Breathing Pattern Analysis

Intensive Care Med (1992) 18:193-201 Intensive Care Medicine Springer-Verlag 1992

Review article

Breathing pattern analysis

M.J. Tobin Loyola Universityof Chicago Stritch School of Medicine, Program Director, Division of Pulmonary and Critical Care Medicine, Edward Hines Veterans Affairs Hospital, Hines, IL 60141, USA Received: 20 October 1991; accepted: 7 April 1992

Clinical examination of the pattern of breathing has gen- relation between subjective estimations of tidal volume erally consisted of counting the respiratory rate over a 15- made by ICU personnel and objective measurements. Of to 60-s period, noting abnormally deep or shallow respi- concern was the widespread tendency to overestimate tid- rations, and documenting well-known patterns such as al volume, which could be particularly dangerous in pa- Cheyne-Stokes or Kussmaul's breathing. In recent years, tients who have low tidal volumes. it has been recognized increasingly that a more detailed analysis of breathing pattern can provide valuable information regarding respiratory system performance [1]. At the time that advances were being made in the analytical approach to breathing pattern, improvements were occur- Direct measurement ring simultaneaously in the instrumentation used to re- In the intubated patient, it is easy to directly measure ven- cord ventilation, especially in the non-intubated patient tilation by attaching a spirometer or pneumotachograph [2]. In this article, I will review the methods that can be to the patient's endotracheal tube. The prototype used to record and analyze the pattern of breathing, and pneumotachograph, the Fleisch instrument, presents a the usefulness of this approach as an investigative tool in problem in this setting because condensation of moist the critical care setting. gases and mucus can easily clog the narrow metal tubes within the instrument, and thus, alter resistance and produce inaccurate results. The development of newer vari- able-orifice pneumotachographs will hopefully diminish this problem. In the non-intubated patient, problems Techniques of measurement arise in attempting to measure ventilation with a spirometer or pneumotachograph because critically ill patients Visual inspection have a low tolerance of devices that require a direct con- Clinicians usually measure respiratory frequency by ob- nection to the patient's airway. In addition, several inves- serving chest wall motion over 15 s and multiplying the tigators [5-7] have shown that use of a mouthpiece and resulting value by 4. Using this approach, an error of 4 nose clips produces spurious alterations in breathing pat- or more breaths/min may easily occur - a rather large tern, causing tidal volume to increase and respiratory fre- error for a parameter that is normally 16-20breaths/ quency to decrease. Although most of the studies demon- min. Despite the simplicity of the measurement, bedside strating the alteration in breathing pattern with the use of assessment of respiratory frequency is often inaccurate. a mouthpiece and noseclips have been conducted in In one study, 34% of nurses' recordings of respiratory healthy subjects during resting breathing, this effect has frequencies deviated by more than 20% from the true val- also been shown in exercising subjects with a markedly el- ue [3]. Thus, underappreciation of the clinical impor- evated minute ventilation (75.5 1/min) [8], and in patients tance of respiratory frequency may be due partly to inac- with moderately severe asthma (forced expiratory volume curacies in its measurement. Furthermore, the discon- in 1 s, 43070 of predicted normal) [9]. In addition, Aska- tinuous nature of bedside measurements may miss sud- nazi et al. [6] reported that mouthpiece-induced changes den changes in critically ill patients. Assessment of tidal in breathing pattern were similar in critically ill patients volume is also usually part of bedside examination, and as in healthy subjects, although supporting data were not clinicians commonly comment on the depth or shallow- included in their report. As a result of these problems ness of breathing. However, in a study conducted in criti- with a mouthpiece and nose clips, several devices have cally ill patients, Semmes et al. [4] noted a very poor cor- been developed to measure ventilation indirectly. 194 Impedance pneumography Impedance pneumography is probably the technique f most often used to non-invasively monitor breathing pattern. Two standard electrocardiographic electrodes placed on the chest wall serve both to pass an alternating current through the chest between the electrodes and to return a voltage for calculation of impedance [6]. The resulting waveform is qualitatively similar to that obtained with spirometry [11]. However, the instrument cannot be calibrated to obtain quantitative volume measurements [2], and measurement of respiratory frequency may even be inaccurate [10].

Mercury-in-silastic strain gauges This transducer consists of rubber tubes filled with mercury which are wrapped around the chest [2]. Changes in Fig. 1. Transducer bands of the respiratory inductive plethysmograph the length and width of the mercury columns alter their (RIP) placed around the rib cage and abdomen. From Tobin MJ [23] resistance. These mercury columns form part of a bal- anced Wheatstone bridge. Variation in electrical resistance alters the balance of the bridge, and these changes forms the basis of the various techniques that can be used are amplified and recorded. Provided that there is no to calibrate the RIP - isovolume, least-squares, and mul- change in body posture, this technique can provide a rea- tiple linear regression techniques. sonably accurate measurement of tidal volume. The isovolume calibration technique requires that the subject voluntarily closes his/her glottis and, at a con- Magnetometers stant lung volume, moves air back and forth between the RC and Ab, without pressurizing thoracic gas volume Magnetometers measure changes in the antero-posterior [16]. Since the volume lost by one compartment is gained diameters of the rib cage (RC) and abdomen (Ab) during by the other, the amplitude of their signals must be made respiration [12]. An alternating current in an exciter coil equal by the operator. The RC and Ab signals are then generates a magnetic field in a receiver coil placed on the added to produce a sum signal, and while the subject opposite body surface. The signals from the RC and Ab breathes from a spirometer the operator adjusts the sum are electronically summed, and when calibrated, they signal to match that of the spirometer. The difficulty that provide a very accurate measurement of tidal volume. many subjects have in performing an isovolume maneu- This technique has proven to be an extremely valuable re- ver led to the development of other calibration techniques search tool and has been widely employed in the study of that could be performed during natural breathing. The respiratory muscle function [13-15]. The major limita- least squares calibration technique requires recordings of tion of magnetometers is that relatively small changes in spirometry, RC and Ab signals in two postures, taking ad- body position alter the antero-posterior diameter with re- vantage of the change in compartmental contribution to sulting loss of calibration and inaccurate volumetric mea- tidal volume that occurs with change in body posture [/7, surements. 18]. The RC/spirometer and Ab/spirometer ratios from the breaths in the two postures are measured, and then Respiratory inductive plethysmography (RIP) cast as coordinate pairs on an X-Y plot of RC/spirometer The RIP is the most widely accepted method of measur- against Ab/spirometer, and a least-squares regression line ing ventilation in a non-invasive and quantitative manner. is fitted through the points. The reciprocals of the inter- The transducers consist of two coils of insulated wire cepts of this line are taken as the calibration factors of the sewn onto bands placed around the RC and Ab (Fig. 1). RC and Ab transducers. Although the least-squares tech- The coils are connected to a small oscillator module that nique is probably the most robust method of calibration puts out frequency-modulated signals that are propor- [/7-19], it can be quite tedious to perform, especially in tional to alterations in the self-inductance of the coil. The critically ill patients. The multiple linear regression tech- self-inductance of the coil, and so the frequencies of its nique avoids the need for variation in body posture [20], oscillator, is proportional to the cross-sectional area en- and is based on the normal variation in compartmental closed by the coil. These signals are sent to a demodula- contribution during tidal breathing [21]. During the ini- tor/calibrator unit which converts the signal into a pro- tial part of inspiration, volume change is produced pre- portional voltage that can be amplified and recorded. dominantly by Ab displacement [22]. The contribution of Calibration of the RIP is based on the assumption of the RC progressively increases so that at the end of inspi- Konno and Mead [/6] that the respiratory system can be ration volume change is predominantly due to RC mo- considered as a simple physical system with two moving tion. Using a computer to sample the signals (at 25 Hz) parts, the RC and Ab. Consequently, volume measured at produced by the spirometer, RC transducer and Ab trans- the mouth is equivalent to the sum of volume change of ducer will field a matrix of 30 to 40 equations in two un- the RC and Ab. This concept of two degrees of freedom knowns with the points from a single breath. Using a se- 195

ries of breaths in a single posture, the RC and Ab calibra- tube while he/she breathed room air for one minute. Of tion factors can be computed. Recently, Sackner et al. the patients who had an f/V T value greater than 100 [23] developed a modification of the single posture tech- breaths/min/1, 95~ failed a weaning trial, whereas 80~ nique that permits calibration of the RIP without the use of the patients with lower f/VT values were successfully of an external spirometer or pneumotachograph. Em- weaned (Fig. 2). As a method of assessing pulmonary ploying this calibration technique, the RIP can provide performance in critically ill patients, the f/V T ratio has a accurate measurements of respiratory timing and RC-Ab number of attractive features; it is easy to measure, it is coordination, and reasonably accurate measurements of independent of the patient's effort and cooperation, it relative changes in tidal volume. appears to be quite accurate in predicting the ability to sustain ventilation, and fortuitously, it has a "rounded off" threshold value (100) that is easy to remember. The mechanism of rapid shallow breathing is un- Analysis known. Roussos [35] has suggested that afferent activity Volume and time components arising in fatiguing respiratory muscles may impinge on the respiratory centers and produce alterations in the In resting healthy subjects, minute ventilation is about breathing pattern. In support of such an occurrence, 6 l/rain [24]. Since PaCO2 is determined by the relation- Gallagher et al. [36] observed rapid shallow breathing in ship between alveolar ventilation and COa production, a healthy volunteers in the period immediately following high minute ventilation in the presence of hypercapnia in- the induction of inspiratory muscle fatigue by resistive dicates the presence of increased deadspace ventilation loading. Since minute ventilation was markedly elevated and/or increased CO2 production [25]. Conversely, after resistive loading, they employed CO 2 rebreathing to hypercapnia associated with a low minute ventilation achieve a similarly elevated minute ventilation during the should arouse suspicion of decreased respiratory drive, control recordings prior to loading. However, such an ap- structural abnormality of the thoracic cage or respiratory proach may not be valid since several investigators have muscle dysfunction. While a minute ventilation of less shown that breathing pattern differs substantially under than 101/min is commonly used as a predictor of wean- non-steady state conditions depending on the stimulus ing outcome [26], one should not rely on this criterion used to augment ventilation [37-40]. To avoid this con- since it is associated with a high rate of false-positive and founding effect, Mador and I [41] recently examined the false-negative results [27, 28]. pattern of breathing before and after inducing respiratory The measurement of minute ventilation should be muscle fatigue, and found that fatigue did not produce partitioned into its respiratory frequency and tidal vol- rapid shallow breathing during either unstimulated ume components. In healthy subjects, respiratory fre- breathing or CO2 rebreathing. In complementary stud- quency is approximately 17 breaths/min and tidal volume ies, Mador and Acevedo [42, 431 observed rapid but not is approximately 0.401 [24]. An elevated frequency is of- shallow breathing when fatigue was combined with an in- ten the earliest sign of impending respiratory disaster [29, creased load. Further investigations are required to deter- 30], and the degree of elevation is proportional to the se- verity of the underlying disease [31]. In patients being discharged from an ICU, tachypnea was more common in patients who subsequently required readmission or died 80- unexpectedly [32]. In a recent study of patients who had 120 / 100 undergone a cardiopulmonary arrest, Schein et aI. [33] found that 53~ of the patients had documented deterio- i 60 - /~ 80 Failure ration in respiratory function in the 8 h preceding the ar- r~ Success rest. Of interest, respiratory frequency was elevated in the ~' 40 "" majority of patients - mean, 29+ 1 (SE) breaths/min - = ql while other routine laboratory tests showed no consistent abnormalities. The observation that detection of ~. 2o tachypnea did not result in changes in patient manage- is f [] ~aa ment (in an effort to prevent an arrest) led the authors to surmise that physicians do not fully appreciate the clini- 0.0 0.2 0 0.6 0 1 0 1.2 cal importance of tachypnea. Tidal Volume (liters) Patients who fail a trial of weaning from mechanical ventilation commonly display an elevated respiratory fre- Fig. 2. Isopleths for the ratio of breathing frequency to tidal volume, representing different degrees of rapid shallow breathing. For the pa- quency (f) and a low tidal volume (VT) [34]. These two tients indicated by the points to the left of the isopleth representing 100 measurements can be combined to produce an index of breaths/rain/I, the likelihood that a weaning trial would fail was 95o70, rapid shallow breathing, viz., the f/V T ratio. In a recent whereas for the patients indicated by the points to the right of this prospective study of patients being weaned from mechan- isopleth, the likelihood of a successful weaning outcome was 80%. The ical ventilation, the f/V T ratio was superior to conven- hyperbola represents a minute ventilation of 101/min, a criterion com- tional predictors of weaning outcome [28]. In this study, monly used to predict weaning outcome; apparently, this criterion was of little value in discriminating between weaned patients (open sym- respiratory frequency and tidal volume were measured bols) and the patients in whom weaning failed (solid symbols). From with a spirometer attached to the patient's endotracheal Yang KL and Tobin MJ [28] 196 mine the mechanisms of this major disturbance in such as that produced by lung inflation, paralysis or air- breathing pattern. way occlusion, produce no appreciable change in the A more detailed approach to breathing pattern analy- slope of integrated phrenic nerve activity [l, 46-48]. sis can provide additional information regarding the re- VT/T~, which is the mechanical transformation of this spiratory control system. In contrast to the traditional ap- neural activity, has been shown to be related to standard proach where minute ventilation (~rE) is analyzed in indices of respiratory center output, such as Po.1 and the terms of tidal volume (VT) and respiratory frequency (f), ventilatory response to hypercapnia [49-51]. An in- i.e., herent disadvantage of VT/TI is that, unlike P0.~, it may underestimate respiratory drive in the presence of marked VE = VTXf (1) abnormalities in respiratory system mechanics. However, Barcroft and Margaria, in 1931, proposed that minute an elevated VT/T~ in this situation reflects elevated respi- ventilation should be analyzed in terms of "(1) the dura- ratory drive, albeit it may be underestimated. For exam- tion of the phases of respiration... (2) upon the rates at ple, in a study of patients who failed a trial of weaning which air is given in and out during the phases" [44]. This from mechanical ventilation, an increase in VT/TI was analytical approach lay dormant for the next 40 years un- observed between the beginning and end of the weaning til the publication of some elegant studies by Clark and trial indicating that an impairment in respiratory center yon Euler in 1972 [45]. This work was later extended by output was not the primary cause of ventilatory failure in Milic-Emili and Grunstein [46] who proposed a further these patients [34] (Fig. 4). mathematical analysis of the breathing cycle. The tradi- Fractional inspiratory time, TI/TTOT, indicates the re- tional equation of minute ventilation (Eq. 1) is rear- lationship between inspiration and expiration and pro- ranged, since respiratory frequency is equal to 60 divided vides a crude measure of the degree of airway obstruction by the time of a total respiratory cycle (TToT) or single [52]. Since the respiratory muscles are normally active breath. only during inspiration, TI/TTOT has also been termed the duty cycle of the respiratory system and the level of ~rE = V T X 60/TToT (2) stress placed on the respiratory muscles is proportional to Conceptually, 60 can be deleted and the equation reduced TI/TTo T. Indeed, TI/TToT and the ratio of mean to transdiaphragmatic pressure (Pdi) per breath/maximum static P~ (i.e., PJP~max) are equally important deter- ~rE = VT • I/TToT (3) minants of diaphragmatic fatigue [53]. The product of these ratios is termed the diaphragmatic tension-time in- Dividing VT by inspiratory time (T 0 while multiplying dex (TTai) and once a critical value of 0.15 is reached, 1/TTo T by T~ gives diaphragmatic fatigue is likely to ensue [53]. When re- ~rE = VT/T I • T (4) porting measurements of tension-time index, it is important to indicate whether it has been computed from vol- The first parameter, VT/TI, has been termed mean in- ume/flow or pressure recordings, as values calculated spiratory flow rate, and the second parameter, TI/TToT, from pressure tracings will be considerably higher than has been called fractional inspiratory time (Fig. 3). the former [54]. Despite the evidence that TI/TToT is a VT/TI has been widely employed as a measure of re- major determinant of respiratory muscle endurance in ex- spiratory drive. The neurogenic basis of doing so has perimental volunteers, and that a reduction in TI/TToT is been demonstrated by a number of investigators (particu- a useful strategy for decreasing the risk of fatigue, in clin- larly von Euler and Cohen and co-workers). Once inspira- ical reality patients rarely display large changes in tion has begun, changes in pulmonary afferent activity, TI/TTOT [30, 39].

Fig. 3. Schematic recording of the respiratory cycle obtained with a respiratory inductive plethysmograph. The sum signal represents tidal volume (VT), which is de- / \ I/ !'\ I/ \ / \ rived from the algebraic sum of the rib I I cage (RC) and abdominal (Ab) excursions.

. _ i~ - i "~ End-K~mtory Level T I = inspiratory time; T E = expiratory time; TTOT = time of a total breath; VT/T I = mean inspiratory flow rate. Altera- tions in the end-expiratory level represent changes in functional residual capacity, provided movement artifact is absent. Max- imum amplitude is the trough-to-peak amplitude of the RC and Ab excursion, irre- spective of their phase relationship to the Ab sum (tidal volume) signal, Their arithmetic Max. Arnpl. sum, designated maximum compartmental amplitude (MCA), is equivalent to the al- I I I o 5 l0 gebraic sum signal (VT) when the RC and Ab move in phase. From Tobin MJ [76] 197

500- 800-

~e~ 400

600 - "E 300 | _I_ 1- , II 200 i p <0.01 O= Start End > Fig. 4. Measurements of mean inspiratory flow (VT/TI), an index of respiratory drive, in 7 patients who failed a trial of weaning from mechanical ventilation. Note that an increase in VT/TI was observed in each patient between the beginning and end of the weaning trial. From Tobin MJ et al [34]

Compared with traditional methods of assessing the v Eucapoia . Hypercapnia respiratory control system, the reproducibility of breath- (n =16) (n = 12) ing pattern measurements is very appealing. The enor- Fig. S. Mean inspiratory flow (VT/TI), an index of respiratory drive, mous inter- and intra-individual variability of the ven- during resting breathing in patients with chronic obstructivepulmonary tilatory and mouth occlusion pressure responses to hyper- disease. The shaded area represents the 95 per cent confidencelimits of capnia and hypoxia makes it difficult to characterize the VT/T~ in healthy subjects. From Tobin MJ [77] response of any single individual as abnormal [55, 56]. Between subjects, the coefficient of variation for the ventilatory responses to chemical stimulation is approxi- the various respiratory muscles [2, 13-15]. In their origi- mately 50% [55]. Within subjects, the coefficient of vari- nal paper, Konno and Mead [16] demonstrated the value ation is 20~ when the ventilatory response is measured of plotting motion of the RC and Ab on a X-Y plot. This over a 2 h period, and this degree of variation increases allows simultaneous assessment of the relative motion of 6-fold when repeated on different days [56]. The enor- the two compartments, and by calibrating motion for vol- mous variability of these indices in healthy subjects ume change it is possible to determine compartmental makes it difficult to characterize the response of any sin- contribution to tidal volume and the degree of breath- gle individual as abnormal, and limits their value for as- to-breath variation in this contribution (Fig. 6). During sessing the response to an experimental intervention [55]. normal breathing both RC and Ab expand on inspiration In contrast, the inter-individual coefficient of variation and decrease back to the resting position during expira- for VT/T I and TI/T~T in healthy subjects is 22O/o and tion. Since the rate of excursion is virtually the same for 8%, respectively, if measured non-obtrusively [57]. When the two compartments, an X-Y plot of RC-Ab motion repeated on different days, the intra-individual coeffi- shows the formation of a closed or a very slightly open cient of variation of VT/T I and Ti/Ta-or are 9~ and 3%, loop. Abnormal motion can be separated into three ma- respectively, compared with 40% for the ventilatory re- jor types: (1) asynchrony, in which the two compartments sponse to hypercapnia [57]. This degree of reproducibility continue to move in the same overall direction, but the allows the construction of a relatively narrow normal rate of motion differs causing a widened loop to form on range for the breathing pattern components, permitting the X-Y plot; (2) paradox, in which one compartment the detection of abnormalities in individual patients, un- moves in an opposite direction to the overall tidal volume like the experience with conventional techniques [55]. For signal, resulting in a negative slope on the X-Y plot; (3) example, although VT/T I is commonly considered an in- an increase in the breath-to-breath variation in the rela- sensitive index of respiratory drive in patients with abnor- tive contribution of the RC and Ab to tidal volume, repre- mal lung mechanics, in a study of patients with COPD senting recruitment and derecruitment of the accessory- it was elevated above the normal 95% confidence limits intercostal muscles and diaphragm [59, 60]. in 71% of the patients [52] (Fig. 5). Paradoxical motion of the RC is a characteristic find- ing in patients with cervical cord lesions, while patients Rib cage-abdominal motion with diaphragmatic paralysis display abdominal paradox. Although investigations of chest wall motion extend back Recently, there has been considerable interest in the study to the last century [58], it is only in the last twenty-five of RC-Ab motion in acutely-ill patients who do not have years, following the landmark study of Konno and Mead diseases that primarily involve the chest wall. Using un- [16], that this approach has been widely employed as a calibrated magnetometers, Ashutosh, Gilbert, and col- means of investigating the activity and coordination of leagues [61, 62] noted that patients displaying asynchro- 198

chanical ventilation [59]. Seven of the patients has a power spectral shift in their diaphragmatic electromyogram (EMG) signal that was interpreted as indicating fatigue. Of these 7 patients 6 has Ab paradox, whereas this was absent in the 5 patients without EMG changes. Electrical 4oo]- interference prohibited the employment of their magne- =If___4 tometers, and paradoxical motion was only characterized 2 BeG by clinical examination. In addition, a shift in the EMG power spectrum has not been shown to bear a relationship to the form of fatigue that is physiologically impor- ,,i,o.,, tant, viz., low-frequency fatigue, and its physiological basis remains unknown [64]. Moreover, no attempt was -0.25L~ + 0.25L made to separate the effect of work of breathing, i.e., load, from muscle fatigue in these patients [59]. In a departure from the descriptive character of most -L -0.25L ~ -0.25L "a-'-O.2~'L previous studies employing Konno-Mead plots, Tobin et Mechanical Weaning Trial al. [65, 66] computed several indices that provided quan- Ventilation Start End titative assessment of the amount of asynchrony, paradox Fig. 6. Analog tracing of the sum (VT) , rib-cage (RC), and abdominal and breath-to-breath variability in compartmental contri- (Ab) volume signals during mechanical ventilation and at the beginning bution to tidal volume. By calculating these indices from and end of a weaning trial in a patient with an unsuccessful weaning a series of breaths at fixed periods in time, it was possible outcome. The terminal portion of the preceding breath and the initia- to obtain a measure of the degree of inter- and intrasub- tion of the subsequent breath are also shown. For clarity, the baselines ject variability and thereby avoid the bias that might re- of the individual analog signals have been adjusted arbitrarily to provide visual separation of the signals. The respective Konno-Mead plots suit with subjective selection of a few breaths. In a study of the RC-Ab relationship are displayed below each of the breaths. Dur- conducted in healthy subjects breathing against resistive ing mechanical ventilation, there is some Ab paradox, which increases loads and using an experimental design that permitted immediately upon discontinuation of the ventilator. There is no pro- the separation of the effect of loading from fatigue, they gression in the extent of abnormal RC-Ab motion from the beginning found that fatigue was neither necessary nor sufficient to to the end of the weaning trial 24 rain later. From Tobin MJ et al [65] induce asynchrony, paradox or increased variability in RC-Ab motion [66] (Fig. 7). In contrast, respiratory loading was sufficient to induce abnormal motion.

Pm Pm Observations in patients being weaned from mechani- Baseline 30% max 60% max cal ventilation also support the view that abnormal RC- s Ab motion does not signify muscle fatigue [65]. Since ventilation cannot be sustained in the presence of respira-

>"6 tory muscle fatigue (by definition), ventilator dependent patients who display Ab paradox should inevitably devel- oCZ: t 0.5 1.5 op ventilatory failure if ventilator support is discontin- I : I ; ued. However, Tobin et al. [65] observed considerable overlap in the degree of Ab paradox in patients who were Ab Volume (I) successfully weaned and in those who failed a weaning Fig. 7. Konno-Mead plots of rib cage (RC)-abdominal (Ab) motion trial (Fig. 8). Another index of RC-Ab motion termed from a subject during resting breathing and while breathing against a maximum compartmental amplitude-to-tidal volume ra- resistive inspiratory load of sufficient magnitude to require the genera- tio (MCA/VT) (Fig. 3) discriminated between the two tion of 30% and 60% of maximum mouth pressure (Pmmax). At 30% groups of patients. The apparent superiority of MCA/ of PmmaX, a loop forms, indicating RC-Ab asynchrony, and some Ab paradox is also evident. At 60% of PmmaX, the degree of Ab paradox V T ratio over other indices may be related to the fact it increases further, as indicated by the leftward shift in the plot. The pres- is the only index that takes both asynchrony and paradox ence of abnormal RC-Ab motion at 30~ of Pmmax is of significance, of both the RC and Ab into account. Patients who failed since this level of pressure can be sustained indefinitely without the de- the weaning trial also displayed considerable breath- velopment of respiratory muscle fatigue. From Tobin MJ et al [66] to-breath fluctuation in the relative contribution of the RC and Ab to tidal volume, with the result that the extent of abnormal motion tended to be divided between the two compartments. Krieger and Ershowsky [67] also re- nous RC-Ab movements had an increased risk of ven- ported that an elevated MCA/VT ratio and increased tilatory failure necessitating mechanical ventilation [62] variability in compartmental contribution to tidal volume and a poor prognosis [61]. Subsequently, investigators at were helpful in predicting weaning outcome. Accordingly, McGilI University suggested that Ab paradox is virtually one can conclude that while abnormal RC-Ab motion is pathognomonic of diaphragmatic fatigue [63] if dia- not due to fatigue per se, monitoring such motion can be phragmatic paralysis and inversion are excluded [59, 60]. clinically useful since it provides a non-invasive reflection This interpretation was largely based on a study of 12 pa- of the balance between respiratory load and the capacity tients exhibiting difficulties during weaning from me- of the respiratory muscles to meet that load. 199

100' PEEP at which end-expiratory lung volume increased,

r they obtained a remarkably close estimate of the patient's =80 original level of auto-PEEP. An attractive feature of this L. L. technique is that it does not disturb expiration, unlike the =6o occlusive technique, in which foreshortening of expirato- r o ry time is unavoidable [73]. External PEEP has recently ,,..,40 =: been advocated as a means of decreasing work of breathing in patients with auto-PEEP [70, 74, 75]. If external r PEEP is used for this purpose, monitoring the end-expi- I:., ratory level offers promise, particularly in spontaneously breathing patients, as a means of determining if the level WS WF WS WF WS WF of external PEEP remains optimal, especially since the Ab Paradox MCAN T RCN T Variability level of auto-PEEP varies over time [70]. In addition, this (> 10%) (> 1,18) (SD > 10%) technique may serve as a simple and convenient test of Fig. 8. The frequency of significant abnormalities of rib cage-abdomi- airflow limitation: in flow-limited patients, end-expirato- nal in patients who were successfully weaned (WS) and in patients who ry lung volume should remain unchanged until the criti- failed a weaning trial (WF). Significant abdominal (Ab) paradox was cal level of external PEEP is exceeded, whereas in pa- considered present when paradoxic volume was > 10% of the total Ab tients without flow limitation, a progressive increase in volume excursion (both in and out phase). The maximum compartmen- lung volume (and peak airway pressures) should occur, tal amplitude to tidal volume ratio (MCA/VT) (see derivation in Fig. 3) starting with even the smallest increments in external was considered significantly increased when it exceeded 1.18. A significant increase in the breath-to-breath variabilityin the relative contribu- PEEP [73]. tion of the rib cage (RC) and Ab to tidal volume (VT) was considered In conclusion, breathing pattern analysis serves as a present when the standard deviation (SD) of the RC/V T ratio exceeded very powerful research tool in investigating the neuro- 10%. Significant differences were observed between the groups of pa- muscular control of breathing, and, in clinical practice, it tients in the occurrence of an increased MCA/VT ratio (t7 < 0.001) and is proving to be a helpful means of monitoring respirato- increased variability of the RC/V x ratio (p<0.05), but not for in- ry performance in critically ill patients. creased Ab paradox. (Based on data published in Tobin MJ et al [65]

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