Canadian Journal of Anesthesia, Vol 7, 127-135, Copyright © 1960 by Canadian Anesthesiologists' Society

Further Comparative Studies of Sodium Methohexital

¹ Department of Anaesthesia, University of Saskatchewan College of Medicine and University Hospital, Saskatoon

One of the most interesting features which has emerged from this study is the apparent discrepancy between our previous study and this one. In the previous study, the identity of each drug was known and was used both for induction and maintenance of anaesthesia. The present study was a blind one and each drug was given for induction of anaesthesia and then during maintenance only if nitrous oxide and oxygen alone were insufficient. These discrepancies emphasize a very basic fact of clinical and of experimental comparisons, namely that when conclusions are drawn as to the comparative properties of two or more agents, it must be clearly stated that the conclusions apply only to the experimental conditions under which the results were obtained. In other words, comparable values are not absolute entities but must be related to circumstances and to the basic experimental assumptions.

The comparison of means has certain limitations which must also be appreciated. In the original study, for instance, following the induction with a barbiturate it was always necessary to give at least one maintenance dose of the barbiturate, whereas in the present study nitrous oxide-oxygen maintenance alone was adequate in four of the thiopental cases and in two of the methohexital cases. Also the cases that remained adequately anaesthetized on nitrous oxide-oxygen alone were by no means always the shortest cases, nor had they received necessarily more than an average amount of barbiturate for induction. Indeed, the two longest cases induced with thiopental and maintained on nitrous oxide and oxygen without further supplement lasted 17 and 19 min. respectively and had received only 125 mg. of thiopental for induction.

Nor is there a clear relation between the wake-up time and the total dose of barbiturate administered. For instance, the longest wake-up time after completion of inhalation anaesthesia supplemented by methohexital was 9 min. and this patient had received a total of 110 mg. of the drug for induction and maintenance; this is very close to the mean total dose. The shortest wake-up time was 30 sec. and this patient also had received 110 mg. of the same drug. The longest wake-up time after nitrous oxide supplemented by thiopental was 46 min. following administration of 375 mg. The patient who had been given the largest total dose of thiopental, 550 mg., awoke within 4 min. of the end of inhalation anaesthesia, the mean being almost double that tiime. This at first glance seems odd until it has been calculated that the first patient received 19.5 mg. of thiopental per square meter body surface area per minute of anaesthesia and the second only 14 mg. This and many more or less tangible factors such as degree of apprehension or sedation, metabolic rate, effect of premedication, age, obesity, effect of nitrous oxide, etc., must be taken into account in relating dose to sleep time so that a mere comparison affords only limited information.

If any conclusion can be drawn from such a comparative study as the one here presented, it is this. If thiopental 2 per cent or methohexital 1 per cent is used for induction of anaesthesia and for supplementation of nitrous oxide-oxygen anaesthesia for a standard operative procedure, and if both drugs are injected at the same rate, the degree of anaesthesia, obtained and its quality are on the average quite similar, except that awakening following methohexital is quicker, and orientation is faster and more complete than with thiopental. These may be significant factors in out-patient practice and where good recovery room facilities are not available.

Note:

Supplies of Methohexital were made available through the courtesy of Eli Lilly and Company Limited, Indianapolis, Ind.