A-V conduction in hyper- and hypothyroid dogs

Abstract

Dogs were made hyper- or hypothyroid by daily feeding of thyroid powder or intraperitoneal injection of 131I, and serum thyroxine levels were determined to confirm their abnormal thyroid states. Conduction time (CT) and functional refractory period (FRP) of the A-V conduction system were studied in these dogs for comparison with those of control dogs. In the hyperthyroid group, A-V CT and FRP were markedly shortened, and the frequency at which the A-V conduction system can transmit rapid atrial impulses was significantly increased. The opposite changes were observed in the hypothyroid group. A-V CT and FRP decreased in the control and hyperthyroid groups during infusion of epinephrine at a rate of 1 mcg. per kilogram per minute, and these changes were comparable in both groups in terms of per cent changes. The results indicate that thyroid hormone markedly facilitates the impulse transmission through the A-V conduction system, and the action appears to be direct and independent of the sympathetic activity. This action may explain extremely rapid ventricular responses and increased digitalis requirements seen in thyrotoxic patients with atrial fibrillation.