Intraoperative transcranial Doppler monitoring of cerebral hemodynamics during complete cardiac arrest and coronary artery bypass procedures

Although cardiopulmonary bypass may profoundly alter cerebral hemodynamics, the exact effects are not well documented. To assess these changes, we monitored transcranial Doppler (TCD) velocity signals from the middle cerebral artery during eight thoracotomies and six craniotomies in which total circulatory arrest was induced for aneurysm clipping. Blood pressure was also continuously monitored in four of these thoracotomies and five of these craniotomies. Both signals were continuously and simultaneously recorded into computer software and graphically analyzed for hemodynamic changes and emboli. Hemodynamic events could readily be seen from comparison of blood pressure signals to velocity. For example, the velocity tracings exactly paralleled those of blood pressure during bypass, indicating dependence of flow upon pressure (i.e., lack of autoregulation). Alterations in cerebral perfusion could be seen during cardiac arrhythmias, with a precipitous fall in velocity and presumably flow followed by a transient hyperemic overshoot as sinus rhythm resumed. In most patients, there was a steady fall in velocity as hypothermia was induced, suggesting a response of flow to diminished metabolism. In five patients there was an unexplained velocity rise after bypass, which may represent postischemic hyperemia. During total circulatory arrest, both velocity and pressure were zero. The pressure signals returned before the velocity signals, suggesting a critical closing pressure phenomenon. Finally, cerebral emboli were frequently recorded during pump maneuvers. We conclude that stable TCD velocity signals can be obtained for long periods during thoracotomy and craniotomy and can be compared with blood pressure to record a variety of hemodynamic events that may otherwise remain undetected.