Central nervous system effects of cardiopulmonary bypass

The spectrum of approaches to the issue of brain injury in cardiac surgical practice ranges from refusal to acknowledge that the problem exists to an overemphasis on cerebral risks that can unduly frighten patients. An appropriate approach to therapeutic and preventive strategies requires a fitting sense of proportion and an understanding of the mechanisms of cerebral injury.

This article reviews the incidence and severity of cerebral injury during cardiopulmonary bypass, the identification of high-risk patients, and the mechanisms of injury, including hypoperfusion, microemboli, and inflammatory response. It discusses the influences of alpha-stat and pH-stat strategies on cerebral blood flow during cardiopulmonary bypass; the use of retinal angiography to image the retinal circulation, thus providing a window on the cerebral microcirculation during bypass; magnetic resonance imaging evidence of an inflammatory response in the brain during bypass; and current efforts to gain better understanding of the molecular mechanisms involved in the inflammatory response.

The current incidence of stroke during cardiopulmonary bypass is somewhat lower than in the 1980s but still remains a significant problem. Levels of cognitive impairment also are unacceptably high. Recognized predictors enable us to identify patients at particularly high risk of stroke. Hypertensive patients are particularly susceptible to ischemic injury during bypass and should be perfused at mean perfusion pressures higher than those for normotensive patients. Under conditions of hypothermia, a pH-stat strategy causes loss of cerebral blood flow autoregulation, and the cerebral blood flow becomes pressure-passive. With both the pH-stat and alpha-stat strategies, cooling of the patient greatly increases the flow to metabolism ratio of the cerebral blood flow; however, this luxury perfusion brings to the brain not just an excess supply of oxygen but also an increased quantity of microemboli. Current investigative efforts are focused on the endothelial cell-leukocyte adhesion cascade, attempting to characterize beta2 and beta1 adhesion molecule expression in patients undergoing cardiac surgery. Hammersmith Hospital is about to complete a study of the effects of high-dose aprotinin on the inflammatory response pattern and on cerebral infarction.

Further progress in the development of therapeutic and preventive strategies with respect to cerebral injury during cardiac bypass depends on an increase in the understanding of the mechanisms involved. Current strategies should include optimizing cerebral perfusion and minimizing macroembolic and microembolic damage. The possibility of modifying the systemic inflammatory response is the most interesting challenge of the next few years.