Magnetic stimulation of the cortex and recording of the motor-evoked potentials (MEPs) by electromyography (EMG) is a well proven method to assess the descending pathways of the spinal cord and detect neurological impairment. We have assessed, in 33 adult patients undergoing spinal surgery, the influence of four total i.v. anaesthesia regimens (TIVA) on this recording technique. In 20 patients, the effect of 50% nitrous oxide was also studied. MEP amplitudes, latencies and success rates of stimulation were obtained in the steady-state after induction of anaesthesia. Combinations of midazolam and ketamine, and alfentanil and etomidate had the least effect on MEPs. Propofol (in combination with alfentanil or ketamine) showed marked depression of the MEP amplitude and the lowest success rates of stimulation. The latencies did not change at all. The addition of nitrous oxide significantly depressed the registered MEPs and lowered the success rates.

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The results indicate that intravenous anaesthetic agents are not likely to influence the cross-sectional area of the major basal cerebral arteries. Therefore TCD seems to be a valid tool to monitor the effects of these agents on the cerebral circulation of neurosurgical patients. This is probably of prognostic and therapeutic value.

13 patients without cerebral or cerebrovascular diseases were investigated by transcranial Doppler sonography during Swan-Ganz catheterisation for cardiologic purposes. Massive increases of mean arterial pressure, heart rate and cardiac index were observed during exercise; flow velocities in the middle cerebral artery increased only to the same extent as the endexpiratory carbon dioxide rose. However, the pulsatility index increased significantly (p less than 0.01) although a decrease should have been expected due to carbon dioxide accumulation. These results are indicative of a functioning autoregulation independent of carbon dioxide; they are relevant for interpretation of TCD results in patients with disturbed autoregulation.