IMPACT OF PLEUROTOMY, CONTINUOUS POSITIVE AIRWAY PRESSURE, AND FLUID BALANCE DURING CARDIOPULMONARY BYPASS ON LUNG-MECHANICS AND OXYGENATION

Objective: To determine effects of surgical pleurotomy, continuous pos itive airway pressure, and fluid balance during cardiopulmonary bypass (CPB) on lung mechanical properties and indices of oxygenation. Desig n: Prospective, descriptive. and interventional study. Setting: Cardio thoracic service at a major university referral center. Participants: Eighteen anesthetized-paralyzed patients undergoing elective coronary artery bypass grafting requiring CPB. Interventions: During CPB, conti nuous positive airway pressure (CPAP) was applied to nine patients; in nine others, no CPAP was applied. Measurements and Main Results: From measurements of airway and esophageal pressures and flow, lung resist ance and elastance were determined before sternotomy and after sternal reapproximation. Measurements were made during forced ventilation ove r a physiologic range of tidal volumes and frequencies, and frequency and volume dependences of lung resistance and elastance were additiona lly identified. in all patients. lung resistance and elastance increas ed after CPB, consistent with models of pulmonary edema. Multiple regr ession analysis showed that these increases were relatively less in pa tients with intact pleurae (p < 0.05) or net negative fluid balance (p < 0.05); however, no difference in these increases was noted between patients receiving CPAP and those receiving no CPAP. Increases in lung resistance were positively correlated to net fluid balance, and negat ively correlated to frequency and tidal volume (p < 0.05). Increases i n lung elastance were positively correlated to body mass index and fre quency, and negatively correlated to tidal volume (p < 0.05). Absolute change in alveolar-arterial oxygen gradient was negatively correlated with net fluid balance, whereas percentage change was positively corr elated to changes in lung elastance (p < 0.05). Conclusions: These fin dings suggest that pleurotomy before CPB and positive fluid balance du ring CPB enhance postbypass pulmonary edema and/or atelectasis. as dem onstrated by acute changes in respiratory mechanics and indices of oxy genation. Low levels of CPAP applied during CPB did not significantly change either mechanical properties or oxygenation. Copyright (C) 1996 by W.B. Saunders Company