THE EFFECT OF EXTERNAL CONSTRICTION OF A BLOOD VESSEL ON BLOOD FLOW

Abstract

The effect of an external constriction of a blood vessel in limiting blood flow has been considered with respect to the relationships of vessel bore to volume flow, and of change in external to change in internal dimensions of the vessel. Expts. with an artificial system and in animals have led to the conclusions that: The effect of a localized reduction in lumen area is primarily that of increasing the fluid friction (viscosity effect) at the site of the constriction, which results in an added ''''peripheral resistance" to the flow of blood and the rate of flow is thereby reduced. The extent of flow reduction will vary in direct relation to the axial length of the constricted area, the velocity of flow and the viscosity of the blood, and in inverse relation to the peripheral resistance of the bed and the lumen area of the vessel constriction. Since, with an intact blood vessel, it is impossible to determine all or even most of the above factors, an estimation of the flow reduction caused by a given constriction will be only as accurate as the estimated values placed upon the determining factors. Without the observer''s knowledge, marked changes in the determining factors may occur, thereby making it impossible to predict within rather wide limits either the immediate or subsequent effects of a known constriction. The findings presented here reveal no justification for the contention that a rather marked degree of external constriction is required to produce a significant reduction in flow through a vessel. In comparison with other external constricting devices, the thermostromuhr cannot be regarded as having any less variable or unpredictable effect in limiting the rate of flow through the vessel to which it is applied.