The prevalence of cardiac arrhythmias was assessed in a group of relatives of patients who had come to necropsy with hypertrophic cardiomyopathy. The validity of an interventricular septal thickness of .gtoreq. 1.3 cm, measured by echocardiography, as a diagnostic criterion of hypertrophic cardiomyopathy among relatives of cases proven at necropsy was also assessed. Close relatives (50) of 8 deceased patients were examined. By the above definition, 22 relatives had hypertrophic cardiomyopathy and 28 did not. A comparison of the prevalence and types of cardiac arrhythmias, as shown by 24-h ambulatory ECG monitoring, was made between the 2 groups and a 3rd apparently healthy group of 40 people. The patients with hypertrophic cardiomyopathy showed a significant increase in supraventricular extrasystoles/24 h, supraventricular arrhythmias, high grade ventricular arrhythmia and the number of patients with > 10 ventricular extrasystoles every 24 h when compared with the other groups. There was no significant difference between normal relatives and controls. The prevalence and types of arrhythmia in these patients were similar to those found by other investigators using different diagnostic criteria. Apparently, these patients have hypertrophic cardiomyopathy; all close relatives of necropsy-proven cases should be examined by echocardiography and subsequently by ambulatory ECG monitoring if the interventricular septal thickness is .gtoreq. 1.3.
