Electrophysiological characteristics of junctional pacemakers in congenital A‐V block and following his bundle cryoablation

The characteristics of the escape mechanism following surgically induced permanent A-V block were investigated and compared with those seen in congenital complete heart block (CHB). Six patients had undergone elective cryothermal ablation of the His bundle for supraventricular arrhythmias unresponsive to pharmacological and pacemaker techniques (group A) and 12 patients had congenital CHB (group B). In the 12 patients in group B the site of block was localized by His bundle electrocardiography to be proximal to the point of recording of the His potential. In 3 patients in group A in whom intracardiac studies were performed it was impossible to record an His potential. There were no significant differences between the control escape rate, junctional recovery time (JRT), and corrected junctional recovery time (cJRT) of the two groups. In group A there were no significant changes in these parameters following the administration of atropine, whereas isoproterenol significantly increased the rate of the subsidiary pacemaker (p less than 0.001) and shortened the JRT (p less than 0.02) and cJRT (p less than 0.02). In group B both atropine (A) and isoproterenol (I) significantly increased escape rate (A, p less than 0.001; I, p less than 0.001) and shortened JRT (A, p less than 0.01; I, p less than 0.001) and cJRT (A, p less than 0.01; I, p less than 0.001). It is concluded that the escape focus in patients with congenital CHB is situated in the A-V node. The escape rhythm following His bundle section is unpredictable and elective permanent pacemaker implantation is indicated.