EFFECTS OF NIFEDIPINE WITH ISOFLURANE, HALOTHANE, OR ENFLURANE ON AUTOMATICITY, CONDUCTION, AND CONTRACTILITY IN ISOLATED GUINEA-PIG HEARTS

Background. Calcium channel blockers and volatile anesthetics have dep ressant effects on cardiac function. Both groups of drugs appear to ex ert both qualitatively and quantitatively different effects on electro physiologic and mechanical function. The aim of this study was to comp are the direct cardiac effects of the calcium channel blocker nifedipi ne in the absence and presence of isoflurane, halothane, or enflurane. Methods. Guinea pig hearts (N = 36) were isolated and perfused with o xygenated Krebs-Ringer solution (pH 7.4, 37-degrees-C). Recording elec trodes were placed in the right atrium and ventricle to measure heart rate and atrioventricular (AV) conduction time. Isovolumetric left ven tricular pressure (LVP) was measured via a latex balloon and transduce r. Hearts were randomly assigned to one of three anesthetic groups at 0.7 and 1.4 minimum alveolar concentration (MAC) and treated with 15 a nd 30 nM nifedipine. Results. Nifedipine alone significantly decreased atrial rate and left ventricular pressure, without prolonging AV cond uction. Nifedipine plus isoflurane, halothane, or enflurane did not si gnificantly prolong AV conduction compared with the respective anesthe tic agent alone, but nifedipine plus isoflurane, halothane, or enflura ne significantly decreased atrial rate compared with the effect of the anesthetic alone. Halothane or enflurane plus nifedipine significantl y decreased atrial rate more than nifedipine alone or isoflurane plus nifedipine. Isoflurane, halothane, or enflurane plus nifedipine signif icantly depressed LVP more than the respective anesthetic agent alone. Halothane or enflurane plus nifedipine also significantly depressed L VP more than isoflurane plus nifedipine or nifedipine alone. Conclusio ns. This study demonstrates that the combined treatment of nifedipine and volatile anesthetics, especially enflurane, additively depresses a trial rate and contractility, but not AV conduction in vitro. In compa rison with results reported previously, these effects appear less pron ounced than those of the combination of volatile agents with diltiazem and, especially, verapamil.