CARDIORESPIRATORY EFFECTS OF POSITIVE END-EXPIRATORY PRESSURE DURING PROGRESSIVE TIDAL VOLUME REDUCTION (PERMISSIVE HYPERCAPNIA) IN PATIENTS WITH ACUTE RESPIRATORY-DISTRESS SYNDROME

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 Pa-CO2 to increase (permissive hypercapnia) has been suggested. Effects of 10 cmH(2)O 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); Pa-O2 increased by 32% and 71% (P < 0.01), respectively, whereas right-to-left venous admixture (Qs/Qt) decreased by 11% and 40%, respectively (P < 0.01). The greatest values of Pa-O2, static compliance, and oxygen delivery and the lowest values of Qs/Qt (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.