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Pressure changes in the pulmonary artery and vein, thoracic vena cava, aorta and intrapleural space as well as the cardiac output were measured in anesthetized dogs subjected to continuous airway pressures varying between -25 to +25 cm H2O. Pulmonary and systemic resistances were calculated. During positive airway pressures the pulmonary artery, vein and vena cava pressures approximately paralleled changes in intrapleural pressures. Cardiac output and aorta pressures decreased; systemic and pulmonary flow resistances markedly increased. During negative airway pressures the pulmonary vessels and vena cava pressures decreased but no longer paralleled the changes in intrapleural pressures. It is assumed that these vessels reached their limit of distensibility. While aortic pressures decreased, cardiac output, systemic and pulmonary resistances remained essentially unchanged. Submitted on January 14, 1960

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