Special Communication RESPIRATORY MONITORING OF CARBON DIOXIDE Weingarten M. Respiratory monitoring of carbon dioxide and oxy- gen: a ten-year perspective. AND OXYGEN: A TEN-YEAR PERSPECTIVE J Clin Monit 1990;6:217-225 Maxwell Weingarten, MD ABSTRACT.During the past 10 years, instrumentation has been developed that can continuously and noninvasively measure changes in carbon dioxide and oxygen. The information gen- erated, which cannot be obtained through the human senses, provides vital clinical data regarding the effectiveness of intu- bation, ventilation, circulation, oxygenation, and the circuit. This instrumentation plays a major role in decision making both in the safe conduct of anesthesia and mechanical ventila- tion as well as in the detection and prevention of potentially catastrophic mishaps. For these reasons, a review of what has been learned regarding the instrumentation, collection, and interpretation of the clinical data, and the clinical value of the information is timely. The clinical significance of the carbon dioxide and oxygen waveforms, inspired to expired carbon dioxide and oxygen differences, alveolar-arterial gradients, and global supply-to-demand oxygen relationships measured by capnography, oxygraphy, and pulse oximetry are ad- dressed in this essay. KEY WORDS.Measurement techniques: capnography; oxyg- raphy; pulse oximetry. During the past 10 years major advances have been made in our ability to monitor and quantify carbon dioxide (CO2) and oxygen (O2) exchange continuously and noninvasively. Since evidence is accumulating that monitoring improves outcome, it may be useful to give a perspective gained from 10 years of monitoring CO2 and 02 during anesthesia and in ventilated patients. MEASUREMENT OF C02 The introduction of capnography by Smalhout and Kalenda [1] defined the relationship of exhaled CO2 to metabolism, circulation, respiration, and the circuit. Under their direction, capnography survived a stormy gestational period as it reached maturity in The Nether- lands. It was introduced in the United States at a small private meeting sponsored by a major instrument man- ufacturer held in conjunction with the World Congress on Intensive Care Medicine in Washington, DC, in May 1978. Five anesthesiologists attended the meeting, two of whom concluded that capnography would prove to be of very little value. Since then, capnography has come to be recognized as an extremely valuable method for the continuous monitoring of respiration and circu- lation in unconscious patients. Recognition of the value From the Department of Anesthesiology, St. Francis Hospital, Mil- ofcapnography for the detection and prevention of mis- waukee, WI 53215. haps has grown to the extent that some states (New Received Sep 15, 1988, and in revised formjan 29, 1990. Accepted for York, New Jersey) have mandated its use on all intubated publication Feb 7, 1990. patients. Copyright 1990 by Little, Brown and Company 217 218 Journal of Clinical Monitoring Vol 6 No 3 July 1990 Capnography refers to the graphic portrayal of the placed in one of the prongs of a nasal 0 2 catheter, or changing concentration of exhaled CO2 during the en- shielded inside a catheter placed in the nose or pharynx. tire respiratory cycle. It also may refer to the interpreta- tion of the waveforms. It should not be confused with INFECTION CONTROL. Sidestream sampling is simpler be- capnometry, which refers to only the digital presentation cause the water trap, connectors, and sampling catheter of the concentration without a waveform. are all disposable. With mainstream detectors the cu- The basis of capnography is rooted in the fact that all vette can be sterilized. mammalian cells, irrespective of their diversity of func- tion, have one common denominator, namely, that they WEIGHT. Sidestream sampling adds very little weight obtain the energy necessary to carry out their specific and bulk to the endotracheal tube connector. Main- functions by continuously using 02 to burn glucose to stream detectors add weight and bulk, which may in- its products of combustion, CO2 and water (H20). For crease the chances of inadvertently pushing the endotra- CO2 to be detected in the exhaled gases there must be cheal tube into the right bronchus if care is not taken. the production of CO2 in the cell (metabolism), the transport of CO2 from the cell to the lung (circulation), SAMPLING PROBLEMS. Moisture, blood, pus, and mucus and the elimination of CO2 as it diffuses into the alveoli may present a problem with both the mainstream and and through the airways (ventilation) [1]. Because the the sidestream techniques. In mainstream monitors, this respiratory cycle alternates the CO2-containing gases of material may cover the cuvette window; in sidestream exhalation with the fresh non-CO2-containing gases of monitors it may be aspirated directly into the detector. inspiration, a characteristic waveform of the changing Line plugging is possible, but these problems are mini- concentration of CO2 is produced. This waveform ac- mized in the better designed monitors of both tech- curately reflects the above sequence of events, as well as niques. Also, in sidestream monitors the aspiration of the total integrity and efficiency of the breathing circuit, air from leaks or fresh inflow gases can dilute the ex- including the ventilator, if one is in use. haled CO2 sample and slur the waveform. This occur- rence is less likely in mainstream monitors because there INSTRUMENTATION is no negative pressure in the circuit. Medical instrument manufacturers have provided a va- RESPONSE TIME. With mainstream monitors the response riety of CO2 monitors that differ widely in response time is less than half a second; in sidestream monitors time, data display, and clinical usefulness [2]. There are the response time is 2 to 3 seconds because of the time two main types, mainstream and sidestream monitors required for the sample to be aspirated through the sam- [3]. pling catheter to the detector. This difference in re- sponse time can be significant because of the need to furnish as early a warning as possible. With sidestream Comparison of Mainstream and Sidestream monitors the waveform can be influenced by the rate of Monitors aspiration, the length and diameter of the sampling In mainstream monitors the detector is designed to be catheter, the flow rate of the incoming gas, and sample attached to an airway adapter that is connected to the catheter turbulence that may distort the proximal and endotracheal tube. The respiratory gases pass by win- distal end of the expired CO2 bolus. dows in the airway adapter, so that the CO2 concentra- tion is measured by the detector without direct contact IMPORTANCEOF THE C02 WAVEFORM with the gases. In sidestream monitors, a small, light- weight Y piece is attached to the end of the endotracheal There is only one normal capnogram (Fig 1), and all tube, and gas is continuously aspirated back through the variations from the normal pattern indicate some ab- detector, which is housed within the monitor. normality that must be recognized and corrected if pos- sible [2]. MONITORING INTUBATED VERSUS NONINTUBATED PA- Any factor that impairs the free exhalation of gas, TIENTS. With the mainstream technique, only those pa- such as a kinked, displaced, or otherwise obstructed en- tients who are intubated can be monitored, since the dotracheal tube; asthma; bronchospasm; or chronic ob- detector is connected to the endotracheal tube. Side- structive pulmonary disease, will produce a change in stream sampling can be used on nonintubated as well as the angle of rise of the ascending limb from about 90 to intubated patients. On nonintubated patients the sam- 160 ~, depending on the severity of the expiratory ob- piing catheter can be placed inside a dental mouth hook, struction. An ascending limb with a long rise time and Special Communication: Weingarten: Carbon Dioxide and Oxygen Monitoring 219 G H 38mm Hg 38ram Hg Om~ Hg Omm Hg E F I dE Fig 2. Capnogram indicating expiratory obstruction. Fig 1. The normal carbon dioxide (C02) waveform. E represents the beginning of expiration; EF represents exhalation of upper air- way dead-space gases, which do not contain C02. FG, the ascend~ ing limb, rises almost at a right angle from the baseline and repre- 38ram Hg sents the increasing concentrations of CO2-containing gases coming flora the more distal airways. GH, the plateau, makes an ahnost right angle turn and parallels the baseline, gently rising a few ram, -L representing the product of the mixed expired COe-containing gases. H represents the end-tidal C02, t, ormally between 35 and Omm Hg 40 mm Hg. This measurement is valid as a trending relationship with arterial COz tension only if a normal plateau has been pres- Fig 3. Capnogram indicating diluted inspired gases. ent. HI, the descending limb, makes an almost right angle turn and rapidly descends to the base line. It represents the inspiratory phase during which fresh non-CO2-containing gases are inhaled and the C02 value falls to O. IE represents the phase of inspiration during tension, will decrease the height of the plateau. In a cir- which the upper airway dead space is filled with fresh non-CO2- culatory crisis the plateau will fall in proportion to the containing gases. Thus, IEF, the baseline, represents both the fresh severity of the event and slowly rise in height as pulmo- gas that filled the upper airway during inspiration and that which nary circulation improves, because the height of the is exhaled in early expiration. plateau is affected by pulmonary perfusion. In fact, ofaU available monitors in use during cardiac arrest, capnog- raphy furnishes the best real-time, continuous informa- no plateau indicates that exhalation has not been com- tion regarding the effectiveness of resuscitative efforts pleted before inhalation occurs. In this situation the arte- [4]. rial-alveolar CO2 [(a - A)Co2] gradient will increase The descending limb reflects the dynamics of inspi- significantly (Fig 2).