The relationship between receptor occlusion and the frequency sweep electromyogram during competitive neuromuscular blockade.

The purpose of this study was to correlate the frequency sweep electromyogram (FS-EMG) responses to various levels of receptor occlusion during partial neuromuscular blockade by d-tubocurarine (dTC). The FS-EMG represents the integrated compound muscle action potential induced by intra-muscular electrical stimulation as the stimulus frequency increases exponentially from 1 to 100 Hz. The FS-EMG response was recorded from the tibialis anterior muscles of six adult cats at different levels of neuromuscular blockade during recovery from intravenously injected dTc, 0.25 mg/kg. The fraction of receptors blocked was estimated by measuring the depolarization produced by proximate intraarterial injection of graded doses of succinylcholine. The FS-EMG became diminished for driving frequencies greater than 50 Hz when receptor occupancy was about 25-30 per cent. The responses corresponding to lower stimulus rates declined only at increasing levels of occlusion. The FS-EMG response to low-frequency stimulation did not diminish until 70 per cent of the available receptor pool was blocked. Simultaneous measurement of muscle force demonstrated a depressed response equivalent to that of the FS-EMG. These data were compared with the sensitivities reported for other tests of neuromuscular block. The frequency sweep electromyogram appears to be about as sensitive as determination of responses to sustained 5-second tetanic stimulation at 200 Hz, but more sensitive than determination of responses to tetanic stimulation at 100 Hz or twitch responses, or train-of-four measurements.