The Effect of Changes in the Pulmonary Vascular Bed Produced by Atropine, Pulmonary Engorgement, and Positive-Pressure Breathing on Diffusing and Mechanical Properties of the Lung*

Abstract

Several factors probably have some role in determining the volume of the effectively ventilated pulmonary capillary bed, including pulmonary blood flow, pulmonary vascular pressures, pulmonary blood volume, alveolar and blood gas tensions, and mechanical effects of ventilation.Others have found that atropine sulfate, 2.0 mg iv, increases cardiac output (1, 2), decreases central venous pressure (1, 2), and affects the distribution of the blood volume as shown by plethysmographic study of extremity venous volume (3). This drug, then, provides an unusual rearrangement of some of the determinants of pulmonary capillary function and, for the present study, was used either alone or in combination with positivepressure breathing or pulmonary vascular engorgement produced by G-suit inflation.This study was carried out to provide further information concerning the interaction of cardiac output, pulmonary vascular pressure, and pulmonary blood volume as determinants of the volume of effectively ventilated capillaries by de-termining the effects of changes in the pulmonary vascular bed produced by atropine, pulmonary engorgement, and positive-pressure breathing on the diffusing and mechanical properties of the lung.

The Pressure-Volume Relationship of the Normal Pulmonary Capillary Bed*

“…Acute pulmonary vascular engorgement increases the volume of the pulmonary capillary bed either by recruiting such previously inactive capillaries or by dilating ones already in use (3,4).…”

“…Changes in right atrial pressure were used as an index of changes in transmural pulmonary capillary pressure. Evidence available from studies in normal men suggests that acutely induced increases in right atrial, pulmonary arterial, and pulmonary "wedge" pressures are equivalent (4,21). Similar results have been produced during acute changes in central vascular pressures in dogs (22).…”

“…Conversely, in normal men, procedures that decrease pulmonary vascular pressure decrease DLco (4,5); yet, the pulmonary capillary bed of the lung chronically subjected to increased intravascular pressure is less able to decrease its volume in response to procedures designed to decrease pulmonary vascular pressure (6). Although this may be the consequence of structural changes in the lung, this observation suggests that the curve relating DLCO to pulmonary vascular pressure may reach a plateau, beyond which changes in pulmonary vascular pressure produce no further changes on DLCO.…”

“…In isolated lung preparations (1,2) and in normal subjects (3,4), acute pulmonary vascular congestion increases breath-holding diffusing capacity for carbon monoxide (DLco) by increasing the instantaneous volume of blood available for CO absorption (Vc). Conversely, in normal men, procedures that decrease pulmonary vascular pressure decrease DLco (4,5); yet, the pulmonary capillary bed of the lung chronically subjected to increased intravascular pressure is less able to decrease its volume in response to procedures designed to decrease pulmonary vascular pressure (6).…”

The Venous Pump in the Legs as a Determinant of Pulmonary Capillary Filling*

“…Previous studies have shown that increases in pulmonary blood flow alone do not increase DLco (1,5,6). Theoretic considerations predict that Dr0o should be relatively independent of pulmonary capillary blood flow but quite sensitive to the instantaneous volume of blood exposed to the alveolar gas (7).…”