Effects of succinylcholine on respiratory and nonrespiratory muscle strength in humans.

Succinylcholine was administered to 10 healthy unanesthetized volunteers to assess its effect on respiratory and nonrespiratory muscle strength and the ventilatory response to CO2. Iso hypercapnia with PETCO2 8-10 mmHg above control was maintained throughout the study, succinylcholine infusion rates were increased from 20 micrograms X kg-1 X min-1 until grip strength (GS) was 20% of control. CO2-stimulated ventilation was 16.1 +/- 1.8 l/min (mean +/- SD), approximately three times control, and remained at that level throughout the study because of nonsignificant decreases in tidal volume and increases in respiratory frequency. Respiratory strength, as measured by maximum inspiratory pressure (IP), maximum expiratory pressure (EP), and forced vital capacity (FVC), was spared relative to GS. When GS = 50% of control, IP = 86 +/- 8% of control, EP = 78 +/- 15%, and FVC = 86 +/- 9%. Wide variation occurred from subject to subject in the succinylcholine versus GS dose-response curve position. However, in all subjects the slope of the dose-response curve was very steep.