Although positive end-expiratory pressure (PEEP) has been widely used in mechanical ventilated patients with acute respiratory distress syndrome (ARDS), how to select the "optimal" PEEP is far from consensus. The application of PEEP may result in beneficial effect by recruiting previously collapsed lung areas, harmful effect by over-distending previously aerated lung areas, or a combination of the both. The net effect of PEEP in a certain patient may depend on the recruitability. Because recruitability varies extremely in ARDS patients and strongly correlates with the response to PEEP, estimation of end-expiratory lung volume (EELV) may be essential for individualized setting of PEEP. Whether the FRC changes at different PEEP levels remains unknown.

Although positive end-expiratory pressure (PEEP) has been widely used in mechanical ventilated patients with acute respiratory distress syndrome (ARDS), how to select the "optimal" PEEP is far from consensus. The application of PEEP may result in beneficial effect by recruiting previously collapsed lung areas, harmful effect by over-distending previously aerated lung areas, or a combination of the both. The net effect of PEEP in a certain patient may depend on the recruitability. Because recruitability varies extremely in ARDS patients and strongly correlates with the response to PEEP, estimation of end-expiratory lung volume (EELV) may be essential for individualized setting of PEEP.

Passive spirometry has long been used to measure the lung recruitment volume (VREC). A prolonged expiration to zero end-expiratory pressure (ZEEP) or airway release maneuver is required and PEEP induced lung volume change above functional residual capacity (FRC) is measured. This technique assumes that FRC does not change at different PEEP levels.

This assumption that PEEP has no effect on FRC can date back to the study of Valta et al in the early 1990s. Using respiratory inductive plethysmography (RIP), they found that in ALI/ARDS patients, after expiring from different PEEP levels to ZEEP, the plethysmography signal returned to the same baseline value. They concluded that FRC does not change with PEEP, and that changes of EELV are attributable only to change in ∆EELV. Ranieri et al arrived at similar conclusions by measuring differences in lung volumes at different PEEP levels using standardized pressure-volume (P-V) curves derived from the ventilator circuit monitors. However, Patroniti et al found an elevation of FRC as increasing of PEEP in patients with ARDS. In this study, FRC was measured with the helium dilution technique, and concluded that neglecting this effect resulted in marked underestimation of VREC. Whether the FRC changes at different PEEP levels remains controversial. The aim of the study is to assess the effect of PEEP on FRC during mechanical ventilation.

Inclusion criteria include:

1. Diagnosed with ARDS according to the Berlin Definition;
2. Age 18-80 years;
3. Ventilated with volume-controlled ventilation using constant flow;
4. Deep sedation (RASS -4 to -5) and absence of spontaneous breathing (i.e., no triggering during tidal breaths and no inspiratory effort during a 5-second end-expiratory hold).

Exclusion criteria include:

1. Evidence of active air leak from the lung, including bronchopleural fistula, pneumothorax, pneumomediastinum, or existing chest tube;
2. Chest wall and/or abdominal injuries;
3. Evidence suggesting reduced chest wall compliance, such as existing large pleural effusion, thoracic trauma and intra-abdominal hypertension (i.e., intra-abdominal pressure > 20 mmHg).
4. Presence of pacemaker, defibrillator, and implantable pumps).