Hemodynamic consequences of carotid-carotid bypass for innominate artery stenosis. The carotid-carotid cervical bypass is one surgical option for symptomatic atherosclerotic lesions of the innominate artery. Controversy exists regarding the necessity of surgically excluding the innominate plaque from the cerebral circuit. A canine study was instituted to characterize the hemodynamic alterations that occur in the right common carotid artery proximal to the bypass graft, termed the critical segment. The direction of flow in the critical segment determines whether emboli originating in the innominate may be propelled cranially despite a patent bypass graft. Six mongrel dogs underwent placement of an autogenous arterial crossover graft as a carotid-carotid bypass. A stenosis of the innominate artery was quantitatively altered, and an electromagnetic flowmeter measured the magnitude and direction of flow in the critical segment at three levels of diameter reduction in the innominate artery. For low-grade stenoses, flow in the critical segment was always prograde. For high-grade stenoses, the flow was always reversed. Stenoses between 57% and 67% yielded flow values of 10 +/- 24 ml/min, and it was in this range that mean flow reversal was found to occur. Even when the mean flow was near zero in the critical segment, flow was not stagnant but oscillated in antegrade and retrograde directions throughout the cardiac cycle. These data indicate that a carotid-carotid bypass causes complete flow reversal in the critical segment when there is high-grade stenosis in the innominate artery. Theoretical analysis of the hemodynamic circuit indicated that arm exercise would augment retrograde flow in the critical segment. 