The Effect of PEEP on Pulmonary Vascular Resistance Depends on Lung Recruitability in ARDS Patients.

Rationale . A U-shape relationship should exist between lung volume and pulmonary vascular resistance (PVR), with minimal PVR at functional residual capacity. Thus, positive end-expiratory pressure (PEEP) in patients with acute respiratory distress syndrome (ARDS) should increase PVR if it induces significant lung distension compared to recruitment. However, this has never been proven in patients. Objectives . To study the effects of PEEP on PVR according to lung recruitability, evaluated by the recruitment-to-inflation (R/I) ratio. Methods . In patients with ARDS, we measured hemodynamic (pulmonary artery catheter), echocardiographic and ventilatory variables (including esophageal pressure), at both low PEEP and higher PEEP by 10 cmH 2 O. Preload responsiveness was assessed by the passive leg raising test at high PEEP. Measurements and Main Results . We enrolled 23 patients, including 10 low recruiters (R/I <0.5) and 13 high recruiters (R/I ≥0.5). Raising PEEP from 4 (2-5) to 14 (12-15) cmH 2 O increased PVR in low recruiters (from 160 (120-297) to 243 (166-380) dyn.s/cm 5 , p<0.01), while PVR was unchanged in high recruiters (from 224 (185-289) to 235 (168-300) dyn.s/cm 5 , p=0.55). Right-to-left ventricular end-diastolic areas ratio simultaneously increased in low recruiters (from 0.54 (0.50-0.59) to 0.64 (0.56-0.70), p<0.01), while remaining stable in high recruiters (from 0.70 (0.65-0.79) to 0.68 (0.58-0.80), p=0.48). Raising PEEP decreased cardiac index only in preload responsive patients. Conclusions . PEEP increases PVR only when it induces significant lung distension compared to recruitment according to the recruitment-to-inflation ratio. Tailoring PEEP on this recruitability index should mitigate its hemodynamic effects.