Transcranial magnetic-evoked potentials under total intravenous anaesthesia and nitrous oxide.

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.

[Transcranial Doppler ultrasound: effects of intravenous anesthetics in neurosurgical patients]. / Transkranielle Dopplersonographie: Effekte intravenöser Anästhetika bei neurochirurgischen Patienten.

AIM: Although adverse effects on cerebral blood flow have been reported, intravenous anaesthetic and sedative agents are often used in neurosurgical patients. Monitoring of these effects by transcranial Doppler sonography remains a questionable procedure as long as the cross-sectional area of the insonated basal cerebral arteries is unknown. This study should evaluate the effects of thiopental, propofol, midazolam and alfentanil on flow velocities and "vessel cross-sectional area" (proportional to the reflected Doppler signal power) measured by transcranial Doppler sonography. METHOD: 19 patients with severe cerebral lesions (Glasgow Coma Scale < 6) were investigated. They were hyperventilated and sedated with fentanyl and flunitrazepam. The Doppler probe was fixed to the temporal bone and focussed to the middle cerebral artery of the more severely lesioned side. Baseline values of flow velocities and vascular cross-sectional area were measured. If routine nursing procedures required a deeper degree of sedation, either thiopental 2.5 mg/kg, propofol 1 mg/kg, midazolam 0.075 mg/kg or alfentanil 0.025 mg/kg were injected intravenously over 30 s. Further measurements were made 60, 120 and 300 s after start of the injection. Mean +/- SD were calculated, statistical evaluation was performed by analysis of variance and paired t-tests using the Bonferroni correction (p < 0.05). RESULTS: The injected agents induced significant decreases of the mean value of flow velocities; the "vessel cross-sectional area" remained unaltered. In some patients paradoxical increases of v were observed. CONCLUSION: 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.

Transcranial Doppler sonography during isoflurane/N2O anaesthesia and surgery: flow velocity, "vessel area" and "volume flow".

Transcranial Doppler sonography (TCD) constitutes an advance in noninvasive monitoring of the cerebral circulation. However, as long as the diameter and cross-sectional area of the insonated middle cerebral artery (MCA) remain unknown, the derived flow velocities (v) are not informative. It is not known how the human MCA is influenced by anaesthetic agents. However, a TCD-modification allows noninvasive determination of "vessel area" (VA) and "volume flow" (VF) in MCA by analysing the backscattered Doppler power. This investigation evaluates the effects of isoflurane (in combination with N2O and surgery) on v, VA and VF. In 14 patients (ASA I) scheduled for minor surgical or gynaecological operations, anaesthesia was induced with droperidol, alfentanil, thiopentone and vecuronium. After intubation ventilation with N2O:O2 = 3:2 was adjusted, to maintain endexpiratory carbon dioxide (FECO2) constant between 4 and 5%. Baseline values of heart rate (HR), oscillometric mean arterial pressure (MAP), and TCD variables (v, VA VF) were measured before adding 2.4% isoflurane to the inspiratory mixture. Further measurements-were made 3, 6, 10, and 20 min after starting isoflurane. Surgery commenced between the sixth and tenth minute after isoflurane application. The MAP, FECO2, and v showed only minor alterations; HR increased after 6, 10 and 20 min. Transcranial "vessel area" and "volume flow" showed increases after isoflurane inhalation. The increase of "vessel area" supports the assumption that isoflurane greater than 1 MAC dilates large human cerebral arteries, so that if flow velocities are considered alone, alterations of cerebral blood flow may easily be underestimated.

[Transcranial Doppler sonography and hemodynamics]. / Transkranielle Dopplersonographie und Hämodynamik.

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.