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To define the long-term natural history of congenital complete heart block, we followed 27 patients prospectively by means of frequent ambulatory electrocardiographic (ECG) recordings for a mean (+/- SD) of 8 +/- 3 years. During that time, 8 of the 13 patients with a mean daytime heart rate below 50 bpm (Group A) had cardiac complications such as sudden death, syncope, presyncope, or excessive fatigue. Six of the eight patients had additional ECG findings that suggested an instability of the junctional escape mechanism. These findings included nocturnal junctional exit block (three patients), little or no change in the junctional rate with physical activity (three patients), and associated tachyarrhythmias (three patients). None of the 14 patients with a mean daytime heart rate of 50 bpm or more (Group B) had an adverse clinical outcome, and 5 of the 13 patients in Group A also remained well. Among the five patients in stable condition in Group A, three had no evidence of an unstable junctional mechanism. We conclude that patients with a mean daytime junctional rate below 50 bpm and other evidence of an unstable junctional escape mechanism should probably undergo prophylactic pacemaker implantation. Since junctional exit block and tachyarrhythmias sometimes appear first during follow-up, the method of risk stratification employed in this study depends on serial ambulatory ECG recordings.

SUMMARYThe circulatory and respiratory responses to five levels of treadmill exercise were recorded for 75 normal males divided equally in the age groups 20 to 29,30