THE EFFECT OF EXTERNAL PRESSURE ON THE VASCULAR VOLUME OF THE FOREARM AND ITS RELATION TO CAPILLARY BLOOD PRESSURE AND VENOUS PRESSURE

Abstract

While the direct measurement of capillary blood pressure in man by means of a micro-pipette (1, 2, 3) is quite accurate, this method has several disadvantages which have prevented wide appli-cation to clinical problems. They are (a) the great variability of readings in single capillaries, which makes it practically impossible to obtain a statistically significant number of readings in a given patient at any one time in his illness; (b) the restriction of this method to the capillaries of the skin of the nail fold where the vessels may not be typical of those in muscle or general con-nective tissue; and (c) the nearness of arterio-venous anastomoses which, by opening and clos-ing, may disturb pressures in the nearby capil-laries of the nail fold. The indirect methods are easily applied to clini-cal problems but, as pointed out previously (4), lead to widely different estimates of blood pres-sure in the capillaries or minute vessels as a whole, as might be expected from the subjective and arbitrary nature of the criteria adopted by each investigator to indicate a state of balance between the externally applied pressure and capil-lary blood pressure. Eichna and Bordley (3) have compared the direct and indirect procedures; they concluded that the usual capsular method, with microscopic observation of the collapse of individual capillaries, was wholly undependable. Fundamentally, nearly all indirect methods for measuring blood pressure in the minute vessels involve the application of external pressure to diminish the volume of one or more capillaries, using visual evidence such as cessation of flow, modification of flow, or changes in skin color to indicate when the applied pressure is great enough to obliterate or at least flatten, the capillaries under observation. It seems possible, however, 1 Commonwealth Fund Fellow, 1940-41. that the average pressure in the minute vessels of human skin and muscle might be estimated plethys-mographically by determining the effect of graded external pressure on the collective vascular volume of the forearm. The plethysmograph, by adding together the infinitesimal changes in volume of many individual vessels, might then provide an average and more objective measure of blood pres-sure in the capillaries, or at least in the minute vessels as a whole. This paper describes a series of observations designed to determine (a) the effect of graded external pressure on the volume of blood in the human forearm; (b) the relation of these pres-sure-volume curves to the previously reported di-rect readings of capillary blood pressure and venous pressure under like conditions; and (c) the effect of known changes of capillary blood pressure on the pressure-volume curves, for the purpose of assessing the validity and usefulness of this procedure as a method of estimating blood pressure in the minute vessels of man during dis-ease. APPARATUS