An Experimental Study on Canine A-V Junctional Pacemaker : Evaluation of the Automaticity by Overdrive Suppression and Autonomic Blockade by Practolol and Atropine

In sick sins syndrome, a sinus arrest is induced by a marked decrease in the automaticity of the sinus node, and a sino-atrial block is also induced by a conduction disturbance of impulse in the region of the sinus node. Consequently, it is not rare that a long cardiac asystole occurs in sick sinus syndrome. In most of these cases, however, an escape beat originated from the atiroventricular junction appears and prevents Adams-Stokes' attack accompanying the cardiac asystole. It is clear, therefore that the automaticity of the A-V junction plays quite an important role in these cases. Studies on the automaticity of the A-V junction, however, have not intensively been carried out. Then the author experimentally treated dogs' hearts so as to beat with A-V junctional rhythm and evaluated the automaticity of the A-V junction on the bases of the following parameters : (1) the spontaneous heart rate of each dog and (2) the degree of overdrive suppression upon each dog. Furthermore, to clarify functioning of the autonomic nerve in the automaticity of the A-V junction, agents for blocking the vagus or the sympathetic nerve were examined for effect upon the parameters. Method : 1. Preparation of dogs with A-V junctional rhythm and control dogs with sinus rhythm. A total of 26 adult mongrel dogs were used. Under anesthesia by intravenous injection with 30mg/kg of pentobarbital, the chest was opened through a midline incision, and a pericardial cradle was made. Then, an electric coagulator was used to induce thermal coagulation at the sinus node region (about one-third of the whole right atrium.) Twenty-two dogs out of twenty-six succeeded in displaying A-V juctional rhythm on the electrocardiogram. A-V junctional rhythm was judged from the following criteria in limb lead electrocardiograms : (1) there was no P wave which would precede QRS complexes, (2) QRS complexes showed essentially the same wave patterns as those presented at the time of sinus rhythm, and (3) the cardiac beats were regular.