The effect of intermittent positive pressure ventilation upon the calibre of cerebral arteries in spasm following subarachnoid haemorrhage--a preliminary communication

Role of large arteries in regulation of cerebral blood flow in dogs

Previous studies have demonstrated a significant pressure gradient from carotid artery to pial or middle cerebral arteries. This pressure gradient suggests that large cerebral arteries contribute to cerebral resistance. We have tested the hypothesis that large cerebral arteries contribute to regulation of cerebral blood flow during changes in blood gases and arterial pressure. Microspheres were used to measure brain blood flow in anesthetized dogs. Resistance of large cerebral arteries was estimated by determining the pressure gradient between common carotid and wedged vertebral artery catheters. Systemic hypercapnia and hypoxia dilated large cerebral arteries, and hypocapnia constricted large cerebral arteries. Resistance of large arteries was 0.6+/-0.1 (mean +/- SE) mm Hg per ml/min per 100 g during normocapnia. During hypercapnia and hypoxia, large artery resistance decreased significantly to 0.2 +/- 0.03 and 0.3 +/- 0.05, respectively. During hypocapnia large artery resistance increased significantly to 1.0 +/- 0.1. In other experiments, we found that large cerebral arteries participate in auto-regulatory responses to hemorrhagic hypotension. When arterial pressure was reduced from 110 to 58 mm Hg, autoregulation maintained cerebral blood flow constant, and resistance of large cerebral arteries decreased significantly from 1.0 +/- 0.2 to 0.6 +/- 0.1 mm Hg per ml/min per 100 g. In absolute terms, we calculated that 20-45% of the change in total cerebral resistance during these interventions was accounted for by changes in large artery resistance. These studies indicate that large cerebral arteries, as well as arterioles, participate actively in regulation of cerebral blood flow during changes in arterial blood gases and during autoregulatory responses to hemorrhagic hypotension.

Vasospasm in response to repeated subarachnoid hemorrhages in the monkey

✓ This study investigates the relationship between vasospasm and repeated subarachnoid hemorrhages in 18 monkeys. Sixteen received weekly 4 cc injections of autogenous blood into the subfrontal subarachnoid space. The weekly mortality rate for 4 weeks was 6%, 33%, 20%, and 37% respectively. The over-all mortality was 75%. The degree of vasospasm did not correlate with the morbidity and mortality. Vasospasm was limited to the intradural cerebral vessels and was diffuse. It never lasted longer than a few hours, late vasospasm did not occur, and the degree of vasospasm did not alter with repeated occasions of “subarachnoid hemorrhage.” Immediate electrocardiogram abnormalities were related to the height of the cerebrospinal fluid pressure rise following the subarachnoid hemorrhage (injected blood). Pathological examination of the vessels shown to be in spasm was normal. The study suggests that the increased mortality associated with repeated subarachnoid hemorrhage is due to cumulative structural damage rather than a heightened vasospastic response to repeated hemorrhages.