Potency of succinylcholine at the diaphragm and at the adductor pollicis muscle

To quantify the differential effect of succinylcholine at the diaphragm and the adductor pollicis muscle, 10 patients were studied during halothane-nitrous oxide anesthesia. Train-of-four stimulation was applied to the ulnar and phrenic nerves. The force of contraction and the electromyographic response of the adductor pollicis were measured and compared with the diaphragmatic electromyogram. Then dose-response curves for both muscles were constructed using incremental doses of succinylcholine with an infusion to replace metabolized or redistributing drug. Linear regressions were obtained between the logit transformation of neuromuscular blockade at the adductor pollicis and diaphragm and the logarithm of the dose. The diaphragm was relatively resistant to succinylcholine. At 90% adductor pollicis block, the diaphragm was only (mean +/- SEM) 37 +/- 3% blocked. The diaphragm required 1.8 +/- 0.2 times as much succinylcholine as the adductor pollicis for an identical 90% block. The ED50 and ED90 values for succinylcholine at the diaphragm were 0.23 +/- 0.04 and 0.40 +/- 0.09 mg/kg, respectively. Corresponding values for the adductor pollicis were 0.14 +/- 0.01 and 0.21 +/- 0.02 mg/kg. The data indicate that the degree of adductor pollicis blockade might overestimate the intensity of diaphragmatic paralysis.