Gas exchange and hemodynamics in experimental pleural effusion

Objective: To investigate the cardiorespiratory effects of graded bilateral pleural effusions in the anesthetized pig. Design: Prospective, randomized, controlled, laboratory study. Setting: Animal laboratory. Subjects: Eleven male Yorkshire pigs. interventions: Animals were anesthetized using inhaled isoflurane. Orotrach eal intubation was followed by mechanical ventilation. Bilateral chest tube s were inserted, and graded increasing pleural effusions were created using saline of 0, 20, 40, and 80 mL/kg, divided equally between each side. At e ach pleural volume, intravascular volume was randomly altered (by phlebotom y or transfusion of colloid) to normal (unchanged), low (decreased by 10 mL /kg), or high (increased by 10 mL/kg). Measurements and Main Results: Hemodynamic parameters, intrapleural pressur es, hemoglobin, and blood gases were measured. At the lowest volume of pleu ral fluid, Pao(2) was reduced by approximately 50% vs. baseline, whereas sy stemic hemodynamics were unchanged. Pao(2) was reduced in a dose dependent fashion as pleural volume increased but was not affected by alterations in intravascular volume. Intrapulmonary shunt was increased both by intrapleur al volume in a dose-dependent fashion and by increases in intravascular vol ume at high levels of pleural volume. Cardiac output and systemic mean arte rial pressure increased with elevated intravascular volume but were not inf luenced by lower levels of intrapleural volume. Mean pulmonary arterial pre ssure, central venous pressure, and pulmonary artery occlusion pressure wer e increased by elevations in both intrapleural volume and intravascular vol ume. Intrapleural pressure and pulmonary vascular resistance were related t o intrapleural volume only. Conclusions: Hypoxemia occurs as an early event in acute bilateral pleural effusions and precedes hemodynamic decompensation. Oxygenation is independe nt of intravascular filling pressures, but hemodynamics are preserved with elevated filling pressures. Clinical studies should be undertaken to examin e the risks/benefits of careful removal of pleural fluid in patients with p leural effusions, when oxygenation is impaired during mechanical ventilatio n.