End-Expiratory Lung Volume in Patients with Acute Respiratory Distress Syndrome: A Time Course Analysis.

Abstract

Lung injury can be caused by ventilation and non-physiological lung stress (transpulmonary pressure) and strain [inflated volume over functional residual capacity ratio (FRC)]. FRC is severely decreased in patients with acute respiratory distress syndrome (ARDS). End-expiratory lung volume (EELV) is FRC plus lung volume increased by the applied positive end-expiratory pressure (PEEP). Measurement using the modified nitrogen multiple breath washout technique may help titrating PEEP during ARDS and allow determining dynamic lung strain (tidal volume over EELV) in patients ventilated with PEEP. In this observational study, we measured EELV for up to seven consecutive days in patients with ARDS at different PEEP levels. Thirty sedated patients with ARDS (10 mild, 14 moderate, 6 severe) underwent decremental PEEP testing (20, 15, 10, 5cm H2O) for up to 7days after inclusion. At all PEEP levels examined, over a period of 7days the measured absolute EELVs showed no significant change over time [PEEP 20cm H2O 2464ml at day 1 vs. 2144ml at day 7 (p=0.78), PEEP 15cm H2O 2226ml vs. 1990ml (p=0.36), PEEP 10 1835ml vs. 1858ml (p=0.76) and PEEP 5cm H2O 1487ml vs. 1612ml (p=0.37)]. In relation to the predicted body weight (pbw), no significant change in EELV/kg pbw over time could be detected either at any PEEP level or over time [PEEP 20 36ml/kg pbw at day 1 vs. 33ml/kg pbw at day 7 (p=0.66); PEEP 15 33 vs. 29ml/kg pbw (p=0.32); PEEP 10 27 vs. 27ml/kg pbw (p=0.70) and PEEP 5 22 vs. 24ml/kg pbw (p=0.70)]. Oxygenation significantly improved over time from PaO2/FiO2 of 169mmHg at day 1 to 199mmHg at day 7 (p<0.01). EELV did not change significantly for up to 7days in patients with ARDS. By contrast, PaO2/FiO2 improved significantly. Bedside measurement of EELV may be a novel approach to individualise lung-protective ventilation on the basis of calculation of dynamic strain as the ratio of VT to EELV.