Correspondence

Use of a mixture of sodium nitroprusside and trimetaphan

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Conjrmation

We would support the findings of MacRae et al. (Anaesthesia 1981; 36: 312-5) that the simultaneous administration of trimetaphan and nitroprusside pro- duces controllable hypotension with low dosages of both drugs.

In 33 of our patients undergoing craniotomy for tumour or aneurysm a reduction in mean arterial pressure of approximately 30% compared with pre- operative levels was achieved with separate infusions of these drugs given in the same 1O:l ratio. The mean duration of hypotension was 6 hours 42 minutes with a range of 2 hours 45 minutes to 11 hours 45 minutes. Despite the many complicating factors involved in major neurosurgical cases we are impressed with the modest doses of nitropwsside, varying from 0.006 to 0.06 mg/kg/hour that were required; in only three cases did the total dose of nitroprusside exceed 20 mg and the highest dose used was 50 mg.

Augmentation of the effects of nitroprusside by ganglionic blockade with tetraethylammonium or hexa- methonium has been demonstrated in dogs’ and a reduction in nitroprusside requirements has been de- scribed in man when trimetaphan is administered by separate infusion? However, the authors are to be congratulated both for illustrating the synergistic effect of this combination of drugs and for their courage in mixing them in the same infusion.

Department of Anesthesiology, RAYMOND MILLER Mount Sinai Medical Centre, HENRY C . TAUSK 100th Street and Fifth Avenue, New York 10029, USA

References 1. PAGB IH, CORCORAN AC, DUSTAN HP, KOPPANYI T. Cardio-

vascular actions of sodium nitroprusside in animals and hypertensive patients. Circuiafion 1955; 11: 188-98.

2. DINMORE P. Combined use of trimetaphan and sodium nitroprusside. British Journal of Anaesthesia 1977; 4 9 1070.

The advantages of separate administration

The mixture of trimetaphan camsylate (TMP) and sodium nitroprusside (SNP) in one infusion as advo- catedby MacRaeetal. (Anaesthesia 1981;36: 312-15)is a simplistic approach to the combined use of these agents, and it is reassuring that they are compatible for up to 4 hours in vitro. There are, however, certain advantages in administering the two agents separately, and also in starting with TMP alone; in the majority of cases TMP is effective by itself in reasonable dosage and SNP is not required.

If the TMP is ineffective the additional administ- ration of an infusion (2 mg%) of SNP reliably induces hypotension in my experience. It is probably helpful also to control tachycardia with a 8-blocker (as little as 1 mg practolol may be effective). It may be given prophylactically, but it is not always required treatment and it is probably preferable to give it only if it is indicated.

If the use of the combination of SNP and TMP is reserved for those cases which are resistant to TMP, the rate of infusion of TMP is not critical (about 1 mg/kg/hour) and it is therefore possible to concentrate on titrating the SNP against the arterial pressure.

Even when the TMP has no apparent effect on the arterial pressure, it still facilitates the subsequent con- comitant use of SNP by increasing its potency by ten- or twenty-fold and also by smoothing its action.

This mode of use would seem likely to minimise the initial rise of intracranial pressure associated with SNP.

A final point is that it has been found beneficial to discontinue the SNP 10-15 minutes before the TMP. There is then a smoother rise of pressure with little of the overshoot one is liable to experience with SNP alone or the simultaneous withdrawal of the two agents.

The value of SNP as I see it, after using it for four years in this manner, is that one is enabled to use TMP cautiously without consideration of the increased inci- dence of tachyphylaxis that unfortunately often accom- panies such caution and the assistance of the myocar- dial depressant effect of halothane is unnecessary as Althesin with moderate intravenous analgesia, a muscle relaxant and controlled ventilation with nitrous oxide or air and oxygen is adequate.

Department of Anaesthesia, PETER DINMORE Hospital of St Cross, Rugby, Warwickshire

A reply

Thank you for letting us see the two letters you have received following publication of our report on the use of a mixture of sodium nitroprusside and trimetaphan (Anaesthesia 1981; 3 6 312-15). We would like to thank Dr Miller and Dr Tausk for their kind comments. It is nice to see that, with regard to the relative potency of these two agents at least, there are no transatlantic differences!

Dr Dinmore describes his technique for the use of these two drugs from separate infusions and we would not disagree with any of his comments. We would very much agree that our approach is simplistic and indeed this is our aim. We feel our method produces good