Unusual observations during pacemaker therapy for complete heart block

Abstract

During the course of management of spontaneously occurring complete heart block with Stokes-Adams syndrome in a 10 year old boy, rare clinical, therapeutic and physiologic observations were made. Initially, cardiac rate was controlled via a catheter electrode. The effect of ventricular rate on right atrial pressure was shown. There was a progressive fall in right atrial pressure as the ventricular rate was increased from 36 to 100/min. A Chardack pacemaker was implanted permanently, and cardiac rate was controlled with a 9-volt stimulus. During steroid therapy, retrograde atrial conduction and normal sinus rhythm developed. Unusual cardiac arrhythmias allowed electrocardiographic data to be obtained concerning the “vulnerable period” of the cardiac cycle in man. During alternating periods of normal sinus rhythm and pacemaker-controlled rate, ventricular premature beats resulted only when stimuli fell in a period of less than 30 msec, preceding the apex and during the descending portion of the T wave. Additionally, it was shown that stimuli in the “vulnerable period” of the cardiac cycle resulted in premature beats with QRS complexes of “multiform” shape. Although a single stimulus (considerably above the premature beat threshold value) placed in the vulnerable period can initiate ventricular fibrillation, a 9-volt stimulus was insufficient to reach ventricular fibrillation threshold in our patient. Steroid therapy was discontinued with subsequent loss of atrioventricular conduction. Pacemaker failure (due to breakage of a resistor wire) occurred 12 months postoperatively. Successful substitution of the abdominal wall pacemaker was made. At the time of pacemaker substitution, the threshold voltage required to control ventricular rate was 4.6 volts indicating that there had been no significant increase in myocardial resistance after one year.