Cardiorespiratory effects of positive end-expiratory pressure during progressive tidal volume reduction (permissive hypercapnia) in patients with acute respiratory distress syndrome

Abstract

Background: In patients with acute respiratory distress syndrome (ARDS), the ventilatory approach is based on tidal volume (V(T)) of 10-15 ml/kg and positive end-expiratory pressure (PEEP). To avoid further pulmonary injury, decreasing V(T) and allowing PaCO2 to increase (permissive hypercapnia) has been suggested. Effects of 10 cmH2O of PEEP on respiratory mechanics, hemodynamics, and gas exchange were compared during mechanical ventilation with conventional (10-15 ml/kg) and low (5-8 ml/kg) V(T). Methods: Nine sedated and paralyzed patients were studied. V(T) was decreased gradually (50 ml every 20-30 min). Static volume-pressure (V-P) curves, hemodynamics, and gas exchange were measured. Results: During mechanical ventilation with conventional V(T), V-P curves on PEEP 0 (ZEEP) exhibited an upward convexity in six patients reflecting a progressive reduction in compliance with inflating volume, whereas PEEP resulted in a volume displacement along the flat part of this curve. After V(T) reduction, V-P curves in the same patients showed an upward concavity, reflecting progressive alveolar recruitment with inflating volume, and application of PEEP resulted in alveolar recruitment. The other three patients showed a V-P curve with an upward concavity; V(T) reduction increased this concavity, and application of PEEP induced greater alveolar recruitment than during conventional V(T). With PEEP, cardiac index decreased by, respectively, 31% during conventional V(T) and 11% during low V(T) (P < 0.01); PaO2 increased by 32% and 71% (P < 0.01), respectively, whereas right-to-left venous admixture (Q̇s/Q̇t) decreased by 11% and 40%, respectively (P < 0.01). The greatest values of PaO2, static compliance, and oxygen delivery and the lowest values of Q̇s/Q̇t (best PEEP) were obtained during application of PEEP with low V(T) (P < 0.01). Conclusions: Although PEEP induced alveolar hyperinflation in most patients during mechanical ventilation with conventional V(T), at low V(T), there appeared to be a significant alveolar collapse, and PEEP was able to expand these units, improving gas exchange and hemodynamics.