Measuring End Expiratory Lung Volume after cardiac surgery

(Acta Anaesth. Belg., 2012, 63, 115-120) Measuring End Expiratory Lung Volume after cardiac surgery G. MICHIELS (*), V. MARCHAL (**), D. LEDOUX (*) and P. DAMAS (*) Abstract. Background : The aim of this study was to evaluate the interest of end expiratory volume (EELV) measurement after cardiac surgery. Methods : After stabilization, four EELV measurements were performed at one hour intervals during three hours using the nitrogen washout technique. EELV was compared to the predicted functional residual capacity (FRC) volume. The relationships between EELV and static compliance of the respiratory system, the PaO2/FiO2 ratio and the body mass index were studied. In addition, a recruitment maneuver was performed using a fixed 45 cm H 2 O pressure control ventilation during 2 minutes between the second and third EELV measurement in half of the patients. Forty one patients were enrolled and 21 of them received the recruitment maneuver (RM). Results : Measured EELV corresponded to 52% of the predicted value. It remained stable during the whole study period. EELV correlated well with the PaO 2 /FiO 2 ratio (r² = 0.40, p < 0.0001) and with compliance of the respiratory system (r² = 0.525, p < 0.0001). EELV was inversely correlated to the body mass index (r² = 0.165, p = 0.008). RM did not significantly improve EELV. Conclusions : EELV is profoundly reduced after cardiac surgery. Measuring EELV is a new tool that is now available during mechanical ventilation. It seems to bring some new robust and possibly useful information. Recruitment maneuvers using sighs does not modify this volume. Key words : Functional residual capacity ; End Expiratory Lung Volume ; mechanical ventilation ; ICU-patient. INTRODUCTION Ventilation capacity and functional residual capacity (FRC) are significantly reduced during the postoperative period of cardiac surgery (1, 2). This restrictive syndrome may be associated with several complications such as postoperative respiratory failure, nosocomial pneumonia, and prolonged ventilatory support. However, until recently, it was not possible to directly measure the FRC during mechanical ventilation. Today, the commercially available Engström Carestation system allows estimating FRC using a nitrogen washout technique. In ventilated patients, however, FRC is influenced by the positive end-expiratory pressure (PEEP) level. For that reason, the expression End Expiratory Lung Volume (EELV) is preferred to FRC. A comparison between the nitrogen washout/washin EELV measurement and volumes estimated by the helium dilution technique or ct-scan has shown good correlation when patients are stable and receive low PEEP levels (3). The aim of this study was to measure EELV in cardiac surgery patients during the postoperative period in order to document the restrictive syndrome and to criticize the consistency of this measurement. The hypothesis of the study was that the Engström Carestation System can confirm the usefulness of a recruitment maneuver. METHODS Following approval by the Ethics Committee of the Medical Faculty of the University of Liege and written informed consent, 41 patients were studied for three hours after cardiac surgery. Unstable hemodynamic conditions necessitating a rapid change in vasoactive drug dosages, post - operative hemorrhage, cardiac ischemia events or arrhythmia were exclusion criteria. All patients underwent a manual lung recruitment maneuver in the operating room at the end of surgery, using a 20 cm H 2 O end expiratory pressure. In the intensive care unit, all patients were sedated with propofol and remifentanyl in order to achieve a Ramsay score of 4 to 5. No muscle relaxants were administrated. Any inspiratory effort during the measurements was noted. Patients were mechanically ventilated in the supine position through an oro-tracheal tube and Grégoire MICHIELS ; Vanessa MARCHAL ; Didier LEDOUX ; Pierre DAMAS. (*) Department of General Intensive Care, University Hospital Centre of Liege, 4000 Liege, Belgium. (**) Cardiovascular Centre of Liege and Huy, 4500 Huy, Belgium. Correspondence address : Grégoire Michiels, Department of General Intensive Care, University Hospital Centre of Liege, domaine universitaire du Sart-Tilman, 4000 Liege, Belgium. Tel. : 32 4 366 74 95. Fax : 32 4 366 88 98. E-mail : gregoire_michiels@msn.com

116 G. MICHIELS et al. using an Engström Carestation (Datex, General Electric, Finland). The ventilation mode was a volume control mode, the inspired fraction oxygen (FiO 2 ) was 0.5, tidal volume ranged between 6 and 8 ml/kg, and the inspiratory/expiratory (I/E) ratio was 1:2 The PEEP level was set at 5 cm H 2 O, and the respiratory rate varied between 12 and 20/min. Ventilatory parameters were adjusted to obtain a PaCO 2 of 40 mm Hg. EELV was measured using an automated procedure available on the ventilator and based on the nitrogen washout method with FiO 2 step changes of 0.1, as previously described by OLEGARD et al. (4). Airway pressure values were measured by the ventilator at the level of the Y piece, immediately proximal to the oro-tracheal tube. Baseline FRC was estimated according to the age, height and sex using the following equation : for men, FRC = 2.34 height (in meter) + 0.009 age (in year) - 1.09 and for women FRC = 2.24 height + 0.001 age 1.00 (5). The inspiratory and expiratory plateau airway pressure were obtained after a 5 sec inspiratory and expiratory pause, allowing calculation of the static compliance of the respiratory system (Crs). Crs was calculated using the following equation : Crs = VT / (Pplat-PEEPtot). Blood gases were measured using Rapidlab 865 from Bayer. All parameters were measured every hour during 3 hours, with first measurement (H1) occurring within 15 minutes following arrival in the intensive care unit and stabilisation. To assess the consistency of the results, the relationship between measured EELV values and time, baseline FRC, or body weight were assessed. In addition, the relationship between EELV and PaO 2 /FiO 2, or Crs was tested. In so far as a decrease in EELV was expected, half of the patients were randomly assigned to undergo a standard recruitment maneuver (RM) described by GATTINONI et al. (6). Between the second (H2) and the third EELV measurement (H3), patients underwent pressure-controlled ventilation for 2 minutes, at a 45 cm H 2 O inspiratory plateau pressure, a 5 cm H 2 O PEEP, a 10 breaths/min respiratory rate, and a 1:1 inspiration-expiration ratio. After RM, the PEEP level was maintained at 5 cm H 2 O. PaO 2 /FiO 2 was again measured at H3 in the group of patients with RM. STATISTICS Normally distributed data are presented as mean ± standard deviation. Statistical comparisons over time were performed using a two way analysis of variance (ANOVA) for repeated measurements, followed by Scheffe's multiple comparisons test. The P threshold for statistical significance was set at 0.05. Correlations between EELV, static compliance, PaO 2 and body mass index (BMI) were evaluated using a Pearson's linear correlation test. RESULTS Characteristics and type of cardiac surgery are summarized in table 1 for the entire group of patients. All patients were uneventfully extubated within 6 to 9 hours after surgery. Mean EELV measured on ICU admission in the 41 patients was 1761 +/- 725 ml. This was significantly lower than the value of FRC predicted by the equations. It corresponded to 52.6 +/- 21.1% of the predicted value. However, no correlation could be observed between EELV and predicted FRC (r² = 0.0034). There was a good correlation between EELV and PaO 2 /FiO 2 ratio (Fig. 1) (r² = 0.37, P < 0.0001) and between EELV and Crs (Fig. 2) (r² = 0.525, P < 0.0001) on admission. An inverse correlation could also be observed between EELV and BMI (Fig. 3) (r² = 0.165, P = 0.008). These correlations were the same or even better when EELV was expressed as a percentage of the FRC predicted value and remained stable. Following, RM there was a slight but not significant increase of EELV. In other words EELV remained stable as a function of time in the 2 groups. Regarding PaO2/FiO2 after RM, the mean change was mild, with high variability among patients (standard deviation of 152 mmhg). The correlation between the change in FRC and the change in PaO 2 /FiO 2 after RM was poor, the slope of the regression line being statistically different from zero (p = 0.04), although this significance was due to only one patient (Fig. 5). DISCUSSION This study confirms that pulmonary function is impaired after cardiac surgery. In particular, EELV appears to be reduced to almost fifty percent of the baseline value. The baseline value was not measured but assessed by a validated equation depending on age and height (5). This is in perfect accordance with other studies (2) after cardiac surgery. This reduction is situated between the EELV measured in ventilated patients with normal lungs

END EXPIRATORY VOLUME AFTER CARDIAC SURGERY 117 Table 1 Characteristics of patients Number 41 Age (year) 65 (61-74) Male sex n (%) 32 (78 %) Weigth (kg) mean ± SD 75.7 +/- 16.7 Heigth (cm) mean ± SD 170.4 +/- 8 BMI (kg/m²) mean ± SD 25.6 +/- 4.5 Comorbidities Smoker n (%) 20 (48.8 %) COPD n (%) 11 (26.8 %) Diabetes n (%) 9 (21.9 %) NYHA>III n (%) 3 (7.3 %) Surgical procedures CABG n (%) 19 (46.3 %) Valve surgery n (%) 17 (41.4 %) Combined 5 (12.2 %) Postoperative ventilatory support (hours) median (IQR) 7 (6-10) ICU length of stay (days) median (IQR) 2 (2-3) In hospital length of stay (days) median (IQR) 10 (9-12) In hospital mortality n (%) 1 (2.4%) BMI : body mass index. COPD : chronic obstructive pulmonary disease. NYHA : New York heart association. CABG : corobary artery bypass grafting. ICU : intensive care unit. and EELV measured in patients with primary lung disorder or secondary lung disorder. In these patients, as evidenced by BIKKER et al. in a very recent study using the same technique, EELV corresponds to 66%, 42% and 34% of the predicted FRC, respectively (7). Whatever the mechanisms proposed to explain this reduction and leading to a true postoperative restrictive syndrome, efforts are made to minimize this alteration (2, 8). Recruitment maneuvers are currently performed at the end of anesthesia in order to improve pulmonary function. This was the case for all 41 patients from our study, who were manually ventilated with a PEEP level fixed at 20 cm H 2 O after the end of circulatory bypass and before leaving the operating room. Despite this procedure, EELV measured after stabilization in the ICU was still profoundly reduced. Interestingly, a new RM performed in the ICU was not able to reverse this restrictive syndrome either. This may indicate that the problem is due to extrapulmonary changes, namely chest wall modifications, supine position, or weight of the abdominal content, but not to postoperative atelectasis. The RM was the one described by GATTINONI et al. (6), in which the PEEP level was not changed. We did not test the effect of increasing PEEP, to limit the risk of hemodynamic effects of high PEEP in the postoperative period, and as well as because all our cardiac patients were easily weaned from mechanical ventilation. EELV measurement was obtained using an automated procedure available on a new ventilator. The procedure is based on the nitrogen washout method with FiO 2 step changes of 0.1, as previously described by OLEGARD et al. (4). This ventilator cannot perform the measurement in spontaneously breathing patient. This is the reason why we could not measure EELV before surgery. The ventilator therefore offers the opportunity to directly measure a parameter of pulmonary function, which is the EELV. The obtained measurements are quite robust for at least three main reasons : first, the values of EELV are similar to those found in the literature after cardiac surgery ; second, the results are reproducible (measurements obtained at H2, and H4 were the same as at H1) ; and third, EELV well correlated with other parameters such as compliance, PaO 2 /FiO 2 ratio and BMI, as shown by Fig. 1-3. The accuracy and reproducibility of the obtained measurements has also been recently demonstrated by DELLAMONICA et al. in acute respiratory distress syndrome patients (9). Noteworthy, the same group reported the ability of the technique to adequately measure the alveolar recruitment induced by PEEP in those patients (10). In our study, there was no correlation between EELV and predicted FRC. This suggests that either the effect of cardiac surgery on EELV is not uniformly distributed throughout the group of patients, or that the value of EELV is questionable. It is not surprising that this correlation was

118 G. MICHIELS et al. Fig. 1. Correlation between EELV and PaO 2/FiO 2 obtained at H1. EELV : End expiratory lung volume Fig. 2. Correlation between EELV and compliance of the respiratory system. EELV : End Expiratory Lung Volume not shown, in so far as patients were in the supine position and heavily sedated. Indeed, the common equation used to predict FRC does not take account of patient weight. This could profoundly affect EELV, as asserted by the good inverse correlation between EELV and BMI (Fig. 3). In addition, and from an individual point of view, changes in PaO 2 /FiO 2 as a function of changes in EELV (Fig. 5) followed the same relationship as the one observed with baseline values (Fig. 1). This last correlation was weak, and may raise question about the exact impact of this measurement on the best ventilatory set up. It would be worth using this new tool in case of ARDS patients (11), in order to predict whether lung recruitment maneuver would lead to any useful change in therapy (12). Further studies are

END EXPIRATORY VOLUME AFTER CARDIAC SURGERY 119 Fig. 3. Correlation between BMI and EELV at H1. BMI : Body Mass Index, EELV : End Expiratory Lung Volume. H1 : data points recorded at H1. needed to determine the usefulness of this device in a strategy aimed at improving ventilation in a variety of clinical situations. CONCLUSION Fig. 4. Evolution of EELV in both recruited and not recruited patients. EELV : End Expiratory Lung Volume, RM Recruitment Maneuver. Data are given as median, interquartile range. EELV is profoundly reduced after cardiac surgery. This new tool is available during mechanical ventilation, and may provide some new useful information. Fig. 5. Correlation between delta EELV and delta PaO 2/FiO 2. EELV : End Expiratory Lung Volume, PaO 2/FiO 2 : oxygen partial arterial pressure / oxygen inspiration fraction.

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