American Journal of Respiratory and Critical Care Medicine

We used optoelectronic plethysmography to study 11 normal subjects during quiet and deep breathing, six sedated and paralyzed patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS) receiving continuous positive pressure ventilation (CPPV) (positive end-expiratory pressure [PEEP] = 10 cm H2O, tidal volume [Vt] = 300, 600, 900 ml), and seven ALI/ARDS patients receiving pressure support ventilation (PSV) (PEEP 10 cm H2O, pressure support = 5, 10, 15, 25 cm H2O). The volumes measured using optoelectronic plethysmography were compared with measurements taken using spirometry and pneumotachography. The three methods were highly correlated. The discrepancies found were 1.7 ± 5.9%, − 1.6 ± 5.4%, and 4.9 ± 6.4% when comparing optoelectronic plethysmography with spirometry, optoelectronic plethysmography with pneumotachography, and spirometry with pneumotachography, respectively. Accuracy of the compartmentalization procedure (upper thorax, lower thorax, and abdomen) was assessed by calculating compartmental volume changes during isovolume maneuvers. The discrepancy from the ideal zero line was − 2.1 ± 48.3 ml. Abdominal contribution to inspired volume was greater for normal subjects than for PSV patients (63 ± 11% versus 43 ± 14%, p < 0.001). It decreased with Vt for normal subjects (48.5 ± 15%, p < 0.05), whereas it increased for CPPV patients (61 ± 10%, p < 0.05). No significant distribution differences were found between 5 and 25 cm H2O PSV. We conclude that optoelectronic plethysmography is a feasible technique able to provide unique data on the distribution of chest wall volume changes in intensive care patients.

Chest wall volumes depend on the regional mechanical characteristics of the lungs and the chest wall. Patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) typically have unevenly distributed lesions throughout the lung parenchyma and altered mechanical properties of the chest wall, especially after abdominal disease (1). Indeed, it may be interesting to assess the chest wall mechanics and distribution of chest wall volume changes of ALI/ARDS patients.

Recently, a plethysmographic method based on optoelectronic measurement was described which can be used to assess absolute chest wall volumes and their variations in the upper and lower rib cage and abdomen (both right and left sides) (2– 4). The method is based on measuring a finite number of displacements of points on the outer surface of the chest wall. It is noninvasive and does not involve connections to the patient. So far, it has only been applied to normal subjects in a seated position (5, 6).

In intensive care units (ICUs), patients are usually covered with multiple devices and lie in a supine position, which limits the visibility of markers on the anterior and lateral parts of the rib cage and abdomen. The applicability of optoelectronic plethysmography in this environment is therefore questionable. In this work, we set out to test the feasibility of this method in an ICU setting by comparing the volumes measured using optoelectronic plethysmography with measurements taken using spirometry and pneumotachography. We studied both normal subjects and patients with ALI/ARDS. Our aim is to report data regarding the feasibility of using optoelectronic plethysmography on ICU patients and some preliminary optoelectronic plethysmography results concerning the distribution of chest wall volume changes found between spontaneously breathing subjects, patients with ALI/ARDS receiving pressure support ventilation (PSV), and sedated and paralyzed patients with ALI/ARDS receiving continuous positive pressure ventilation (CPPV).

Study Population

The study population consisted of 24 subjects. Eleven were healthy volunteers (5 men and 6 women, age 28.0 ± 4.5 yr, weighing 65.3 ± 11.5 kg and 1.71 ± 0.09 meters tall, body mass index [BMI] 22.18 ± 2.08 kg/m2). Thirteen were patients who, either before or during the study, had ALI/ARDS, defined according to the American European Consensus Conference Criteria (7). The patients' most relevant clinical data are summarized in Table 1.

Table 1. PATIENTS' CHARACTERISTICS

PatientsSexAgeWeightBMIOnset ALI/ARDSStudy DayDiagnosisOutcome S/D
PaO2 /Fi O2 PaO2 /Fi O2 Day of Study
CPPV
 1F707529.3152136 1PneumoniaS
 2F737027.3190230 2PneumoniaS
 3F717027.314120422PneumoniaD
 4M817024.2218335 1Postanoxic comaS
 5F404821.3292383 3PneumoniaS
 6F647533.314422820PneumoniaS
Mean ± SD66.5 ± 14.168 ± 10.127.1 ± 4.1189.5 ± 58.6252.7 ± 90.48.2 ± 10.0
PSV
 1M606019.6 7919015SepsisD
 2M764819.516837621PneumoniaS
 3M557020.518830276SepsisS
 4M558024.710723451PolitraumaS
 5M677524.522524528Hemorrhagic shockS
 6F607529.312322538Septic shockS
 7M658026.1146308 7Cardiac arrestS
Mean ± SD62.6 ± 7.569.7 ± 11.823.4 ± 3.7148.0 ± 50.0268.6 ± 63.333.7 ± 23.7

Definition of abbreviations: BMI = body mass index, kg/m2; CPPV = continuous positive pressure ventilation; PSV = pressure support ventilation; S = survived; D = deceased.

Experimental Set-up (Figure 1)

Optoelectronic plethysmography. Optoelectronic plethysmography was carried on the whole study population by analyzing the movements of retro-reflective markers using four television cameras connected to an automatic motion analyzer (Elite system; BTS, Milan, Italy). All markers were simultaneously visible to at least two television cameras so that their three-dimensional positions and displacements could be reconstructed using stereo-photogrammetric methods (8). After a series of preliminary experiments, we found that the best set-up for the supine position involved using 45 markers. The markers were placed in order to define three chest wall compartments: upper thorax, lower thorax, and abdomen, as shown in Figure 2 (see for details).

Spirometry. The normal subjects were connected to a water displacement spirometer fitted with a potentiometer (Model 308; Spectrol Electronics, City of Industry, CA, linearity ± 0.25%). The spirometer was filled with room air at ambient temperature, and the subject–spirometer circuit was closed without CO2 absorption or oxygen supply. Spirometry was carried out on normal subjects for 30 to 40 s. For four CPPV and four PSV patients, who were ventilated using a Siemens Servo 900 C ventilator (Siemens-Elema, Solna, Sweden), the spirometer was connected to the ventilator exhaust port. Expiratory gases were collected over a 60-s period. The analogue spirometer signal (calibrated beforehand using a 1.5-L calibration syringe) was sent to an A/D board (RTI800; Analog Devices, Norwood, MA), synchronized with the automatic motion analyzer and digitally recorded at 100 Hz.

Pneumotachography. For four CPPV patients and four PSV patients, a pneumotachograph (HR 4700-A; Hans Rudolph Inc., Kansas City, MO) was inserted between the filter and the connection to the ventilator. Its output signal was sent to an A/D board, synchronized with the automatic motion analyzer and digitally recorded at 100 Hz. Time integration of this signal provided the volume measurement.

Esophageal pressure. For the seven patients receiving PSV, esophageal pressure (Pes) was measured by using an esophageal balloon (Bicore, Irvine, CA) modified to allow connection to the transducer (SCX01; Sensym, Milpitas, CA). During measurements, the balloon was inflated with 0.5 to 1 ml of air. The balloon positioning was verified by chest radiography.

Experimental Procedures

After an adaptation period, the 11 normal subjects were asked to breathe normally and quietly for about 30 s, then to take four deep breaths. The six sedated paralyzed patients receiving CPPV were ventilated at the fraction of inspired oxygen (Fi O2 ) in use (0.44 ± 0.11), (positive end-expiratory pressure [PEEP] 10 cm H2O) and studied at Vt = 300, 600, and 900 ml. Their respiratory frequency was kept constant, and the data collected in each test were averaged over 60 s. The seven patients receiving PSV were ventilated at the Fi O2 in use (0.38 ± 0.08), (PEEP 10 cm H2O) and studied at 5, 10, 15, and 25 cm H2O PSV.

To assess the accuracy of optoelectronic plethysmography in measuring volume distribution between the three chest wall compartments (upper thorax, lower thorax, and abdomen), the seven patients receiving PSV were made to make isovolume movements by occluding the expiratory line (pressing expiratory hold on the Servo 900 C) at the end of expiration. The patient's inspiratory effort caused either a contraction of the belly and an expansion of the thoracic cage (“belly in”) or an expansion of the belly and a contraction of the rib cage (“belly out”). Simultaneously, the esophageal pressure swings were recorded.

Data Analysis

Volume comparison. A detailed description of the optoelectronic method used to analyze chest volume in standing and seated positions has already been published (2-4). The changes made in order to analyze subjects in a supine position are summarized in the . Three-dimensional reconstruction was based on stereo-photogrammetry (8) using the data from two or more television cameras to improve calculation accuracy. On the basis of previously defined geometrical models that describe the whole chest wall and its compartments, dedicated software was used to calculate the absolute volumes and their changes during respiration from changes in the x, y, and z coordinates of the markers.

The expiratory changes in volume (ΔVolume = volume at end of inspiration − volume at end of expiration) measured using optoelectronic plethysmography, spirometry, and pneumotachography were averaged for each test (i.e., over approximately 8 to 20 breaths for each test) and the three methods were compared by linear regression and Bland-Altman analysis (9). In addition, the mean percentage discrepancy between any two methods was calculated as:

discrepancy%=[(ΔVXΔVY)/ΔVX]×100

where ΔVX is the volume change measured using spirometry or pneumotachography and ΔVY is the volume change measured using optoelectronic plethysmography. When comparing pneumotachography with spirometry, ΔVY is the pneumotachographic volume change and ΔVX, the spirometric volume change.

Accuracy. To test the accuracy of the compartmentalization procedure, we compared the volume changes in each chest wall compartment (upper, lower thorax and abdomen) measured during occlusion tests on patients receiving PSV. The compartmental volume changes measured in each test were averaged. Ideally, the sum of the volume changes in the three compartments (upper thorax, lower thorax, and abdomen) should equal zero.

Distribution of Chest Wall Volume Changes

Optoelectronic plethysmography was used on the whole study population, both normal subjects and CPPV and PSV patients, to study volume changes in the upper thorax, lower thorax, and abdomen. Distribution of chest wall volume changes calculated for all subjects in all ventilatory conditions was expressed as percentage of Vt.

Statistical Analysis

All data are expressed as mean ± SD unless otherwise indicated. The three methods (optoelectronic plethysmography, spirometry, and pneumotachography) were compared by linear regression and Bland-Altman analysis (9). Spirometry, pneumotachography, and optoelectronic plethysmography accuracy data were compared using Student's t test, as were the distribution of chest wall volume changes of normal and mechanically ventilated patients. Probability values of less than 0.05 were taken as significant.

Figure 3 shows typical experimental tracings for normal subjects during spontaneous breathing (left panel ) and patients receiving PSV (right panel ).

Comparisons between Spirometry, Pneumotachography, and Optoelectronic Plethysmography

Figure 4 shows the relationships among chest wall volume changes as measured by optical plethysmography, spirometry, and pneumotachography. Note the excellent correlation of the several comparisons, the small intercept of the regression line, and the tight data around that line. There is an increase in the difference between volumes measured at the mouth and at the chest wall. This difference increases absolutely (while it decreases fractionally) within increasing Vt (upper two panels), but no such relationship for the comparison of spirometric and pneumotachographic measurements.

The average discrepancy between pneumotachography and optoelectronic plethysmography was −1.6 ± 5.4%, which was similar to the discrepancy between spirometry and optoelectronic plethysmography (1.7 ± 5.9%, p = not significant [NS]). The largest discrepancy found was between spirometry and pneumotachography (4.9 ± 6.4%, p < 0.05). The discrepancies between pneumotachography, optoelectronic plethysmography, and spirometry were not correlated with the BMI.

Accuracy

The patients in PSV reacted to the “occlusion” maneuver either with a “belly-in” (four patients) or a “belly-out” (three patients) movement. The results are shown in Figure 5. As can be seen, the discrepancy from the ideal zero line was on average −8.3 ± 52.5 ml (i.e., 12.7% of the thoracic volume change) for patients who performed belly-in movements. The same discrepancy was on average −10.6 ± 56.7 ml (i.e., 8.9% of the thoracic volume change) for patients who performed belly-out movements (not significant).

We found a significative difference (p < 0.05) in averaged esophageal pressure swings during occlusion between belly in and belly out patients (−7.8 ± 5.0 cm H2O and −13.8 ± 7.4 cm H2O, respectively), suggesting that belly out is associated with a greater inspiratory effort. Moreover, we found a correlation between the amplitude of the esophageal pressure swings and the deviation from the ideal zero line, i.e., more negative was the esophageal pressure swings more positive was the difference between the observed changes in chest wall volume and the ideal zero line (slope −3.5 ml/cm H2O, intercept −40.8 ml, r = 0.63, p < 0.01).

Compartmental Volume Changes

We observed significant differences in distribution of chest wall volume changes between normal subjects and mechanically ventilated patients and, in the latter case, between patients receiving CPPV and patients receiving PSV. Moreover, distribution of chest wall volume changes appeared to change according to Vt. The relevant data are summarized in Figure 6.

As can be seen, abdominal contribution to total chest wall volume changes was 63.1 ± 11.4%, 56.4 ± 9.5%, and 43.3 ± 14% for normal subjects during quiet breathing (Vt = 631.3 ± 148.4 ml), CPPV patients at the lowest Vt used (Vt = 300 ml), and PSV patients at the lowest pressure support used (5 cm H2O, Vt = 341.8 ± 81.1), respectively (normal subjects versus CPPV, p = NS; normal subjects versus PSV, p < 0.001; and CPPV versus PSV, p < 0.05). Increasing Vt for normal subjects (by asking them to breathe deeply, Vt = 2,695.1 ± 655.0 ml), CPPV patients (Vt = 900 ml), and PSV patients (at 25 cm H2O, Vt = 770.2 ± 287.2) led to different abdominal contributions: 48.5 ± 15%, 61.1 ± 10%, and 47.9 ± 10%, respectively (normal subjects versus CPPV p < 0.05; normal subjects versus PSV, p = NS; and CPPV versus PSV, p < 0.05). Indeed, when Vt was increased, abdominal contribution to total chest wall volume changes decreased significantly for normal subjects (p < 0.05), but increased significantly for CPPV patients (p < 0.05). No significant differences in abdominal contribution to total chest wall volume changes were found between 5 and 25 cm H2O PSV. It is interesting to note, however, that there was greater upper thorax (and lower abdomen) contribution to total chest wall volume changes for patients receiving PSV than there was for patients receiving CPPV at all Vt levels.

The primary aim of this study was to test the feasibility of using optoelectronic plethysmography on ICU patients, who are usually covered with multiple devices which could potentially interfere with measurements. We found that optoelectronic plethysmography is as feasible for these patients as it is for normal subjects.

Comparison between Spirometry, Pneumotachography, and Optical Plethysmography

To assess the effectiveness of optoelectronic plethysmography in measuring volumes, we used spirometry as a reference method throughout the whole study population, and in the case of patients with ALI/ARDS we used both spirometry and pneumotachography. It is important to point out, however, that these comparisons are not straightforward. Optoelectronic plethysmography, in fact, measures changes in the chest wall, whereas spirometry and pneumotachography measure changes in lung gas volume. These changes are not necessarily the same because there may be blood shifts from the thorax/ abdomen to the periphery or vice versa.

In fact, as has been described for positive pressure ventilation (10), blood shifts to the periphery may lead to greater gas lung volume changes than chest wall volume changes, and vice versa during negative pressure ventilation. Indeed, there is a potential physiological source of discrepancy between lung gas volume changes and chest wall volume changes. Interestingly, gas volume changes were found to be systematically significantly higher than chest wall volume changes when Vt was increased (Figure 4, upper and middle panels). This may suggest a progressive increase in blood shift from the thoracic/abdominal complex to the periphery when intrathoracic pressure is increased.

Another potential source of discrepancy stems from the differences between actual lung gas volume changes and the volume changes measured using spirometry which are caused by temperature, humidity, and pressure differences between the lungs and the spirometer. In our experimental conditions, spirometer gas humidity and pressure corrections, in order to bring them in line with the standard body conditions of normal subjects (i.e., 37° C and 100% humidity), imply a 6 to 7% gas expansion, according to standard formulas (11). On the other hand, during mechanical ventilation, the positive pressure applied to the lungs induces a gas compression of approximately 1 to 2%, depending on the pressure used. Furthermore, spirometry was carried out through closed-circuit rebreathing (without CO2 absorption) on normal subjects, and through expired gas collection on mechanically ventilated patients. During rebreathing, oxygen consumption exceeds CO2 output, which results in an overall decrease in lung–spirometer system gas content (12). This does not occur for mechanically ventilated patients, because the ratio between oxygen consumption and CO2 output remains unchanged during the test. Faced with all these confounding factors, we chose not to introduce any corrections, because most of them are mutually compensating (i.e., temperature and humidity increase gas volume, whereas pressure and O2 consumption above CO2 output decrease gas volume). In fact, we have previously shown that these corrections are not necessary in a similar situation where the volume–pressure curve is performed with a super-syringe (13).

Indeed, even without any correction, the changes in lung gas volume and the changes in chest volume were highly correlated with a discrepancy of approximately 2%. Interestingly, the highest and most significant discrepancy was found when comparing two accepted methods: spirometry and pneumotachography (4.9 ± 6.4%).

With all these limitations in mind, among which possible blood shift is the most important physiologically, our data suggest that optical plethysmography is an excellent estimate of lung gas volume changes even in the case of ICU patients.

Compartmentalization Procedure Accuracy

To assess the accuracy of the compartmentalization procedure in patients receiving PSV, we used the occlusion test which resulted in belly-in or belly-out movement. Ideally (absence of blood shifts and negligible lung gas compression/decompression) changes in thoracic volume should be opposite and equal to changes in abdominal volume, and their sum should be zero. Considering the overall data, the magnitude of the discrepancy from the ideal zero line was −2.1 ± 48.3 ml. This, on average, is a satisfactory result; however, we have to explain the rather large standard deviation we observed. This could be the result of a technical problem in the method or of physiological phenomena. Interestingly, we found that the amplitude of the negative esophageal pressure swings was associated with a chest wall volume change more positive than expected. This may suggest a blood shift from periphery into the chest and/or greater lung gas expansion.

Distribution of Chest Wall Volume Changes

We observed significant differences in distribution of chest wall volume changes between normal subjects and mechanically ventilated patients and, in the latter case, between patients receiving CPPV and patients receiving PSV. For spontaneously breathing subjects, distribution of chest wall volume changes is dictated by the mechanical characteristics of the respiratory system and the relative activity of the diaphragm and inspiratory rib cage muscles, whereas for paralyzed CPPV patients, only the mechanical characteristics of the system are involved. The case of PSV is more complicated, because spontaneous and mechanical ventilation are combined to various degrees depending on the amount of respiratory support.

Chest wall volume changes for normal subjects during quiet breathing (631.3 ± 148.4 ml) were preferentially distributed to the abdominal compartment. This suggests greater compliance of this compartment and/or a preferential activation of the diaphragm. Taking a normal functional residual capacity (FRC) of 2,200 ml (14) for our normal subjects, “quiet breathing” implies lung expansion of approximately 25 to 30%. Shifting to deep breaths (Vt = 2,695.1 ± 655.0 ml), i.e., expanding the lung by more than its FRC, led to redistribution of chest wall displacement from the abdomen to the thorax. Assuming that abdomen/thorax compliances remain the same, this suggests greater activation of rib cage inspiratory muscles. The final result is a more uniform distribution of inflation between the thorax and the abdomen at high lung volumes, as has been previously described for normal subjects (15).

We measured the FRC of four of the seven patients receiving CPPV using helium dilution (16): it was found on average to be 1,364 ± 448 ml. In fact, CPPV patients ventilated with Vt = 300 ml expanded their respiratory systems by approximately 25%, the same order of magnitude as normal subjects during quiet breathing; their abdominal contribution to total chest wall volume changes was also found to be similar. However, unlike in the case of normal subjects, expanding their respiratory systems to nearly 100% of their FRC (Vt = 900 ml) led to increased contribution of the abdominal compartment. These data suggest that increasing the Vt and pressures used to ventilate patients with ALI/ARDS at 10 cm H 2O PEEP may cause increased compliance of the abdominal compartment, possibly due to intratidal recruitment (17).

The most interesting results, however, were seen in the case of PSV patients. At 5 cm H2O PSV (Vt = 341.8 ± 81.1 ml), chest wall volume changes were more highly distributed to the upper thorax than for both ALI/ARDS patients receiving CPPV and spontaneously breathing normal subjects. It may be supposed that this preferential contribution of the upper thorax was mainly due to the activity of inspiratory rib cage muscles. Increasing the support pressure to 10, 15, and 25 cm H2O (and increasing the Vt) led to an increase in abdominal and a decrease in upper thorax contribution to total chest wall volume changes. This may be the result of lower inspiratory rib cage muscle activity and/or an increase in abdominal compliance through mechanisms similar to those reported for CPPV patients. However, even at 25 cm H 2O PSV (Vt = 770.2 ± 287.2 ml), a level at which ventilation should mainly be passive, the distribution was significantly different from that found for CPPV patients. This finding deserves further investigation.

In conclusion, we believe that optoelectronic plethysmography is an excellent tool for measuring chest wall volume changes in ICUs and that it may be used to assess the physiological importance of blood shifts and to study distribution of chest wall volume changes in different ventilatory conditions.

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Correspondence and requests for reprints should be addressed to Andrea Aliverti, Ph.D., Centro di Bioingegneria, Politecnico di Milano-Fond.Pro Juventute Don Gnocchi, Via Gozzadini 7, I-20148 Milano, Italy. E-mail:

This work was partly supported by the European Commission – BIOMED II programme (biomedical technology research project “BREATH” – Biomedical technology for REspiration Analysis THrough optoelectronics) and by MURST - Cofin '98.

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