Experimental Mechanical Arterial Stimulation at the Circle of Willis

CORDING to various clinical observations, cerebral vessels may develop local spasm in reaction to operative manipulation? ,6,8 This clinical experience is supported by a number of laboratory experiments.l,2,5.7 Although mechanical stimulation is obviously only one of several factors that may cause cerebrovascular spasm at operation, it is more subject to control and modification than other factors such as intracerebral chemical irritation 4,9 or various extracerebral effects. However, the means whereby gentle mechanical stimuli cause changes in cerebral vessel size are not clearly understood. Except for the closure of an occlusive clip, the forces applied to vessels are kept to a minimum which may be considered fairly consistent when applied by the same operator during the same dissection. Nonetheless, even when the force is minimal, local spasm may occur. We wondered if perhaps the direction in which the force is applied with relation to the vessel wall actually modifies or even determines the nature of the reaction. Specifically, would there be any noticeable difference in resulting spasm if similar minimal forces were applied separately at right angles to and parallel with the long axis of the vessel wall? With this question in mind, we exposed the circle of Willis in the cat by microdissection. Then, using the micromanipulator, we applied a single gentle stroke to the vessel wall in one of two directions, either at right angles to the long axis or parallel with it. In some cases, any subsequent reactions were tested against topical application of vasoconstrictors or vasodilators. In others, the stimuli were applied after stellate ganglionectomy or cervical sympathectomy.