Importance of transmural pressure and lung volume in evaluating drug effect on pulmonary vascular tone.

MEASUREMENT of pulmonary vascular resistance has frequently been used in the study of vasoactive drugs to indicate changes in pulmonary vascular tone.' However, as pointed out by Borst et al. 4 and Fowler, it is not possible in many of these studies to separate vasoactive drug effects from purely mechanical effects resulting from changes in transmural pressure,* and lung volume. It is the purpose of this study to separate these two effects by using a method applicable to intact animals and man. Pulmonary vascular resistance (PVR) is defined as the ratio of the mean pressure drop across the pulmonary vascular bed to the mean blood flow through the bed. PVR depends on the geometry of the vascular bed and the physical properties of the blood. The geometry of the vascular bed is related to the volume of the lung as well as to the distribution of transmural pressure along the vessels, the number of parallel vessels that are open, and the physical properties of the vessel walls." Most vasoactive drugs will affect the intravascular pressure, the blood flow, and the properties of the vessel walls. Assuming the physical properties of the blood to remain constant, it should be possible to infer the vasoactive effect of a drug on the pulmonary vasculatnre from measurement of PVR, provided the lung volume and transmural pressures are held constant.