Interaction between negative inotropic effects of halothane and nifedipine in the isolated rat heart

Isobolographic analysis was used to assess quantitatively the combined direct negative inotropic effect of halothane and nifedipine as well as to define the type of interaction between these two agents. Experiments were performed on an isolated rat heart preparation. Developed left ventricular pressure and dP/dtmax were used as indices of inotropic action. It was found that the combination of halothane and nifedipine gives mostly additive direct negative inotropic effect. However, with a small level of myocardial depression (ED25), there was some deviation from the additive interaction toward an infra-additive effect (p less than 0.01), and with a high level of depression (ED75), some deviation toward a supra-additive effect (p less than 0.05).