Effects of halothane on mechanical response of skeletal muscle from malignant hyperthermia susceptible patients

The purpose of this investigation was to compare the effects of halothane o n malignant hyperthermia (MN) and normal isolated muscle bundle performance during isometric contraction and relaxation phases. Mechanical parameters were measured: peak tension (PT), time to peak tension (TPT) and positive p eak of isometric tension derivative (+dP/dt(max)) characterized the contrac tion phase. Half-relaxation time (RT1/2) and negative peak of isometric ten sion derivative (-dP/dt(max)) characterized the relaxation phase. The ratio R = (+dP/dt(max))/(-dP/dt(max)) was used to study the coupling between con traction and relaxation under isometric condition. In normal muscle, haloth ane increased PT by nearly 40% without altering TPT. The +dP/dt(max) value increased concomitantly with the -dP/dt(max) values, thus no changes in R w as observed. In MH muscle, PT was first potentiated (0.5-1.0 vol% halothane ) and then depressed (2.0-3.0 vol% halothane). TPT and +dP/dt(max) were not altered whereas RT1/2 increased progressively with concomitant decrease in -dP/dt(max), thus R increased by nearly 40%. The amplitude of MH muscle co ntracture with stepwise concentrations of halothane was correlated with the increase of RT1/2 and R and the decrease of -dP/dt(max). These results sug gest that halothane alters the relaxation phase more than the contraction p hase in MH human skeletal muscle compared to normal muscle.