Cerebral cross-perfusion and the Circle of Willis: does physiology trump anatomy?

Abstract

Introduction: Cerebral cross-perfusion is essential for ipsilateral brain viability during unilateral insult. Aortic arch and great vessel procedures depend on its function for safe practice, unless adjuncts like shunts are used. This paper assesses the contribution of cerebrovascular anatomy against physiology in determining requisite hemispheric perfusion during carotid endarterectomy (CEA). Materials and methods: A review of shunting requirements for CEAs under locoregional anesthesia (LA) at the John Radcliffe Hospital during 1999-2013 was performed. A PubMed search for "Circle of Willis" was screened for all original articles defining cerebrovascular anatomy by postmortem or angiography. Results: Over 14 years, 1137 CEAs were performed under cervical plexus block; during this period, the departmental standard of practice evolved from exclusively general anesthesia to predominantly LA. CEAs performed under LA during the early phase (1999-2003) had a shunt rate of 15.1%, compared to 20% as predicted by stump pressures alone. However, shunting decreased as higher perioperative systolic pressures were routinely practiced; shunt rates were 8.0% during the intermediate (2004-2007) and 6.4% during the later (2008-2013) phase. By comparison, 25 articles characterizing 6414 brains report an intact circle of Willis in 33-35% of people, with a complete hemi-circle anteriorly (77%) seen more commonly than posteriorly (42%), and 11-16% deficient in both hemi-circles with no cross-flow. Conclusion: Cerebral cross-perfusion is fundamental for safe CEA. Anatomy of the circle of Willis alone does not itself determine adequacy. Physiological mechanisms are important in overriding apparent deficiencies, and these can be manipulated perioperatively. Consequently, only 1 out of 15 patients requires shunting during CEA.