The actions of neuromuscular relaxants at hyperbaric pressures

Previous studies have shown that high pressures increase the twitch tension of directly or indirectly stimulated mammalian muscle, while depressing transmission at the neuromuscular junction. The present paper explores the interaction between high pressures and muscle relaxants on the indirectly stimulated rat phrenic nerve--diaphragm preparation. Two muscle relaxants, d-tubocurarine and succinylcholine, were studied. Measurements of the electromyogram and twitch tension amplitudes were made before and after application of the muscle relaxants and compared with results of measurements of those two variables under hyperbaric conditions. High pressures tended to enhance neuromuscular blockade by d-tubocurarine more than that by succinylcholine, as indicated by electromyographic suppression, and high pressures tended to antagonize the succinylcholine effect on twitch tension more than that of d-tubocurarine. The findings are in agreement with previous observations that high pressures increase muscular twitch tension and depress excitatory synapses. They further demonstrate a complex interaction between hyperbaric pressures and neuromuscular blocking drugs.