The role of different components of balanced anaesthesia in tolerance to endotracheal intubation

In order to demonstrate the role of anaesthesia, analgesia and muscle paralysis in suppressing the responses to nociceptive stimuli during balanced anaesthesia, the effect of tolerance to endotracheal intubation was used as a model during recovery from a suxamethonium block after different combinations of thiopental and fentanyl. The induction groups were: Thiopental, 4, 6 or 8 mg/kg, and thiopental, 4 mg/kg, supplemented with fentanyt, either 1 microgram/kg or 2 microgram/kg. All 107 patients received suxamethonium 1.5 mg/kg, after precurarization and recovery of muscle strength had been recorded by measuring the twitch tension of thumb adduction caused by indirect supramaximal stimulation of the ulnar nerve. With 4 mg/kg thiopental 60% of the patients reacted against the tube before the suxamethonium block had subsided. Increasing the dose to 8 mg/kg or supplementing the 4 mg/kg thiopental with 1 microgram/kg fentanyl reduced the reactions to about 30%. After supplementing with 2 microgram/kg fentanyl, 90% of the patients tolerated the tube. Reactions against the tube, if any, usually occurred before the twitch tension had recovered by more than 30%. After 4 and 6 mg/kg thiopental, about 10% of the patients started reacting before there were signs of recovery of twitch tension. The results show that if bucking or coughing are used as an indication of inadequate muscle paralysis overcurarization can easily occur. Tolerance to the endotracheal tube is more rationally achieved by small doses of narcotic analgesics (e.g. fentanyl, 0.5 to 1 microgram/kg) than by increasing the dosage of thiopental. This simple model has been found useful in demonstrating the basic principles of balanced anaesthesia at the beginning of training in the specialty.