Pressure-flow relations in canine collateral-dependent cerebrum.

BACKGROUND AND PURPOSE The pressure-flow relation has been characterized in normal cerebrum, but it has not been studied in cerebrum dependent on collateral flow. The purpose of this investigation was to examine the relation between systemic pressure, regional cerebral blood flow, and middle cerebral artery branch pressure in collateral-dependent and normal brain. METHODS In 10 anesthetized dogs, we cannulated a branch of the middle cerebral artery and identified collateral-dependent tissues using the shadow flow technique. We measured pressure in normal and cannulated branches of the middle cerebral artery and regional cerebral blood flow. The common carotid arteries were occluded, and hemorrhage reduced mean arterial pressure to 50 and 25 mm Hg. RESULTS When aortic pressure was 25 mm Hg, blood flow to the collateral-dependent zone decreased from a baseline of 87 +/- 5 to 6 +/- 1 ml/100 g per minute (mean +/- SE), and pressure in the cannulated branch of the middle cerebral artery decreased from 49 +/- 4 to 2 +/- 0.4 mm Hg (p less than 0.05). Small-vessel resistance in the collateral-dependent region decreased from 0.56 +/- 0.09 to 0.38 +/- 0.04 mm Hg/ml/100 g per minute (p less than 0.05) after carotid occlusion but did not change significantly during hypotension. In normal brain, however, small-vessel resistance continued to decrease as systemic pressure was reduced. CONCLUSIONS This study provides pressure-flow relations in normal and collateral-dependent cerebrum during carotid occlusion and progressive hypotension. In collateral-dependent tissue, concurrent occlusion of middle cerebral and carotid arteries produces autoregulatory dilatation of small vessels, resistance fails to decrease as mean arterial pressure is reduced to 25 mm Hg, and profound focal hypoperfusion is produced.