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 pulm onary injury, decreasing V-T and allowing Pa-CO2 to increase (permissi ve hypercapnia) has been suggested. Effects of 10 cmH(2)O of PEEP on r espiratory mechanics, hemodynamics, and gas exchange were compared dur ing mechanical ventilation with conventional (10-15 ml/kg) and low (5- 8 ml/kg) V-T. Methods: Nine sedated and paralyzed patients were studie d. V-T was decreased gradually (50 ml every 20-30 min). Static volume- pressure (V-P) curves, hemodynamics, and gas exchange were measured. R esults: During mechanical ventilation with conventional V-T, V-P curve s on PEEP 0 (ZEEP) exhibited an upward convexity in six patients refle cting a progressive reduction in compliance with inflating volume, whe reas PEEP resulted in a volume displacement along the flat part of thi s curve. After V-T reduction, V-P curves in the same patients showed a n upward concavity, reflecting progressive alveolar recruitment with i nflating volume, and application of PEEP resulted in alveolar recruitm ent. The other three patients showed a V-P curve with an upward concav ity; V-T reduction increased this concavity, and application of PEEP i nduced greater alveolar recruitment than during conventional V-T. With PEEP, cardiac index decreased by, respectively, 31% during convention al V-T and 11% during low V-T (P < 0.01); Pa-O2 increased by 32% and 7 1% (P < 0.01), respectively, whereas right-to-left venous admixture (Q s/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 h yperinflation in most patients during mechanical ventilation with conv entional V-T, at low V-T, there appeared to be a significant alveolar collapse, and PEEP was able to expand these units, improving gas excha nge and hemodynamics.