A CLINICAL APPRAISAL OF THE ADEQUACY OF BRAIN CIRCULATION DURING ANAESTHESIA (WITH PARTICULAR REFERENCE TO CAROTID THROMBOENDARTERECTOMY)

Abstract

Studies in 30 patients (out of 100) having carotid thromboendarterectomy are described. 1. The validity of using internal jugular venous oxygen tension (Pvo2) as a measure of cerebral blood flow (cbf) in these clinical situations was tested in 30 anaesthetized patients from whom 90 paired values of arterial carbon dioxide tensions (Paco2) and Pvo2 were obtained during carotid thromboendarterectomy. A significant relationship, Pvo2 = 1.21 Paco2 + 1.2, was obtained over a wide range of Pao2 and Paco2 values, indicating that Pvo2 was a reliable measure of cbf under these circumstances. Since cbf has a linear relationship with Paco2, and Pvo2 is similarly related to Paco2, it follows that cbf is also related to Pvo2. The physiological basis and validity of this relationship (Fick principle) is outlined. No significant differences were found to result from three types of anaesthetic agents used: cyclopropane, nitrous oxide and halothane, and halothane and oxygen. This indicated that any of these three agents provided adequate cbf under the clinical circumstances described in this study. There were no differences in samples obtained during the use of a temporary internal carotid artery shunt. This indicated that the shunt provided blood flow sufficient to maintain cerebral circulation. 2. Although the anaesthetic agent per se did not appear important, it appeared essential that the conduct of anaesthesia with the agent provide stability. Monitoring of the electrocardiogram, central venous pressure, and arterial blood pressure provided a ready assessment of cardiovascular stability. Ophthalmodynamometry provided a useful means of accurately assessing the patency of the carotid circulation in the early postoperative period following thromboendarterectomy by comparison with the preoperative retinal artery diastolic pressure. It proved too cumbersome a technique to be of value during the surgical procedure. CNS stability was maintained by attention to “light” adequate general anaesthesia, near-normal body temperature, and adequate systemic arterial blood gas Po2, Pco2, pH, and acid-base balance (with emphasis on the prevention of hypocarbia and tendency towards hypercarbia). Continuous eeg monitoring had limited usefulness as an index of this stability and in assessing adequacy of cbf. Because our surgical shunting procedure was effective in all patients studied, as indicated by the stability of our Pvo2 values during the shunt period, the eeg was also of limited value as a diagnostic index of cerebral ischaemia or a prognostic influence on the ultimate course of the patient postoperatively.