Sharbrough, Frank W.

Regional cerebral blood flow (CBF) was measured (intra-arterial injections of 133 Xe) and electroencephalograms (EEG) were recorded periodically before, for two hours during, and for one and one-fourth hours after middle cerebral artery (MCA) occlusion in 20 squirrel monkeys ( Saimiri sciureus ). A CBF-Pa co co2 response curve for these animals under barbiturate anesthesia was created from CBF values prior to MCA occlusion and during the time a steady state was being achieved. The animals were subdivided into four groups (five monkeys in each) on the basis of Pa co co2 values: 20, 36, 40, and 60 mm Hg. CBF values from this study were compared to previous results obtained with 35 Kr. The phenomenon of "look through" and the importance of recognizing this artifact and its significance in analyzing CBF results in areas of focal ischemia are discussed. The present results were correlated with cerebral ATP and lactate concentrations in ischemic regions determined in previous studies using this preparation at these Pa co co2 values and at comparable time intervals before, during, and after MCA occlusion. The EEG appears to reflect the state of ischemic brain accurately. However, CBF measured by the 133 Xe method can be misleading in regard to the true degree of ischemia resulting from occlusion of an intracranial vessel and cannot be relied on to demonstrate accurately "steal" or "reverse steal" due to changes in Pa co co2 .

During an 11-month period, 81 endarterectomies under a carefully controlled level of general anesthesia were monitored with continuous electroencephalograms (EEG) and intermittent regional cerebral blood flow (CBF) measurements. There was a high correlation between the CBF (milliliter per 100 gm per minute) during carotid occlusion and alterations in the EEG: no EEG change was seen with the flow above 30 ml/100 gm brain per minute, major changes were not seen with a flow between 18 and 30 ml, and changes invariably occurred with a flow below 17 ml. The degree of EEG change reflected the severity of flow reduction but was always reversible with the placement of a shunt. The EEG at the termination of the surgery corresponded with the patient's neurological state in that all EEG tracings were normal or unchanged as compared to the preoperative tracing and no neurological worsening occurred in any patients studied. The EEG is a valuable monitoring technique that indicates when a shunt is required and informs the surgeon of the state of cerebral function not only during occlusion but also throughout the entire operative procedure.

Data from the records of patients who underwent 2223 carotid endarterectomies at the Mayo Clinic between January 1, 1972, and December 31, 1985, were abstracted to compare the effects of isoflurane, enflurane, and halothane on the critical cerebral blood flow (CBF) (i.e., the CBF below which the majority of patients develop EEG ischemic changes within 3 min of carotid occlusion), the incidence of EEG ischemic changes, and the neurologic outcome. In a total of 2196 of these procedures, the patient received one of the three volatile anesthetics and, in 2010 of these, both the EEG and the CBF were monitored. Chronologically, halothane was the primary agent from 1972–1974; enflurane progressively replaced halothane during 1975–1981; and isoflurane was used almost exclusively since 1982. This analysis confirmed a previous study that the critical CBF during isoflurane anesthesia (703 procedures) was approximately 10 ml ± 100 g−1 • min−1, as contrasted to that of approximately 20 ml ± 100 g−1 • min−1 during halothane anesthesia (467 procedures). This analysis also established that the critical CBF during enflurane anesthesia (840 procedures) was approximately 15 ml ± 100 g−1 • min−1. The incidence of EEG ischemic changes was significantly less (P < 0.001) during isoflurane anesthesia (18%) than during either enflurane (26%) or halothane (25%) anesthesia. This difference occurred despite the fact that the preoperative risk status was greater in the patients given isoflurane. There was no difference in neurologic outcome between the three anesthetics, and none was expected, since all patients with EEG changes were immediately shunted, if possible. The authors conclude that relative to halothane and enflurane, isoflurane does offer a degree of cerebral protection for transient incomplete regional cerebral ischemia during carotid endarterectomy.