Impact of PEEP on V/Q mismatch according to patient position and recruitability: a clinical prospective study

High positive end-expiratory pressure (PEEP) may improve ventilation-perfusion (V/Q) mismatch in acute respiratory distress syndrome (ARDS) patients with high recruitability in supine position. However, impacts of PEEP on V/Q mismatch in prone position relative to supine position remain poorly understood. We aimed to compare PEEP effects between positions, and evaluate its relationship with lung recruitability. A total of 50 patients with moderate-to-severe ARDS were consecutively enrolled. Recruitment-to-inflation (R/I) ratio was used to measure baseline lung recruitability. V/Q mismatch, gas exchange, and respiratory mechanics were evaluated at PEEP 5 cm H 2 O and PEEP 15 cm H 2 O in supine position, followed by PEEP 5 cm H 2 O and PEEP 15 cm H 2 O in prone position. Median R/I ratio was 0.60 [0.38, 0.72], separating patients with higher and lower R/I ratio. In patients with higher R/I ratio, PEEP 15 cm H 2 O (compared to PEEP 5 cm H 2 O) reduced shunt without significantly increasing dead space in both supine and prone positions, thereby improving V/Q mismatch (supine: 30.7 [28.6, 36.8]% at PEEP 15 cm H 2 O vs.38.0 [34.9, 45.4]% at PEEP 5 cm H 2 O, P < 0.001; prone: 25.7 [21.4, 30.4]% at PEEP 15 cm H 2 O vs. 32.8 [27.5, 36.7]% at PEEP 5 cm H 2 O, P < 0.001). However, in patients with lower R/I ratio, PEEP 15 cm H 2 O (compared to PEEP 5 cm H 2 O) in supine position did not improve V/Q mismatch (37.8 [34.7, 42.1]% at PEEP 15 cm H 2 O vs. 41.3 [34.8, 45.4]% at PEEP 5 cm H 2 O, P = 0.078), as the reduction in shunt was accompanied by a significant increase in dead space; in prone position, PEEP 15 cm H 2 O significantly worsened V/Q mismatch (35.9 [28.3, 43.4]% at PEEP 15 cm H 2 O vs. 31.7 [24.3, 37.6]% at PEEP 5 cm H 2 O, P < 0.001), as it failed to reduce shunt while significantly increasing dead space. In patients with higher R/I ratio, PEEP 15 cm H 2 O improved V/Q mismatch in both supine and positions. In patients with lower R/I ratio, PEEP 15 cm H 2 O did not impact V/Q mismatch in supine position but significantly worsened it in prone position.