorespiratory assessment because of the common (and unpredict- Pro Summary able) discrepancy between P and P aCO2 ETCO2

Capnography noninvasively and continuously monitors Patient Position

CO2 elimination throughout the respiratory cycle. Both time-based and volume-based capnography allow for me- In a prospective study by Grenier et al,74 the ability of chanical ventilation and extubation strategies to be de- P to accurately estimate P during neurosurgical ETCO2 aCO2 signed with clear, precise, objective criteria. With the ob- procedures was assessed in relation to surgical position. jective data provided by capnography, adequate gas Patients were classified into groups according to position: delivery, optimal PEEP, effective ventilation, and timing supine, lateral, prone, or sitting. The mean P was (a-ET)CO2 for extubation can be established regardless of clinician or 6 Ϯ 4 mm Hg. Grenier et al compared 624 simultaneous institutional preferences. measurements of P and P and found no differ- aCO2 ETCO2 Continuous capnography as an additional clinical tool ence in P ,P ,orP for the supine, prone, aCO2 ETCO2 (a-ET)CO2 from intubation to extubation quickly alerts clinicians to or sitting groups. In the lateral-position group, P was ETCO2 important changes in a patient’s cardiorespiratory status. significantly lower, whereas P was not different, which aCO2 Although there are few data to support continuous cap- resulted in a significantly higher P (7 Ϯ 3mmHg, (a-ET)CO2 nography in the intensive care unit, extrapolating the sub- p Ͻ 0.05). stantial data from the operating room setting is reasonable. Negative P values, where P is greater than (a-ET)CO2 ETCO2 Additionally, basic physiologic principles support the rou- P , were first reported by Nunn and Hill in 1960.75 aCO2 tine use of capnography. However, education is a key In the study by Grenier et al,74 a negative difference was component of the successful use of capnography. The mul- found in 4% of the comparisons, but only in the prone and tidisciplinary bedside clinical team must know how to in- sitting positions. A more frequent occurrence was changes terpret the individual data points, the trends over time, and in opposite directions (decrease in P with increase in ETCO2 the capnograms. P or vice versa), which occurred in 25% of the mea- aCO2 Overall, capnography is a safe, cost-effective, noninva- surements. These changes occurred more frequently in the sive monitoring technology that enhances patient safety sitting, lateral, and prone groups. Grenier et al also found and may optimize mechanical ventilation while minimiz- large difference variations (up to 19 mm Hg) in the values ing the duration of ventilation. The bottom line is that a from individuals. In all, 11% of measurements obtained simple, relatively inexpensive monitor can prevent airway varied by Ͼ 5 mm Hg, especially in the prone and sitting disasters and save lives. groups. Grenier et al74 concluded that, though the mean P (a-ET)CO2 Con: Not Every Mechanically Ventilated Patient was similar to values reported in the literature, P could aCO2 Should Be Monitored With Capnography not be reliably estimated by P because of scattering ETCO2 From Intubation to Extubation of individual values (in Bland-Altman analysis), negative P values, large variations in opposite directions, (a-ET)CO2 Even from the con position it must be acknowledged large difference changes between successive measure- that capnography has demonstrated its value in confirming ments, and instability of P values over time, ac- (a-ET)CO2 correct placement of the ETT and monitoring the integrity cording to position. of mechanical ventilation equipment. Furthermore, in the operating room it is common practice to adjust ventilator Cyanotic Congenital Heart Disease settings based on P , assuming a more or less constant ETCO2 relationship between P and P .73 The purpose of P is generally determined by 4 components: al- aCO2 ETCO2 (a-ET)CO2 this review is to use the medical literature to demonstrate veolar ventilation, pulmonary perfusion, CO2 production, that this practice is not applicable to other areas of the and V˙ /Q˙ . In children with cyanotic congenital heart dis- hospital, especially the intensive care unit. ease, alveolar ventilation typically is normal, whereas pul-

When assessing the applicability of capnography for all monary perfusion and CO2 production may be abnormal, mechanically ventilated patients, the disease states or con- resulting in V˙ /Q˙ mismatch. In congenital heart disease, ditions that affect the relationship between P and P low pulmonary perfusion and/or right-to-left shunting of aCO2 ETCO2

RESPIRATORY CARE • APRIL 2007 VOL 52 NO 4 431 CAPNOGRAPHY FOR MECHANICALLY VENTILATED PATIENTS low-CO /high-CO venous blood leads to cyanosis and, Simultaneous P and P measurements in children 2 2 aCO2 ETCO2 theoretically, an increased P . The following stud- with saturations of 65–97% (median 85%) while breathing (a-ET)CO2 ies demonstrate the difficulty in using capnography as a room air were obtained during surgery. P was great- (a-ET)CO2 surrogate for arterial blood gas analysis in children with er-than-predicted, despite corrections for right-to-left cyanotic congenital heart disease. shunting. The patients had a mean P of (a-ET)CO2 In one of the earliest studies to assess the accuracy of 8.1 Ϯ 4.4 mm Hg, versus a mean predicted value of capnography in congenital heart disease, Burrows76 simul- 2.9 Ϯ 2.3 mm Hg (p Ͻ 0.001). Short et al concluded that taneously assessed P and P in patients with cy- the P difference in children with congenital heart ETCO2 aCO2 (a-ET)CO2 anotic and acyanotic lesions. P significantly under- disease is difficult to predict, and they hypothesized that ETCO2 estimated P in all cyanotic patients; however, P this greater-than-predicted P indicated that factors aCO2 ETCO2 (a-ET)CO2 correlated closely with the P in the acyanotic group. other than right-to-left shunting were contributing to this aCO2 VD/VT values from the cyanotic patients were significantly difference in gas exchange. greater than those from the acyanotic group. Burrows con- De Vries and colleagues73 evaluated the relationship cluded that, while P may be an acceptable estimate of between P and arterial oxygen saturation measured via ETCO2 CO2 P in children with acyanotic congenital heart disease, pulse oximetry (S ), and, if a consistent relationship was aCO2 pO2 P significantly underestimates P in children with found, to assess whether S could be applied as a cor- ETCO2 aCO2 pO2 cyanotic congenital heart disease. recting factor to estimate P from P in patients aCO2 ETCO2 In a follow-up study, Lazzell and Burrows77 evaluated with congenital heart disease. The study consisted of (1) a the stability of P during surgery in children with retrospective study to evaluate the relationship between (a-ET)CO2 congenital heart disease. Children were divided into 4 equal S and the change in P to arrive at a more reliable pO2 CO2 groups: (1) no interchamber communication and normal estimate of P from P (ie, derivation set to correct aCO2 ETCO2 pulmonary blood flow (normal group); (2) acyanotic chil- for degree of hypoxia), and (2) a prospective observational dren with increased pulmonary blood flow (acyanotic shunt- study to test the clinical usability and reliability of the ing group); (3) cyanotic children with mixing lesions and correction formula (ie, validation set). The correlation be- normal or increased pulmonary blood flow (mixing group); tween P and the raw P values was r2 ϭ 0.17 aCO2 ETCO2 and (4) cyanotic children with right-to-left intracardiac (p Ͻ 0.05), whereas the correlation between P and the aCO2 shunt, which indicates decreased and variable pulmonary corrected P values was r2 ϭ 0.94 (p Ͻ 0.001). There ETCO2 blood flow (cyanotic shunting group). Simultaneous P was no significant difference between the actual values aCO2 and P measurements were obtained from each patient and the corrected values. Accurate decision making on ETCO2 during identifiable intraoperative events, including arterial ventilator settings would have been supported in 92% of line placement, patient preparation, post-sternotomy, post- the cases with the corrected P values, compared to ETCO2 heparin-administration, and immediately after aortic can- just 5% when using the raw P values. De Vries and ETCO2 nulation. colleagues73 concluded that, while capnometry is a useful The cyanotic children had a greater P than the tool in respiratory monitoring, it has important physiologic (a-ET)CO2 acyanotic children in the initial measurement comparison. limitations in estimating P in children with congenital aCO2 No differences existed in P during the course of heart disease. The authors believed that the utility of cap- (a-ET)CO2 surgery in the control, acyanotic shunting, or mixing groups. nometry in these children can be enhanced by correcting P in children with cyanotic shunting lesions was the P value for the degree of hypoxia. (a-ET)CO2 ETCO2 significantly greater after patient preparation and sternotomy than during the initial comparison. Lazzell and Bur- Transcutaneous Monitoring rows77 found considerable inter-individual and intra-individual P variability in the children who had mixing Wilson et al79 prospectively compared 2 methods of (a-ET)CO2 and right-to-left shunting congenital heart disease. noninvasive CO measurement: P and transcutane- 2 ETCO2 Lazzell and Burrows77 concluded that P is not ously measured CO (P ) in infants and children sched- (a-ET)CO2 2 tcCO2 stable in children with cyanotic congenital heart lesions uled for repair of congenital heart disease (32 cyanotic and and cannot be used during surgery to reliably estimate 21 acyanotic patients). Before cardiopulmonary bypass, P . Though the P in children with acyanotic they obtained simultaneous arterial blood gas, P , and aCO2 (a-ET)CO2 ETCO2 shunting and mixing congenital heart lesions is stable in- P measurements. The mean difference between the tcCO2 traoperatively, mixing congenital heart lesions can cause P and P values (P )(2Ϯ 1 mm Hg) was ETCO2 tcCO2 (a-tc)CO2 large individual variations, which brings into question the significantly less (p Ͻ 0.001) than P (5 Ϯ 3 mm Hg). (a-ET)CO2 reliability of P monitoring in these children. P was lower in 6 patients, whereas P was ETCO2 (a-ET)CO2 (a-tc)CO2 The accuracy and reliability of P monitoring in lower in 39 patients. P and P were equally ETCO2 (a-ET)CO2 (a-tc)CO2 infants and children with congenital heart disease were accurate in 8 patients. The difference was most significant also brought into question in a study by Short et al.78 in patients with cyanotic congenital heart disease, in whom

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P (2 Ϯ 1 mm Hg) was significantly more accurate concordant directional changes in end-tidal and arterial tcCO2 than P (7 Ϯ 3 mm Hg, p Ͻ 0.001). This study by CO are not assured in a linear fashion. ETCO2 2 Wilson et al79 indicates that transcutaneous monitoring Mechanical ventilation of patients with head injury, who provides a better estimation of P than does capnogra- are at risk for intracranial hypertension, requires high aware- aCO2 82 phy. They also noted that this difference was especially ness of CO2 level. Kerr et al conducted a prospective, apparent in patients with cyanotic congenital heart disease repeated-measures study to examine the agreement be- and in those Ͻ 1 year of age. tween P and P and the magnitude of P ,to aCO2 ETCO2 (a-ET)CO2 In another prospective comparison, Tobias and col- track P prior to and after endotracheal suctioning, ETCO2 leagues80 investigated the efficacy and accuracy of trans- and to identify factors that affect the accuracy and useful- cutaneous monitoring in infants and children after cardio- ness of capnography in the clinical management of me- thoracic surgery. They studied 33 consecutive patients who chanically ventilated adults with severe head trauma 48–72 had undergone cardiothoracic surgery. P was deter- hours after injury. They studied 35 consecutive patients ETCO2 mined via infrared spectroscopy with a sidestream aspira- (62 observations) admitted to the emergency department tor. Within the first postoperative hour, if the P and with severe head injury (Glasgow Coma Scale score Յ 8). ETCO2 P values did not correlate (difference Ն 5 mm Hg), a The Pearson correlation coefficient indicated a weak rela- aCO2 P electrode was placed. In only 3 cases was P tionship between P and P , regardless of whether tcCO2 (a-ET)CO2 aCO2 ETCO2 Ͻ 5mmHg,soaP electrode was placed in 30 pa- the measurement was made immediately prior to or after tcCO2 tients. In 3 patients, all of whom exhibited cardiovascular endotracheal suctioning (r2 ϭ 0.09 and 0.11, respectively). instability, P was Ն 5 mm Hg. In the remaining 27 Following the Bland-Altman technique, the mean P (a-tc)CO2 (a-ET)CO2 patients, a total of 101 pairs of P and P values was 5.8 Ϯ 5.9 mm Hg before suctioning and tcCO2 aCO2 were analyzed. Mean absolute P was 7.1 Ϯ 6.4 mm Hg after suctioning. The estimated upper (a-tc)CO2 1.7 Ϯ 1.4 mm Hg (range 0–9 mm Hg). In the 101 paired and lower limits of agreement showed that 95% of the values, P was Յ 2 mm Hg in 82 (81%), 3–5 mm Hg P differences were between Ϫ5.9 mm Hg and (a-tc)CO2 (a-ET)CO2 in 18 (18%), and Ն 6 mm Hg in 1 (1%). Linear regression 17.5 mm Hg prior to suctioning and between Ϫ5.5 mm Hg analysis revealed a slope of 0.90 and an r2 of 0.885 and 19.7 mm Hg after suctioning. This large range in the (p Ͻ 0.001). limits of agreement confirms a lack of agreement between In summary, Tobias and colleagues80 found that trans- P and P . aCO2 ETCO2 cutaneous monitoring provides clinically acceptable esti- Kerr et al82 noted that larger P values (maxi- (a-ET)CO2 mates of P in infants and children after cardiothoracic mum 25 mm Hg) were in patients with atelectasis, pneu- aCO2 surgery, unlike end-tidal CO2 monitoring. The authors did monia, or a chest tube. PEEP also significantly affected note that P may be inaccurate in patients who require the difference between P and P . As PEEP in- tcCO2 ETCO2 aCO2 vasoactive agents (especially in large doses) for cardio- creased, P increased. In patients who were not (a-ET)CO2 vascular dysfunction. breathing spontaneously there was a stronger correlation 2 ϭ between PETCO and PaCO (r 0.25), which was even 2 ϭ 2 2 Ͻ Trauma stronger (r 0.77) when PEEP was 5cmH2O or when the ratio of P to F was Ͻ 250 mm Hg. aO2 IO2 Trauma patients frequently have cardiopulmonary in- Kerr and colleagues82 concluded that P was less ETCO2 volvement that requires mechanical ventilation. The abil- valid as a surrogate for P in patients with the following aCO2 ity to monitor P to detect changes in cardiorespira- characteristics: spontaneously breathing, receiving assist- ETCO2 tory status and/or to adjust ventilator settings could have a control ventilation, PEEP Ͼ 5cmHO, low P /F ,or 2 aO2 IO2 major impact on patient care. Russell and Graybeal81 as- any combination of the above. The P bias raises (a-ET)CO2 sessed the accuracy and reliability of 171 P measure- serious questions about the clinical utility of this instru- ETCO2 ments, relative to P values, from 9 mechanically ven- ment in spontaneously breathing patients and patients de- aCO2 tilated trauma patients. The mean P was veloping pulmonary complications. (a-ET)CO2 14 Ϯ 11 mm Hg and showed a positive correlation (p ϭ 0.001, r2 ϭ 0.41); however, only 40% of the changes Obesity reflected a linear relationship. Though 78% of the individual patients had significant correlations (p Ͻ 0.02–0.001) V˙ /Q˙ abnormalities are an important factor in capnogra- of the P differences, P erroneously predicted phy. V˙ /Q˙ variations can greatly occur in situations where (a-ET)CO2 ETCO2 changes in 27% of the comparisons. To make matters functional residual capacity is reduced. Griffin et al83 stud- worse, the errors were not consistent, as 15% of the errors ied patients with severe obesity (body mass index Ͼ 40 kg/ were false decreases and 12% were false increases. The m2) undergoing gastric bypass surgery to determine the authors concluded that, for this trauma population, trends accuracy of P and P monitoring. Clinically in- ETCO2 tcCO2 in P magnitude cannot be reliably monitored, and dicated arterial blood gas samples were obtained with si- (a-ET)CO2

RESPIRATORY CARE • APRIL 2007 VOL 52 NO 4 433 CAPNOGRAPHY FOR MECHANICALLY VENTILATED PATIENTS multaneously recorded P and P readings. The comparisons (p ϭ 0.038). Four transcutaneous values and ETCO2 tcCO2 differences between the end-tidal and arterial values were 12 capnography measurements were 5 mm Hg Ͼ P aCO2 averaged. (p ϭ 0.06). Linear regression analysis of the P (a-ET)CO2 The absolute difference was significantly lower for the differences revealed a slope of 0.43 and an r2 of 0.77. P readings than for the P readings Linear regression analysis of the P differences re- tcCO2 ETCO2 (a-tc)CO2 (1.5 Ϯ 1.5 mm Hg vs 5.3 Ϯ 2.9 mm Hg, p Ͻ 0.001). P vealed a slope of 0.91 and an r2 of 0.90. Nosovitch and tcCO2 more closely correlated with P in 83% of patients, colleagues85 concluded that transcutaneous CO monitor- aCO2 2 whereas P was closer in just 13% (no difference in ing was more accurate than capnography during pediatric ETCO2 one patient). Griffin et al83 created Bland-Altman plots intraoperative care. and found that obese patients with the greatest hypercap- Russell and colleagues86 prospectively studied 59 post- nia also had the largest P values. It was presumed operative cardiac surgery patients for P differences (a-ET)CO2 (a-ET)CO2 that these patients had greater V˙ /Q˙ inequality. The authors from admission to extubation in the intensive care unit. A concluded that transcutaneous monitoring was more accu- total of 382 individual difference comparisons were made. rate than capnography in patients with a body mass index The relationship between P and P was assessed aCO2 ETCO2 Ͼ 40 kg/m2. during widely differing conditions of cardiorespiratory support. No respiratory, hemodynamic, or pharmacologic fac- Surgery Patients tors assessed significantly changed P . Mean P (a-ET)CO2 aCO2 was 36.5 Ϯ 5.9 mm Hg, and mean P was ETCO2 Casati and colleagues84 prospectively studied 17 con- 31.0 Ϯ 6.4 mm Hg, resulting in a mean P of (a-ET)CO2 secutive geriatric patients (age Ͼ 60 y) who were receiv- 5.5 Ϯ 5.2 mm Hg (range –8 to ϩ21.9 mm Hg). For the ing general anesthesia to evaluate the accuracy of capnog- population as a whole, the correlation between P and aCO2 raphy and transcutaneous monitoring. After 30 min of P , determined by regression analysis, was maintained ETCO2 hemodynamic stability (systolic arterial pressure within (r ϭ 0.64, p Ͻ 0.001). However, the P values (a-ET)CO2 20% of baseline) and constant ventilatory variables, arte- analyzed for many individual patients did not have a sta- rial blood was drawn and immediately analyzed for P . tistically significant correlation (p values ranged from aCO2 At the same time, both P and P values were Ͻ 0.001 to 0.90). Negative differences constituted 8.1% of ETCO2 tcCO2 recorded. The P values ranged between 21 mm Hg and the measurements. Despite a significant P differ- aCO2 (a-ET)CO2 58 mm Hg. The mean P was 2 Ϯ 4mmHg ence correlation, Russell et al86 concluded that individual (a-tc)CO2 (95% confidence interval Ϫ6 to 9 mm Hg), whereas the variation observed in postoperative cardiac patients neces- mean P was 6 Ϯ 5 mm Hg (95% confidence in- sitates periodic assessment of ventilation with arterial blood (a-ET)CO2 terval Ϫ3to16mmHg,pϭ 0.001). The P dif- gas analysis. (a-ET)CO2 ference was Յ 3 mm Hg in only 7 (15%) of the 45 P aCO2 versus P comparisons, whereas P was Specialty Gases ETCO2 (a-tc)CO2 Յ 3 mm Hg in 21 (46%) of those 45 comparisons (p ϭ 0.003). Linear regression analysis for P gave Since 1935, heliox has been used as a therapeutic bridge (a-tc)CO2 a slope of 0.84 (r2 ϭ 0.73), whereas linear regression to improve airflow past pulmonary obstructions.87 Over analysis for P gave a slope of 0.54 (r2 ϭ 0.50). the past 2 decades, the use of heliox has gained support in (a-ET)CO2 Casati et al84 concluded that in a population of ventilated, many emergency departments and intensive care units.88 elderly patients, transcutaneous monitoring provided a sta- Ball and Grounds89 developed a device to generate vari- tistically more accurate estimation of P than did capable mixtures of helium, O , and CO within the normal aCO2 2 2 nography. physiologic range, and they tested the performance of 2 Nosovitch et al85 prospectively compared the accuracy sidestream capnographs and one in-line capnograph.89 The of capnography and transcutaneous monitoring in 30 pe- addition of helium caused all 3 monitors to underestimate diatric surgery patients. When clinically indicated, arterial the CO2 concentration. This underestimation increased pro- blood gases were obtained and simultaneous P and portionally as the concentration of helium increased. ETCO2 P measurements were recorded. The absolute differ- Though further data are needed, these results could lead to tcCO2 ences (ie, no negative numbers) were calculated to avoid an inappropriate reduction in ventilator settings (and hy- artificially lowering the mathematical mean of the differ- percapnia) in patients mechanically ventilated with heliox ences. and managed with capnography. The P values ranged from 21 mm Hg to 122 mm Hg. aCO2 The mean P was 4.4 Ϯ 7.1 mm Hg. The mean Transport (a-ET)CO2 P was 2.8 Ϯ 2.9 mm Hg (p ϭ not significant). (a-tc)CO2 P was Յ 3 mm Hg in 58% of those pair compar- Tingay and colleagues90 conducted a prospective, un- (a-ET)CO2 isons, whereas P was Յ 3 mm Hg in 77% of those blinded study to assess the accuracy and reliability of P (a-tc)CO2 ETCO2

434 RESPIRATORY CARE • APRIL 2007 VOL 52 NO 4 CAPNOGRAPHY FOR MECHANICALLY VENTILATED PATIENTS monitoring during neonatal transport. Specifically trained absolute P value was Յ 4 mm Hg in 96% of the (a-tc)CO2 transport pediatricians started P and P monitor- comparisons. The absolute P value was Յ 4mmHg tcCO2 ETCO2 (a-ET)CO2 ing before the first arterial blood gas measurement in 26 in 38% of the comparisons (p Ͻ 0.001). The authors con- infants Յ 28 days old who had capillary refill time of cluded that, in infants and toddlers who require ventilation Ͻ 2 s. Paired arterial, transcutaneous, and capnography for respiratory failure, transcutaneous monitoring is more CO2 measurements were recorded every 20 min, starting accurate than capnography. P inaccuracy was great- ETCO2 at stabilization and continuing throughout the transport. est at higher CO2 values in patients with severe lung dis- Twenty-one P and P values and 82 (a-tc)CO2 (a-ET)CO2 ease managed with permissive hypercapnia. P values were obtained. The mean P was 93 (tc-ET)CO2 (a-ET)CO2 Berkenbosch et al prospectively compared the accu- 7.8 mm Hg (p Ͻ 0.001). Only 48% of the P record- ETCO2 racy of PtcCO and PETCO in 25 pediatric patients who ings were within 7 mm Hg of the paired P values. This 2 2 aCO2 required mechanical ventilation for respiratory failure. capnography bias was independent of P . There was no aCO2 When clinically indicated, arterial blood gas measurements significant difference for the P difference (mean (a-tc)CO2 were performed, with simultaneous recording of PETCO Ϫ0.97 mm Hg, p ϭ 0.4). Compared to the paired P Ϯ 2 aCO2 and PtcCO . The mean P(a-ET)CO was 6.4 6.3 mm Hg. values, 67% of the transcutaneous readings were within 2 Ϯ 2 Ͻ The mean P(a-tc)CO was 2.6 2.0 mm Hg (p 0.001). In 5 mm Hg, and 81% of P readings were within 7 mm Hg. 2 tcCO2 71% of the measurements transcutaneous monitoring was There was no significant difference between P and tcCO2 more accurate, whereas in 20% of the measurements cap- P as the CO level changed. Tingay and colleagues90 aCO2 2 nography was more accurate. In 9% of the measurements concluded that, for neonates who require ventilation dur- PETCO and PtcCO were equally accurate. In a subgroup ing transport, P more accurately reflects P than 2 2 tcCO2 aCO2 analysis in patients who weighed Ͼ 40 kg, the mean does PETCO . Ϯ 2 P(a-ET)CO was 8.4 7.7 mm Hg, compared to a mean Belpomme and colleagues91 conducted a single center 2 Ϯ P(a-tc)CO of 2.8 2.4 mm Hg. prospective descriptive study of the accuracy of capnog- 2 Berkenbosch et al93 concluded that transcutaneous mon- raphy in 100 consecutive adult patients who required in- itoring more accurately estimated P than did capnog- aCO2 tubation and ventilation outside the hospital. The P(a-ET)CO 2 raphy in pediatric patients who required mechanical ven- values showed important differences, between tilation for respiratory failure. If it is accepted that a Ϫ19.7 mm Hg and ϩ75 mm Hg at the time of intubation, monitored PCO value within 3–5 mm Hg of the actual and Ϫ11.8 mm Hg to ϩ98 mm Hg on arrival at the hos- 2 P is acceptable for clinical decision making, then 93% aCO2 pital. Even when the mean P(a-ET)CO was not extremely 2 of the transcutaneous measurements were within that range, elevated, it differed widely among the patients: 36% of whereas capnography provided that degree of accuracy patients at intubation and 27% on arrival to the hospital less than 60% of the time. had P values Ͼ 10 mm Hg. Several patients had (a-ET)CO2 negative P values Ͼ 10 mm Hg. The P In a prospective nonrandomized consecutive enrollment (a-ET)CO2 (a-ET)CO2 94 difference did not differ according to disease pathology study, McDonald et al evaluated the correlation between P and P in mechanically ventilated critically ill but was significantly more important (p Ͻ 0.001) in hy- ETCO2 aCO2 infants and children to determine whether P reliably percapnic patients than in normocapnic or hypocapnic pa- ETCO2 estimated ventilation. They analyzed 1,708 pairs of P tients. The study concluded that capnography is not reli- ETCO2 2 ϭ and PaCO values. PETCO correlated with PaCO (r 0.72, able to accurately estimate PaCO and is not sufficient by 2 2 2 2 ϭ itself to adjust respiratory settings for patients outside the p 0.001), and they concluded that their data strongly hospital setting. supported the utility of capnography as a noninvasive method to assess ventilation in critically ill patients with Pediatric Respiratory Disease minimal-to-moderate lung disease. However, closer review of their data calls into question the clinical utility of the 92 results. The absolute P values were Ͻ 5mmHgin Tobias and colleagues studied 25 infants and toddlers (a-ET)CO2 who required intubation for respiratory failure, to deter- only 54% of the comparisons, and were Ͼ 10 mm Hg in mine the accuracy of capnography and transcutaneous 20% of the comparisons. And in 1,423 consecutive blood gas analyses, P and P changed in the opposite monitoring. When clinically indicated, arterial blood gases aCO2 ETCO2 were obtained in conjunction with P and P val- direction 26% of the time. A multivariate analysis by the ETCO2 tcCO2 investigators showed that P /F and the ventilator in- ues. Only absolute values (ie, no negative numbers) were aO2 IO2 dex (ie, (P – peak inspiratory pressure) – respiratory used to avoid artificially lowering the mathematical mean aCO2 of the differences in the values from the noninvasive rate)/1,000) significantly influenced (p ϭ 0.001 and 0.01, monitors. respectively) P . The authors also noted that (a-ET)CO2 The mean P was 6.8 Ϯ 5.1 mm Hg, whereas the P becomes more variable as the number of pairs (a-ET)CO2 (a-ET)CO2 mean P was 2.3 Ϯ 1.3 mm Hg (p Ͻ 0.001). The available for analysis increases for a single subject. (a-tc)CO2

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Con Summary 7. Holland R, Webb RK, Runciman WB. The Australian Incident Mon- itoring Study. Oesophageal intubation: an analysis of 2000 incident reports. Anaesth Intensive Care 1993;21(5):608–610. In summary, several factors can dramatically alter the 8. Birmingham PK, Cheney FW, Ward RJ. Esophageal intubation: a accuracy of capnography. Both sampling errors and alter- review of detection techniques. Anesth Analg 1986;65(8):886–891. ations in V˙ /Q˙ status can cause inaccuracy. Diseases and 9. Knapp S, Kofler J, Stoiser B, Thalhammer F, Burgmann H, Posch M, conditions that increase dead space and/or intrapulmonary et al. The assessment of four different methods to verify tracheal tube (eg, parenchymal diseases or conditions) or extrapulmo- placement in the critical care setting. Anesth Analg 1999;88(4):766– 770. nary (eg, cyanotic congenital heart diseases) shunt increase 10. Roberts WA, Maniscalco WM, Cohen AR, Litman RS, Chhibber A. the difference between P and P . Furthermore, ETCO2 aCO2 The use of capnography for recognition of esophageal intubation in alterations in V˙ /Q˙ matching, secondary to increased dead the neonatal intensive care unit. Pediatr Pulmonol 1995;19(5):262– space or shunt fraction, limit the accuracy of capnography 268. in patients with abnormal pulmonary function. 11. Repetto JE, Donohue PK, Baker SF, Kelly L, Nogee LM. Use of capnography in the delivery room for assessment of endotracheal tube placement. J Perinatol 2001;21(5):284–287. Summary 12. Grmec S. Comparison of three different methods to confirm tracheal tube placement in emergency intubation. Intensive Care Med 2002; 28(6):701–704. Despite an extensive review of the medical literature, 13. Katz SH, Falk JL. Misplaced endotracheal tubes by paramedics in an there does not appear to be a definitive answer to the urban emergency medical services system. Ann Emerg Med 2001; question, “Should every mechanically ventilated patient be 37(1):32–37. monitored with capnography from intubation to extuba- 14. Kannan S, Manji M. Survey of use of end-tidal carbon dioxide for tion?” The lack of a clear answer is because no definitive, confirming tracheal tube placement in intensive care units in the UK. randomized studies have attempted to address this ques- Anaesthesia 2003;58(5):476–479. 15. American Heart Association. 2005 American Heart Association tion. The pro position is supported by the physiology of ˙ ˙ (AHA) guidelines for cardiopulmonary resuscitation (CPR) and emer- the V/Q relationship in the lung and capnography’s ability gency cardiovascular care (ECC) of pediatric and neonatal patients: to prevent airway mishaps. On the other hand, the con pediatric advanced life support. Pediatrics 2006;117(5):e1005–e1028. position is supported by literature that brings into question 16. Bhende MS, Thompson AE, Cook DR, Saville AL. Validity of a the relationship between P and P in the clinical disposable end-tidal CO2 detector in verifying endotracheal tube ETCO2 aCO2 setting. placement in infants and children. Ann Emerg Med 1992;21(2):142– Thus, in conclusion, it seems reasonable to continuously 145. 17. Campbell RC, Boyd CR, Shields RO, Odom JW, Corse KM. Eval- monitor at least a subset of critically ill patients with cap- uation of an end-tidal carbon dioxide detector in the aeromedical nography to ensure the integrity of the ventilatory appa- setting. J Air Med Transp 1990;9(11):13–15. ratus and the ETT. However, at this point, definitive data 18. Puntervoll SA, Soreide E, Jacewicz W, Bjelland E. Rapid detection are not yet available to clearly support continuous capnog- of oesophageal intubation: take care when using colorimetric cap- raphy to optimize mechanical ventilatory support. We hope nometry. Acta Anaesthesiol Scand 2002;46(4):455–457. that as new data become available, the answer to this 19. 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Discussion about that. But I think the catastro- get a number that we think matches phes capnography can help us avoid the patient condition, we typically ac- Steinberg: It seems there are 2 uses are relatively rare. Ira [Cheifetz] cept it as accurate and move on. That for capnometry: preventing catastro- showed us Joint Commission data on is potentially problematic in some sit- phes, and titrating mechanical venti- 23 patients, but that is only 23 out of uations with patients who have a P - aCO2 lation. Tim [Myers], though the tech- how many thousands intubated during to-P difference to start with and ETCO2 nology might not be good enough for that period? And it wasn’t a question who get worse with certain conditions. us to titrate therapies, is the technol- of not having monitoring capabilities; If we could use it for a training mech- ogy good enough to help us monitor rather, it was inappropriately-set ven- anism, that might be beneficial, but for disasters? That seems to me the tilator alarms or alarms that were the data shows that in 10–25% of the more important reason to use capnog- turned off. cases cited the P value trends in ETCO2 raphy. I think the bigger question is that the opposite direction of the P . aCO2 we come to look at various monitors— Myers: Yes, I think it’s cost-effec- electrocardiogram, pulmonary graph- Cheifetz: With regard to using cap- tive from a disaster standpoint, and ics, waveforms, pulse oximetry—as nography for clinical management, there are probably no major arguments the accepted standards, and when we which is what we do every day

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for every ventilated patient in our ICU, think the problem with capnography would contend that if you give them I presented preliminary data that indi- is the technology; it’s the physiology more numbers, they will find more cated significantly shorter duration of and understanding the physiology. ways to sue us, and it won’t necessar- mechanical ventilation in a heteroge- Most clinicians want to use capnog- ily reduce legal costs. neous group of pediatric ICU patients. raphy as a substitute for an arterial 1. Neff TA. Routine oximetry. A fifth vital The key in using capnography for clin- blood gas, so when looking at the sign? (editorial) Chest 1988;94(2):227. ical management is to go back to the P -P difference, most clini- aCO2 ETCO2 basic physiology. In most of the stud- cians are not trying to assess the phys- Deem: With regard to pulse oxim- ies that Tim mentioned that claimed a iology and the dead space and so etry, we assume it is the standard of significant difference between P forth—they want to make a ventilator aCO2 care, but in the only large randomized and P , the difference was about adjustment because the P goes ETCO2 ETCO2 trial that has compared pulse oxime- 5 mm Hg. Well, 5 mm Hg represents up or down and eliminates the need try to no pulse oximetry,1,2 which was the expected airway dead space, so for an arterial blood gas part. And I done in Scandinavia and enrolled more the key here is that you need to em- think that is where problems can arise, than 20,000 patients, there was no ben- ploy capnography within the context as Tim pointed out, because the num- efit. But we still assume that pulse of your overall knowledge of the pa- bers can move in opposite directions oximetry is the standard of care. And tient, the patient’s pathophysiology, in a significant number of cases. so, given that there were 26 cases, and and basic physiology. As dead-space that your assumption is that those cases ventilation changes, the difference be- Rubin: A lot of this is centered would have been prevented by cap- tween P and P changes as around physiology and the appropri- ETCO2 aCO2 nometry—which we don’t really know well, and that represents a key com- ate interpretation. I’d like to address to be true—in terms of the economic ponent in the clinical management of the issue with a cost/benefit analysis, impact that means there were only 26 a patient. It does not mean that the particularly if you are basing this on hospitals that were affected across the monitor is wrong. In fact, the monitor legal costs. Lawyers like numbers, and country. For the individual hospital the is telling you what is going on—you the more numbers you give them, the cost would be considerable to buy a must use all of the available informa- more they are going to be able to pick capnometer for each patient, and very tion to manage your patient accord- things apart and use the numbers to few if any of those patients would ac- ingly. talk about appropriate or inappropri- tually benefit. For that matter, very ate response. Many would expect that few hospitals would benefit. So I think Kallet: Two comments. First, we use oxygen saturation measurements are you have to look at the economics a pulse oximetry as a standard monitor- the standard of care. There was an ed- little bit differently, as opposed to ing. We wouldn’t get rid of that, and itorial in Chest a few years back call- looking at the legal savings. there are definite situations where ing oxygen saturation the “5th vital 1. Moller JT, Johannessen NW, Espersen K, there is artifact in pulse oximetry that sign.”1 Ravlo O, Pedersen BD, Jensen PF, et al. doesn’t match the S [oxygen satu- I’m currently an expert in a legal Randomized evaluation of pulse oximetry aO2 ration measured via blood-gas analy- case where there is a practice that con- in 20,802 patients: II. Perioperative events and postoperative complications. Anesthe- sis], but we work around that; we live sistently measures oxygen saturation siology 1993;78(3):445–453. with it! Second, I think the future in in children who come in. There is a 2. Moller JT, Pedersen T, Rasmussen LS, capnography is probably going to be child who came in, doing well, had a Jensen PF, Pedersen BD, Ravlo O, et al. volumetric, because a lot of the things bit of pneumonia, had oxygen satura- Randomized evaluation of pulse oximetry Tim alluded to, I think, can be solved tion that they measured at 93%, sent in 20,802 patients: I. Design, demography, pulse oximetry failure rate, and overall com- when we start measuring mixed ex- the child home, appropriately on an plication rate. Anesthesiology 1993;78(3): pired CO2 and also being able to de- antibiotic, and a few days later the 436–444. termine the slope of phase 3 for alve- child died. The concern from the law- olar dead space. So, again, when the yers was that they didn’t admit the Cheifetz: I presented the financial gradients change, we need to have a child immediately to the hospital with argument as a preemptive strike, ex- therapeutic intervention. Particularly oxygen saturation of 93%. I said this pecting Tim to come back at me with in ARDS, in the future I think these is unusual that they are grabbing on to finances, but it did not happen. There tools might be helpful in titrating that number. And the lawyer that I’m are advantages and disadvantages of

PEEP, VT, or recruitment in lung-pro- working with said, “Not at all. Here not sharing your talks ahead of time, tective ventilation. are some publications in the legal lit- and we had not shared ours. My im- erature telling lawyers how to use ox- portant arguments for capnography are Hess: I agree with the issues about ygen saturation measurements to their that it can prevent airway disasters and dead space and the gradient. I don’t benefit in malpractice suits.” So I help optimize mechanical ventilation.

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When it comes to lawyers and admin- Hess: I’m trying to think of a disas- istration, where every step of the or- istrators, and what they are going to ter that would occur on a ventilator dering and administration process has do with the numbers, we can argue that a capnograph would pick up rather a double-check to prevent a medica- about that all day; but there are data than appropriately-set ventilator tion disaster. The aviation industry also and experience that you can prevent alarms. For example, if a patient self- has redundancy built into their safety disasters with capnography. Prevent- extubates, there should be a discon- checks and alarm systems. ing an airway disaster obviously goes nect alarm that sounds on the ventila- a long way. Imagine if that patient is tor virtually right away. Or if a tube Hess: Correct, and then you add in your ICU or is a loved one in an- occludes, there should be a high-pres- more ICU monitors that produce spu- other ICU. In terms of spreading the sure or low-VT alarm. I can’t think of rious alarms, and then nobody pays costs around, yes, the financial impact a disaster that a capnograph might pick attention to any of them. of the litigations may hit just a few up before an appropriately-set venti- hospitals, obviously not all hospitals, lator alarm would. Rubin: We’ve already stated that but what if it’s your hospital and your good clinical assessment is more im- unit? MacIntyre: If I may preemptively portant, so I would argue that, rather strike here, I remember a case in which than getting capnographs, it would be MacIntyre: It will hit your premi- a young person became disconnected more effective to spend the money on ums. from the ventilator, but the circuit fell an additional well-trained respiratory on the patient’s chest, and the pres- therapist in the ICU, to provide the Cheifetz: It will definitely hit you. sure and volume alarms were not tight eyes and ears and careful assessment Somehow, an airway disaster is going enough to pick up the fact that venti- skills to determine what’s going on, to affect you. In our ICU we have had lation was not being delivered to the when, and with which patients. several cases where kids were inad- patient. Now, you could argue, maybe the alarm should have been set a little Cheifetz: I think that would be great vertently extubated, generally small bit tighter, and that’s reasonable. But if we had an abundance of respiratory sick babies. In each case, the problem that’s an example of why at one point care practitioners to put in the hospi- was caught quickly and the patient did the Food and Drug Administration was tals. Ray [Masferrer, Associate Exec- fine. But the first warning sign of an suggesting redundant alarm systems utive Director, American Association inadvertent extubation to the bedside for life-support systems. for Respiratory Care], how many re- staff is the end-tidal capnography spiratory therapists are we short right alarm; it generally is more sensitive Hess: Let me give you another sce- now in the United States, based on the than the ventilator alarm. nario. You have your capnograph recent AARC work survey study? Ray hooked onto the endotracheal tube; the says many! Fessler: I’ve never believed that we ventilator disconnects from the cap- suffer from too little information in nograph, which is still attached to the Myers: Ira pointed out that a lot of the ICU. I’m skeptical when anyone endotracheal tube; the patient is able those P -P differences were aCO2 ETCO2 tells me that I need to monitor some- to do some breathing on their own, 5–6 mm Hg, which is right on the thing more, and I’m even more skep- and the capnograph would not be use- cusp of what is normal anatomical tical when they tell me that it’s going ful in determining that there is no CO2 physiology for a gradient. Interestingly to come with no costs and no risks. or there’s no ventilation coming from enough, most of these studies were One of the additional costs is all the the ventilator. designed not to track negative num- unnecessary tests that will be done to bers, to make it statistically “nice” to track down spurious inaccurate results. Cheifetz: Redundancy in life-sup- calculate their mean. So that could be I think you estimated the acquisition port monitoring systems is important. 6 in the opposite direction of the dif- costs for hospitals throughout the Any time anyone investigates any pa- ference you’re expecting. If they had United States to be about $85 million. tient disaster, there is never a single looked at negative numbers, the pCO2 For a simple cost/benefit analysis, if root cause of the incident. There is values could have been in the 40–50 we consider a best-case scenario that always more than one underlying prob- mmHg range. With normal arterial we would prevent 17 deaths by spend- lem. If you rely on one alarm, one CO2 values, where you are in there ing that $85 million, that’s $5 million single process, it may not be enough, actually increasing ventilator settings per death. That’s no one’s definition so redundant alarm systems and and those types of things to bring your of a cost-effective intervention. It several layers of safety checks are es- CO2 back into range, it’s a negative sounds like a tremendous waste of sential to prevent disasters. This is no correlation or a negative gradient that health-care dollars. different from chemotherapy admin- you are really dealing with. Those were

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kind of statistical flaws, I think, in raphy also enabled us to continuously respiratory distress syndrome. N Engl J Med 2000;342(18):1301–1308. some of those studies that didn’t look monitor VT at the endotracheal tube, at negative numbers or negative gra- which is important for infants and 2–4 MacIntyre: It always worries me dients in most of them. small children, and the goal VT was 6 mL/kg for patients with acute lung when we have the “usual care” con- MacIntyre: Ira, can I switch gears injury.5 I realize that the need to mon- trol groups, especially in weaning trials. As Ely found many years ago in from monitoring disconnects? Obvi- itor VT at the endotracheal tube is more 1 ously, what’s critical is how you use of an issue for infants and children adult patients, and which was subse- 2 the monitor, and in your weaning than for adults. quently confirmed by Kollef et al, if you protocolize your weaning with study1 you only reported those who I also must acknowledge that it is rules that have aggressive spontane- had the monitor and those who didn’t possible that the patients in the inter- ous breathing trials and regular assess- have the monitor. What did you do vention group received more attention ments, you will shorten weaning time, with the capnograph that caused the from the bedside staff, although we compared to physician-guided wean- shorter ICU stay? will never be able to prove or dis- prove that. Staff may have paid more ing. So getting the staff on board to do 1. Hamel D, Cheifetz I. Continuous monitor- these things in a regular fashion is im- ing of volumetric capnography reduces attention to the numbers, leading to length of mechanical ventilation in a het- more interactions with the ventilators, portant. That should be the control erogeneous group of pediatric ICU patients and being more involved with the pa- group, not the uncontrolled behavior (abstract). Respir Care 2005;50(11):1517. tients in the capnography group. This patterns. is obviously a risk in any unblinded 1. Ely EW, Baker AM, Dunagan DP, Burke Cheifetz: That is the commonly study. HL, Smith AC, Kelly PT, et al. Effect on asked question about our volumetric We are now working to assess pat- the duration of mechanical ventilation of capnography study. We compared du- ˙ identifying patients capable of breathing terns of how VCO , alveolar minute ration of ventilation between 2 groups: 2 spontaneously. N Engl J Med 1996;335(25): ventilation, and the other volumetric 1864–1869. a volumetric capnography group and capnography variables relate to 2. Kollef MH, Shapiro SD, Silver P, St John a standard of care group. Our goal was changes in patient management and RE, Prentice D, Sauer S, et al. A random- to study the effects of monitoring volu- the implications on duration of venti- ized, controlled trial of protocol-directed metric capnography on routine venti- versus physician-directed weaning from lation. We are also working to develop mechanical ventilation. Crit Care Med lator management and duration of ven- protocols to better define the role of 1997;25(4):567–574. tilation. We intentionally did not volumetric capnography to optimize protocolize the 2 groups. We used mechanical ventilation. So in sum- Cheifetz: When we designed the volumetric capnography, based on the mary, we still have many questions as study, we considered using detailed available data. First, we used capnog- well. protocols for both groups, but we raphy to predict the likelihood of suc- 1. Hubble CL, Gentile MA, Tripp DS, Craig feared that we would be studying the cessful extubation. We used Hubble’s DM, Meliones JN, Cheifetz IM. Dead space protocols rather than the technology. study of the VD/VT ratio as a predictor to tidal volume ratio predicts successful ex- Either way, the study could be faulted. 1 tubation in infants and children. Crit Care of extubation. AVD/VT less than 0.5 So what you have to do is perform the was associated with a 96% successful Med 2000;28(6):2034–2040. 2. Castle RA, Dunne CJ, Mok Q, Wade AM, study one way and then come back extubation rate, a VD/VT of 0.51–0.65 Stocks J. Accuracy of displayed values of and confirm the results the other way, was associated with a 67% successful tidal volume in the pediatric intensive care which we are planning to do. unit. Crit Care Med 2002;30(11):2566– extubation rate, and a VD/VT greater than 0.65 was associated with only a 2574. MacIntyre: Protocol with a ma- 3. Cannon ML, Cornell J, Tripp-Hamel DS, 20% success rate. In our study, we Gentile MA, Hubble CL, Meliones JN, chine and protocol without a machine, trended VD/VT over time. It may have Cheifetz IM. Tidal volumes for ventilated I think would be a pretty good study. been that our results were based on infants should be determined with a pneu- simply pulling the endotracheal tubes motachometer placed at the endotracheal Branson: Nobody has mentioned a little quicker. Of note, the extuba- tube. Am J Respir Crit Care Med 2000; 162(6):2109–2112. the actual devices. We have Dra¨ger tion-failure rates were identical in the 4. Chow LC, Vanderhal A, Raber J, Sola A. ventilators, which have a sensor that 2 groups. Are tidal volume measurements in neona- goes in the airway, and whenever I For weaning we used carbon diox- tal pressure-controlled ventilation accurate? put it in the airway, I guarantee you ˙ Pediatr Pulmonol 2002;34(3):196–202. ide elimination (VCO ) and alveolar when I come back in 4 hours, the sen- 2 5. The Acute Respiratory Distress Syndrome minute ventilation (and their trends Network. Ventilation with lower tidal vol- sor will be out, because it alarms, say- over time) to optimize patient-venti- umes as compared with traditional tidal vol- ing “clean the cuvette,” because of the lator interactions. Volumetric capnog- umes for acute lung injury and the acute humidity. How much problem is there

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with failure of side-stream versus what happens when the end-tidal CO2 1. Helm M, Schuster R, Hauke J, Lampl L. mainstream technology in anesthesia? is very low, because of the large V˙ /Q˙ Tight control of prehospital ventilation by My biggest limitation is that I can’t mismatch because the patient also capnography in major trauma victims. Br J get a therapist to leave the sensor in has aspirated or has a pulmonary con- Anaesth 2003;90(3):327–332. 2. Davis DP, Dunford JV, Ochs M, Park K, line. tusion? Then do you have the para- Hoyt DB. The use of quantitative end-tidal medic not actually ventilating the pa- capnometry to avoid inadvertent severe hy- Cheifetz: For years the policy in our tient adequately, to provide adequate perventilation in patients with head injury unit has been to monitor capnography CO2 elimination and oxygenation after paramedic rapid sequence intubation. J Trauma 2004;56(4):808–814. from “intubation to extubation.” It is trying to get the CO2 to rise? And rare that the capnostat is taken out of what happens when it is very high— ˙ ˙ line by the bedside staff because of a again because of the V/Q mis- Branson: What it shows is that if technical issue or secretions. I can match—and you have the patient be- it’s normal the patients do better. It count on my fingers how many times ing very aggressively ventilated and doesn’t show that if people actively it happens in a year. The key is edu- killing the cardiac output and the make it normal that it makes a dif- blood flow to the brain? That’s my cating the staff to keep the sensor clean ference. And therein lies the prob- and to keep the tubing vertical instead big concern. I think Dean probably lem: if it’s normal in the normal of horizontal, so secretions do not en- said it very well: it’s not the moni- range, patients do have better out- ter the lines. With staff education, we tor; it’s the physiology and under- comes. But if it’s very low, meaning have not had a problem. standing behind it. high VD/VT, they don’t do very well Branson: I recently reviewed a pa- Kallet: There were a couple of stud- because of lung injury. If it is very 1,2 high, perhaps for another reason, per about controlling end-tidal CO2 in ies in which they looked at capnog- head-injured patients, especially dur- raphy in the pre-hospital setting with they also don’t do very well. But ing the pre-hospital phase. And the head-injured patients, and in terms of there is nothing that says that forc- ing it into a range improves the out- issue becomes, if you control it—let’s trying to maintain a normal CO2, sta- say the normal range is 35– tistically there were better outcomes. come in those head-injured patients. 45 mm Hg—this issue of the physiol- It was a useful tool in the pre-hospital That particular study has not been ogy and not matching the arterial CO2, setting with head-injured patients. done.

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