Background: Our aim was to quantify human involuntary isometric skeletal muscle strength during anaesthesia with propofol, sevoflurane, or spinal anaesthesia using bupivacaine.
Methods: Thirty-three healthy patients undergoing anaesthesia for elective lower limb surgery were investigated. Twenty-two patients received a general anaesthetic with either propofol (n=12) or sevoflurane (n=10); for the remaining 11 patients spinal anaesthesia with bupivacaine was used. We used a non-invasive muscle force assessment system before and during anaesthesia to determine the contractile properties of the ankle dorsiflexor muscles after peroneal nerve stimulation (single, double, triple, and quadruple stimulation). We measured peak torques; contraction times; peak rates of torque development and decay; times to peak torque development and decay; half-relaxation times; torque latencies.
Results: Males elicited greater peak torques than females, medians 6.3 vs 4.4 Nm, respectively (P=0.0002, Mann-Whitney rank-sum test). During sevoflurane and propofol anaesthesia, muscle strength did not differ from pre-anaesthetic values. During spinal anaesthesia, torques were diminished for single-pulse stimulation from 3.5 to 2.0 Nm (P=0.002, Wilcoxon signed rank test), and for double-pulse from 7.6 to 5.6 Nm (P=0.02). Peak rates of torque development decreased for single-pulse stimulation from 113 to 53 Nm s(-1) and for double pulse from 195 to 105 Nm s(-1). Torque latencies were increased during spinal anaesthesia.
Conclusions: At clinically relevant concentrations, propofol and sevoflurane did not influence involuntary isometric skeletal muscle strength in adults, whereas spinal anaesthesia reduced strength by about 20%. Muscle strength assessment using a device such as described here provided reliable results and should be considered for use in other scientific investigations to identify potential effects of anaesthetic agents.