The effect of intracranial hypertension on cerebrovascular resistance--an experimental study.

Pulse transmission through cortex between a vein and artery in the middle cerebral distribution has been used as an index of vascular resistance in the middle cerebral field in baboons. Following catheterisation of these small vessels, the skull has been reconstituted and the effect of intracranial hypertension produced by cisternal infusion has been studied. An earlier linear relaxation of resistance in relation to rising intracranial pressure has been succeeded by a second phase in which pulse transmission increases quite profoundly despite minimal further increase in intracranial pressure. This appears to represent the exhaustion of the compliance of the vascular compartment, and corresponds closely to levels of intracranial pressure at which failure of autoregulation to increased intracranial pressure has been observed in previous experiments. Spontaneous bursts of transient increased pressure made their appearance in a number of animals at this level of pressure. The possible significance of increased pulse transmission following exhaustion of compliance in relation to breakdown of the blood brain barrier and potential spreading oedema is discussed.