Lack of correlation between direct and indirect measurements of arterial pressure in unanaesthetized rats

Abstract

The tail-cuff method for measuring blood pressure in awake rats, while used as a standard procedure, is not as reliable as is often assumed. Tail-cuff measurements made daily in trained, unanaesthetized rats with chronically implanted aortic catheters did not correlate significantly with simultaneously-recorded intra-arterial pressure. On the other hand, correlation between direct and indirect measurements improved when rats were anaesthetized with ether. Constriction of the tail artery during measurement of arterial pressure could account for the lack of correlation. To examine this possibility, simultaneous measurements were made in unanaesthetized rats after intraperitoneal injection of pressor doses of noradrenaline. During the first 20 min, elevation of arterial pressure caused by injected noradrenaline was clearly evident with either method of measurement. After this, in most experiments, tail pressure either diminished, or could not be detected, while intra-arterial pressure remained high. When sympathetic vasomotor activity was increased in anaesthetized rats by bilateral carotid occlusion, similar results were obtained. These findings indicate that tail-cuff measurements are not valid in certain circumstances in unanaesthetized rats.