Accuracy and precision of end-expiratory lung-volume measurements by automated nitrogen washout/washin technique in patients with acute respiratory distress syndrome

Abstract

IntroductionEnd-expiratory lung volume (EELV) is decreased in acute respiratory distress syndrome (ARDS), and bedside EELV measurement may help to set positive end-expiratory pressure (PEEP). Nitrogen washout/washin for EELV measurement is available at the bedside, but assessments of accuracy and precision in real-life conditions are scant. Our purpose was to (a) assess EELV measurement precision in ARDS patients at two PEEP levels (three pairs of measurements), and (b) compare the changes (Δ) induced by PEEP for total EELV with the PEEP-induced changes in lung volume above functional residual capacity measured with passive spirometry (ΔPEEP-volume). The minimal predicted increase in lung volume was calculated from compliance at low PEEP and ΔPEEP to ensure the validity of lung-volume changes.MethodsThirty-four patients with ARDS were prospectively included in five university-hospital intensive care units. ΔEELV and ΔPEEP volumes were compared between 6 and 15 cm H2O of PEEP.ResultsAfter exclusion of three patients, variability of the nitrogen technique was less than 4%, and the largest difference between measurements was 81 ± 64 ml. ΔEELV and ΔPEEP-volume were only weakly correlated (r2 = 0.47); 95% confidence interval limits, -414 to 608 ml). In four patients with the highest PEEP (≥ 16 cm H2O), ΔEELV was lower than the minimal predicted increase in lung volume, suggesting flawed measurements, possibly due to leaks. Excluding those from the analysis markedly strengthened the correlation between ΔEELV and ΔPEEP volume (r2 = 0.80).ConclusionsIn most patients, the EELV technique has good reproducibility and accuracy, even at high PEEP. At high pressures, its accuracy may be limited in case of leaks. The minimal predicted increase in lung volume may help to check for accuracy.

Recruitment Maneuvers and PEEP Titration

“…[83][84][85] A modified nitrogen washout technique is available on one commercially available ventilator. If EELV is measured on 0 PEEP and then again after PEEP is added, it is possible to calculate strain, an important determinant of lung injury.…”

Measurement of Alveolar Recruitment at the Bedside: The Beginning of a New Era in Respiratory Monitoring?

“…The procedure lasts much longer than the classical PV curve method because it requires steady state conditions (10 min at least) between the different PEEP levels, in order to ensure nitrogen washout/wash-in. 18 It has the limitations of the dynostatic and nitrogen wash-out/wash-in methods. Measurement of intratracheal pressure may be associated with errors in pressuremeasurement(end-holevsside-holecatheters,intraluminal diameter, catheter obstruction by tracheal secretions).…”

“…By excluding inspiratory resistance of the endotracheal tube, measuring intratracheal pressure makes possible the accurate measurement of dynostatic PV curves in dynamic conditions. 19,20 For routine clinical use, a 2-mm intraluminal and air-filled catheter with end-hole should be positioned in the trachea, 2 cm beyond distal tip of the endotracheal tube, 18 and left in place throughout the period of measurement. Before starting automatic measurement of PEEP-induced alveolar recruitment, the clinician selects the different PEEP levels at which alveolar recruitment will be measured (a maximum of 5) according to an ascending PEEP trial, the tidal volume, and the maximal inflation pressure (up to 100 cm H 2 O).…”

“…16 The PEEP releasing maneuver is replaced by the measurement of endexpiratory lung volume using the nitrogen wash-out/ wash-in technique. 17,18 The ventilator is equipped with a sensor specifically designed for measuring intratracheal pressure. By excluding inspiratory resistance of the endotracheal tube, measuring intratracheal pressure makes possible the accurate measurement of dynostatic PV curves in dynamic conditions.…”

Monitoring of lung function in acute respiratory distress syndrome

“…In contrast, the open circuit multiple breath nitrogen washin-washout technique that is now incorporated in one ventilator brand is quite easy to handle and gives acceptable estimates of EELV (26). However, one needs to consider that the EELV obtained is solely the gas volume in lungs and if EELV increases with increasing PEEP, it cannot be determined whether the increase in EELV is due to overdistension of open units, an increase in normal lung volume, or a recruitment of previously collapsed lung units or a combination of all.…”