Hemodynamics of carotid back pressure and cerebral flow during endarterectomy

Abstract

This study analyzes the hemodynamics of cerebral hemisphere blood flow by application of first principles of fluid mechanics, with the specific objective of quantitating the relationship between ipsilateral collateral cerebral blood flow and carotid back pressure. The variables arise from the fluid mechanics equations as nondimensional, normalized ratios: the ratio of collateral cerebral flow to normal flow and the ratio of carotid back pressure to mean arterial pressure (carotid back pressure index). The results show that the relationship between these two variables depends on two things: the cerebral venous pressure and the cerebral vascular reserve. The predicted safe lower limit of the carotid back pressure index for carotid operations without the use of an external shunt is 0.32 to 0.39 for patients with normal cerebral vascular reserve and venous pressure and 0.64 to 0.68 in patients with no vascular reserve. When the known effects of PCO 2 and arterial blood pressure on the cerebral circulation are combined with the theoretical results of this study, it is shown that hypocarbia and mild systemic hypertension increase collateral blood flow at any carotid back pressure index during carotid occlusion. Conversely, hypercarbia and hypotension are predicted to decrease flow.