Halothane and Isometric Contractions of Isolated Pregnant Rat Myometrium

The effects of halothane on isometirc contractions of isolated pregnant uterine muscle strips were evaluated in tissue obtained from 13 midpregnant rats. Peak developed tension was depressed in a dose-related manner at halothane concentrations above 0.8 vol per cent, but was not affected at lower halothane concentrations. Time to peak tension was reduced 10-20 per cent, and relaxation time, 10 per cent, by halothane concentrations ranging up to 2.2 per cent. Total resting tension consisted of a passive component and a calcium-dependent component. In concentrations above 0.8 per cent, halothane rapidly removed 100 per cent of the calcium-dependent resting tension. At lower concentrations, halothane reduced it 50 per cent. The passive component of resting tension was unaffected by halothane. These actions of halothane can prevent postpartum hemostasis. They occur even with very low anesthetic concentrations and can be detected soon after introduction of anesthetic into the muscle bath. This indicates that the hemostatic hazards associated with the use of halothane for delivery may not be prevented by limiting the concentration of halothane or the duration of anesthetic exposure.