Effect of halothane on sinus rhythm of dog heart in situ

Effect of halothane on sinus rhythm was investigated using a direct perfusion technique of the canine SA node artery in situ. Halothane was dissolved in a fat emulsion and administered to the SA node artery in doses between 148.8 micrograms and 2976 micrograms. Both the solvent itself and a stabilizer of halothane produced no change in the heart rate. Halothane induced a biphasic chronotropic response in all doses: an initial sinus acceleration of 2 to 5 sec duration followed by a long-lasting sinus deceleration. Both responses were dose-related. The positive response was not substantially affected by prior administration of propranolol or tetrodotoxin or in reserpinized dogs. Atropine or tetrodotoxin also failed to cause any change in the negative response. Therefore, both responses were assumed to have resulted from a direct action of halothane on the SA node, but the detailed mechanism underlying the positive response remains to be further clarified.