The influence of positive end-expiratory pressure on pulmonary blood volume changes in adult respiratory distress syndrome

Abstract

Pathophysiologic changes associated with the Adult Respiratory Distress Syndrome (ARDS), such as the presence of pulmonary vascular occlusions, decreased vascular compressibility, or inceased intrapulmonary shunting, could limit the effect of positive end-expiratory pressure (PEEP) upon pulmonary blood volume (PBV). Accordingly, we determined the effect of 5 and 15 cm H 2O PEEP upon the PBV changes of normal volunteers (n = 9) and patients with moderate or severe ARDS associated with acute pulmonary hypertension (n = 10). Changes of PBV were estimated from equilibrium blood pool scans using Technetium-99m-labeled erythrocytes. The change of PBV induced by PEEP was assessed by measuring the count density over a region of the left lung during 0, 5, and 15 cm H 2O continuous positive airway pressure in the volunteers and during mechanic ventilation with 5 and 15 cm H 2O PEEP in the ARDS patients. Biventricular ejection fractions using gated blood pool angiocardiography and central hemodynamics were also measured in the ARDS patients. In volunteers, 5 and 15 cm H 2O continuous positive airway pressure decreased pulmonary activity by 10% ± 4% and 24% ± 9%, respectively (mean ± SD, P = .0001). In ARDS patients, PBV appeared to be unaffected by decreasing PEEP from 15 to 5 cm H 2O, despite an increased stroke volume, biventricular end-diastolic volume, and venous admixture. Pulmonary vascular resistance and right and left ventricular ejection fraction were unchanged. The ability of PEEP to reduce PBV appears to be decreased during acute lung injury.