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SUMMARY This report details our experience with documented chronic second-degree atrioventricular (AV) nodal block (proximal to His [H] ) in 56 patients. Forty-six men (82%) and 10 women (18%), ages 18-87 years, were studied. Nineteen of the patients (34%) had no organic heart disease (including seven trained athletes) and 37 (66%) had organic heart disease. ECGs in all patients demonstrated episodes of type I seconddegree block; five patients also had periods of 2:1 block. Prospective follow-up of patients with no organic heart disease (157-2280 days, mean 1395 636 days) revealed one patient with clear indication for permanent pacing because of bradyarrhythmic symptoms (permanently placed on day 220 of follow-up). Two patients died nonsuddenly.In patients with organic heart disease (prospective follow-up of 60-2950 days, mean 1347 ± 825 days), pacemakers were implanted in 10 patients, primarily for treatment of congestive heart failure in eight and syncope in two. Sixteen patients died three suddenly, seven with congestive heart failure, two of an acute myocardial infarction and four of causes unrelated to cardiac disease.In summary, chronic second-degree AV nodal block has a relatively benign course in patients without organic heart disease. In patients with organic heart disease, prognosis is poor and related to the severity of underlying heart disease.SECOND-DEGREE atrioventricular (AV) nodal block is most often recognized in the acute clinical setting, complicating inferior wall myocardial infarction, digitalis intoxication, acute myocarditis or recovery after open heart surgery.' In such circumstances, second-degree AV nodal block is usually reversible with time, and usually plays no major role in determining clinical outcome.Chronic second-degree AV nodal block is also seen in a variety of circumstances.2-1 Although generally it is considered a benign conduction defect,12 13 few systematic data have been reported. In this report, we describe our experience in 56 consecutive patients with chronic second-degree AV nodal block and report clinical, electrocardiographic and electrophysiologic findings. We also report the follow-up data and summarize the clinical significance of chronic seconddegree AV nodal block. No. 5, 1981. crease in PR or AH intervals from the first conducted beat of a sequence to that of the last conducted beat before the dropped beat." Type II block was defined as an episode of second-degree block with no measurable increase in PR from the first to the last conducted beat of a sequence. Two-to-one (2:1) block was not classified by type. Patterns of left and right bundle branch block were diagnosed using standard electrocardiographic criteria.'5 The duration of the QRS complex was considered narrow when less than 0.12 second and was considered wide when at least 0.12 second. Patient SelectionFifty-six consecutive patients with chronic seconddegree AV nodal block were detected, studied and followed between January 1970 and March 1980. Approximately 50% of patients were detected within...

The effect of cycle length on atrial vulnerability was studied in 14 patients manifesting reproducible repetitive atrial firing during atrial extra-stimulus (A2) testing. Repetitive atrial firing was defined as the occurrence of two or more premature atrial responses with return cycle (A2-A3) of 250 msec or less and subsequent mean cycle length of 300 msec or less, following A2. The zone of repetitive atrial firing could be defined in terms of its longest and shortest A1-A2 coupling intervals. Each patient was tested at a long cycle length (CL1) (mean 884 msec) and a short cycle length (CL2) (mean 557 msec). CL1 was sinus rhythm and CL2, an atrial paced rhythm. Repetitive atrial firing occurred in two patients at CL1 and in all patients at CL2. Of the former two patients (group 2), the zone of repetitive atrial firing was markedly widened in one at CL2 due to a shortening of atrial functional refractory period (FRP) at CL2. In the other, zone of repetitive atrial firing could not be totally defined due to induction of sustained atrial flutter preventing definition of atrial FRP. The occurrence of repetitive atrial firing at only CL2 in 12 patients (group 1) reflected: 1) a shortening of atrial FRP from 294 +/- 11 msec at CL1 to 242 +/- 10 msec at CL2 (mean +/- SEM; P less than 0.01), allowing delivery of A2 at shorter coupling intervals (9); 2) the new occurrence of repetitive atrial firing at A1-A2 coupling intervals achievable at both cycle lengths (1); or 3) both effects (2). In conclusion, decrease of cycle length potentiated atrial vulnerability. This demonstration implies that atrial pacing could potentiate occurrence of paroxysmal atrial fibrillation or flutter.

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