TRANSCRANIAL DOPPLER SONOGRAPHY: EFFECTS OF HALOTHANE, ENFLURANE AND ISOFLURANE ON BLOOD FLOW VELOCITY IN THE MIDDLE CEREBRAL ARTERY

Systolic, diastolic, and mean blood flow velocity in the middlecerebral artery (Vs,mca; Vd,mca; Wm,mca) and pulsatility (Vs- Vd)/Vm of the waveform obtained were recorded in 51 patientsbefore, during and after general anaesthesia. Transcranial Doppler(TCD) sonographic variables were measured in the awake patientand after induction of anaesthesia with thiopentone 5–6mg kg^–1. After tracheal intubation, 17 patients received0.8% halothane and 66% nitrous oxide in oxygen for 30 min (15minnormoventilation; 15min hyper-ventilation). The inspired halothaneconcentration was then increased to 1.6% for 45min (15min normoventilation;15 min hyperventilation; 15 min normoventilation with nitrousoxide replaced by oxygen). Enflurane (1.7% for 30 min and 3.4%for 45 min) was given to another 17 patients; 17 other patientsreceived isoflurane (1.2% and 2.4%). Mean arterial pressure(MAP), nasopharyngeal temperature, end-tidal carbon dioxideconcentration, inspired and end-tidal anaesthetic agent concentrations,haemoglobin concentration, PVC and TCD variables were measuredat the end of each 15 min period. After recovery from anaesthesia,TCD variables were measured again. There were no intergroupdifferences in changes in MAP, nasopharyngeal temperature, haemoglobinconcentration and PCV. Halothane, enflurane and isoflurane atlow doses and normoventilation had little influence on TCD variablescompared with awake values. In large concentrations with nitrousoxide in oxygen and normoventilation, there were differencesbetween the volatile agents. Halothane increased blood flowvelocities, but enflurane and isoflurane caused little change.Hyperventilation always decreased blood flow velocities andincreased pulsatility. Six of 17 patients undergoing hyperventilationwith 2.4% isoflurane and nitrous oxide in oxygen showed zerodiastolic flow. This phenomenon was not observed in any patientreceiving halothane or enflurane. With high doses of the volatileagents and normoventilation, blood flow velocities decreasedin all groups after discontinuation of nitrous oxide. Our TCDresults are compatible with the known effects of halothane,enflurane and isoflurane on cerebral blood flow (CBF). However,the relative changes in blood flow velocity and CBF may notbe proportional with large doses of volatile agents, as a constantdiameter of basal cerebral arteries may not be assumed underthese conditions.