Regional vascular reactivity in the middle cerebral arterial distribution

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Abstract
✓ Regional vascular reactivity has been assessed in anesthetized baboons by comparing the arterial and venous pulse heights in the pial circulation within the middle cerebral arterial field. The ratio of venous to arterial pulse height (the “pulse index”) provided some indication of the state of the regional cerebral vascular resistance within the field of the middle cerebral artery. The pulse index increased in response to increased arterial pCO2 and to the effects of vascular occlusion, indicating vasodilatation in response to biochemical or autoregulatory stimuli. The pulse rose sharply for a short time following release of temporary vascular occlusion; this rise was used to assess the phenomenon of reactive hyperemia following occlusion. Occlusion of the same artery produced hyperemia of approximately the same extent irrespective of the length of occlusion, but longer occlusions produced longer lasting hyperemia. Vascular resistance steadily decreased to very low levels of input arterial pressure in progressive exsanguination experiments, indicating that the failure of cerebral blood flow autoregulation at low levels of arterial pressure is probably not due to exhaustion of myogenic mechanisms within the small resistance vessels themselves.