Relation between lung volume and pulmonary vascular resistance

SEVERAL studies have suggested the pos-sibility of a causal relationship between changes in lung volume (LV) and changes in pulmonarj ' vascular resistance (PVR).1"0 In general, data indicate that an increase in LV above normal results in increased PVR.1 On the other hand, there is relatively little information on the effect of decreasing LV below normal. Cloetta was the first to suggest that decreased LV might also result in ele-vated PVR, giving a "U-shaped " curve re-lating resistance to LV, but his data were not conclusive.2 Burton and Patel recently presented data indicating that this relation-ship held for the saline-perfused lung of the open-chest rabbit.3 Simmons and Heming-way1 observed markedly increased PVR fol-lowing induction of pneumothorax in closed-chest dogs, also consistent with Cloetta's hypothesis. Because the effect of both increased and decreased LV on PVR had never been investi-gated simultaneously in the intact animal, the present study was undertaken. Methods Ton experiments were conducted on mongrel dogs (8.6 to 27.4 Kg.) anesthetized with 25 mg./Kg. of sodium pentobarbital intravenously, with small additional doses as needed. Normal ventilation was maintained with a Starling res-piration pump (positive-pressure breathing), small doses of intravenous succinylcholine chloride being given for muscle relaxation. The inspired gas was pure oxygen to prevent hypoxia and its effects on cardiac output and pulmonary vascula