The 'induction' dose of thiopentone. A method of study and preliminary illustrative results.

Study of the minimal dose requirements for induction of anaesthesia poses great problems which are solved by the use of a standard administration technique and the abolition of the eyelash reflex as an endpoint. This has been used in 2206 consecutive unselected inductions, in which variables considered to be likely to influence the dosage were recorded. Milligram per kilogram is the most acceptable method of expressing the average dosage of thiopentone. Doses follow a right skew distribution. Women required a significantly lower average dose of thiopentone than men, while obese patients required less than others. Moderate or heavy drinking increased the induction dose but the use of tobacco did not have any influence. The most important factors governing dosage are the physical fitness of the patient and the premedication used. Patients in ASA grades 1 and 2 required significantly more thiopentone than those in grades 3 and 4. This effect is as great as that of premedication in which an opiate with a phenothiazine or hyoscine markedly reduced the induction dose. It was more important than the patient's pre-operative condition with respect to sedation or apprehension. Small doses of opiates or benzodiazepines do not have as much effect on dosage.