We have studied the effect of nitrous oxide on cerebral haemodynamics in 24 healthy male volunteers. Hemispherical cerebral blood flow (CBF) was measured using the xenon-133 inhalation technique, blood flow velocities in the right middle cerebral artery were calculated using transcranial Doppler ultrasound and the pulsatility index (PI)--the inverse of which is theoretically proportional to flow in the vessel under investigation--was derived from analysis of the spectrally analysed velocity pulse wave form obtained from the middle cerebral artery. Each variable was measured with the subject inhaling 100% oxygen (1st baseline), 30% nitrous oxide in oxygen, 100% oxygen (2nd baseline) and 60% nitrous oxide in oxygen. CBF was significantly greater with 30% (0.01 > P > 0.001) and 60% nitrous oxide (P < 0.001) compared with baseline, although the difference between 30% and 60% nitrous oxide was not significant. Changes in 1/PI correlated closely with those in hemispherical CBF. Blood flow velocities increased significantly with 30% (P < 0.001) and 60% nitrous oxide (0.005 > P > 0.001), the difference between 30% and 60% nitrous oxide also being significant (0.005 > P > 0.001). We observed a plateau in the change in CBF caused by nitrous oxide and suggest that this may be explained by activation of intact autoregulative mechanisms in healthy human brain.
SUMMARYThe effects of high concentrations of inspired oxygen on cerebrovascular haemodynamics were studied in healthy human volunteers using transcranial Doppler measurements of middle cerebral artery blood velocity. If the end-tidal carbon dioxide level was kept constant, the measured blood velocity and the calculated Pulsatility Index showed no significant change when subjects were breathing 21, 40, 75 or 100% oxygen.
SummaryWe describe the successful management of general anaesthesia for Caesarean section in a patient with poorly controlled paroxysmal ventricular tachycardia of pregnancy. The use of alfentanil before laryngoscopy and tracheal extubation ensured cardiostability without compromising maternal or fetal wellbeing. General anaesthesia allows prompt cardioversion. We believe that in the presence of a life-threatening unstable cardiac rhythm this consideration outweighs any theoretical advantage of regional blockade.
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