Significance of common carotid back pressure measurements

Pressure in the common carotid artery distal to a clamp occluding the artery was measured and compared to the internal carotid back pressure. In most patients the difference between the two pressures was trivial (mean 1.45 mm Hg, SD 10.45 mm Hg), but in 25.6% the pressure difference exceeded 10 mm Hg. In 2.6% the difference was greater than 30 mm Hg. Flow in the internal carotid artery reversed during common carotid clamping in 10.3% of the studies; the maximal rate of reversed internal carotid flow recorded was 62 ml/min. Significant retrograde maximal flow could be anticipated when the carotid bifurcation was relatively free of stenosis and the internal carotid back pressure exceeded the common carotid back pressure by 20 mm Hg or more. Reversal of internal carotid flow during common carotid clamping only occurred when the internal back pressure was 50 mm Hg or greater, suggesting that retrograde internal carotid flow is a manifestation of high intracranial collateral blood pressure and not a cause of cerebral ischemia. The results have application to carotid and other extracranial arterial reconstructions and to ocular plethysmographic assessment of colateral hemispheric pressure.