Thalamonal and Cerebral Circulation

Sir,—We read with much interest the fin-: study by Drs Sari, Okuda and Takeshita (Brit. J. Anaesth. (1972) 44, 330) as it relates closely to our own work. As they state, Fitch and his colleagues (1969) reported that fentanyl and droperidol in combination cause a small reduction in intracranial pressure, suggesting that this might be due to a fall in c.b.f. and CMRo,. This led us to study the affect of fentanyl and droperidol in combination on c.bj. and CMRo, in anaesthetised (trichloroethylene), ventilated dogs (Miller and Barker, 1969); c.b.f. was reduced significantly even wh;n correction was made to the c.b.f. value for the fall in Paoo, which in some cases followed drug administration. There was a small (not significant fall in arterial pressure and CMR02 was not changed. Michenfelder and Theye (1971) also studied the effects of fentanyl and droperidol on cb.f. and CMRo, in the dog. At normocapnia, c.b.f. was reduced by 40^50% and CMRo, by 23% and they conclude that droperidol is a cerebral vasoconstricting agent. The disparity between these results and those of Dr Sari and his colleagues may relate to specits differences, but this also raises the question of the validity of the use of Reivich's data (obtained from monkeys) to correct human c.b.f. results for changes of Paoo,. Studies in progress in baboons and humans by the MRC Certbral Circulation Group in Glasgow suggest that the change in c.bi. produced by a 1 mm Hg change in Paoo, around 40 mm Hg may be closer to 3 ml/100 g/min than to the 1 ml/lOOg/min found by Reivich (1964) and used by Sari and his colleagues. On this basis, their corrected c.b.f. after Thalamonal would then be 43.7 ml/lOOg/min, a reduction of over 9% from their control value of 48.3 ml/lOOg/min.