Endocrine Pacemaker for Complete Heart Block

Abstract

In dogs with complete heart block, adrenal medulla or thyroid autografts, or tri-idiothyronine tablets implanted into the myocardium produced a sustained elevation in heart rate for 24 to 96 hours. Exercise of the animal caused a further increase in heart rate. Electrocardiographic studies suggested that the faster pacemaker activity originated from the area of implantation of the grafts or the tablets. After implantation of either thyroid grafts or tri-iodothyronine tablets, a latent period of 8 to 12 hours occurred before the new pacemaker became active. Data suggest that the pacemaker shift and the increase in heart rate are due to a local increase in hormones provided by the autografts or by the tri-iodothyronine tablets. There was no effect on the heart rate when similar or higher doses of tri-iodothyronine were given systemically. The method of tri-iodothyronine implantation into the myocardium is valuable in the study of the action of this hormone. The technique has two unique advantages: the reaction remains localized, and the endpoint is signaled electrically. Reserpine suppresses the ability of implanted tri-iodothyronine to increase the heart rate. This suggests that normal catecholamine levels are required for the positive chronotropic action of tri-iodothyronine. Myocardial catecholamine concentrations were not increased around the implanted tri-iodothyronine tablets. If catecholamines are necessary for the action of tri-iodothyronine on the myocardium, this suggests that tri-iodothyronine causes sensitization of the myocardium to the catecholamines, rather than causing an accumulation of the catecholamines themselves. Fibrous encapsulation of endocrine transplants appears to prevent permanent pacemaker activity. In lieu of successful thyroid transplants, myocardial implantations of tri-iodothyronine as slow release pellets are potentially applicable for patients with complete heart block.