MEASUREMENT OF EVOKED DIAPHRAGM TWITCH STRENGTH DURING ANAESTHESIA Adaptation and Evaluation of an Existing Technique

The respiratory muscles are particularly vulnerable during and after anaesthesia, but objective investigation of their function during this period has received little attention. In conscious subjects the strength and endurance of the diaphragm have been assessed [1-4] by the measurement of the pressure gradient across the muscle during voluntary ventilatory manoeuvres and loaded breathing; these tests cannot be applied easily after anaesthesia and surgery. The production of an involuntary contraction of the diaphragm by electrical stimulation of the phrenic nerve has been used to make an objective assessment of diaphragm strength [5], to investigate the development of diaphragmatic fatigue [6] and to examine the effects of drugs on both of these [7]. The first report of stimulation of the phrenic nerve, with observation of the response of the diaphragm, in an anaesthetized patient was probably that of Dripps [8], who investigated whether prolonged apnoea was peripheral or central in origin. The electromyographic (EMG) response of the diaphragm, after stimulation of the phrenic nerve, has been recorded during anaesthesia [9] and used to calculate dose-response curves for pancuronium in the dia-phragm. Chauvin [10] recorded the diaphragm twitch strength and the effect of vecuronium using needle electrodes for bilateral phrenic nerve stimulation. We describe a technique of recording the mechanical twitch response of the diaphragm after unilateral transcutaneous stimulation of the phrenic nerve, which is applicable during an