Relation Between Lung Volume and Pulmonary Vascular Resistance

Ten experiments were conducted on anesthetized dogs ventilated with a Starling pump and breathing oxygen. While tidal volume, respiratory rate, and arterial pH and pCO2 were kept constant, lung volume was varied by using either a negative, zero, or positive end-expiratory pressure, leading to average changes in lung volume of −32, 0, and +75 per cent. Pulmonary vascular resistance (PVR) increased with either decreased or increased lung volume, indicating that the relationship between resistance and lung volume is a U-shaped curve. Since the transmural distending pressures of large pulmonary vessels either increased or remained unchanged during these procedures, changes in PVR cannot be ascribed to changes in systemic circulatory dynamics, such as cardiac output. It was noted that cardiac output increased with negative end-expiratory pressure (effectively, negative-pressure breathing) and decreased with positive end-expiratory pressures (effective, positive-pressure breathing), as previously reported. A hypothesis is presented for explaining the U-shaped curve relating resistance and lung volume.