Dynamic hyperinflation and intrinsic positive end-expiratory pressure in ARDS patients

In ARDS patients, changes in respiratory mechanical properties and ventilatory settings can cause incomplete lung deflation at end-expiration. Both can promote dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEP). The aim of this study was to investigate, in a large population of ARDS patients, the presence of intrinsic PEEP, possible associated factors (patients' characteristics and ventilator settings), and the effects of two different external PEEP levels on the intrinsic PEEP.

We made a secondary analysis of published data. Patients were ventilated with a tidal volume of 6-8 mL/kg of predicted body weight, sedated, and paralyzed. After a recruitment maneuver, a PEEP trial was run at 5 and 15 cmH₂O, and partitioned mechanics measurements were collected after 20 min of stabilization. Lung computed tomography scans were taken at 5 and 45 cmH₂O. Patients were classified into two groups according to whether or not they had intrinsic PEEP at the end of an expiratory pause.

We enrolled 217 sedated, paralyzed patients: 87 (40%) had intrinsic PEEP with a median of 1.1 [1.0-2.3] cmH₂O at 5 cmH₂O of PEEP. The intrinsic PEEP significantly decreased with higher PEEP (1.1 [1.0-2.3] vs 0.6 [0.0-1.0] cmH₂O; p < 0.001). The applied tidal volume was significantly lower (480 [430-540] vs 520 [445-600] mL at 5 cmH₂O of PEEP; 480 [430-540] vs 510 [430-590] mL at 15 cmH₂O) in patients with intrinsic PEEP, while the respiratory rate was significantly higher (18 [15-20] vs 15 [13-19] bpm at 5 cmH₂O of PEEP; 18 [15-20] vs 15 [13-19] bpm at 15 cmH₂O). At both PEEP levels, the total airway resistance and compliance of the respiratory system were not different in patients with and without intrinsic PEEP. The total lung gas volume and lung recruitability were also not different between patients with and without intrinsic PEEP (respectively 961 [701-1535] vs 973 [659-1433] mL and 15 [0-32] % vs 22 [0-36] %).

In sedated, paralyzed ARDS patients without a known obstructive disease, the amount of intrinsic PEEP during lung-protective ventilation is negligible and does not influence respiratory mechanical properties.