Bradycardia-dependent long QT syndrome, sudden death and late potentials.

The report by Tobe et al. (1) in this issue of the Journal about late potentials in patients with familial long QT syndrome offers a suggestion that the signal-averaged electrocardiogram (ECG) may be a useful diagnostic tool in the detection and risk stratification of patients with QT prolongation. The association between long QT syndrome and late potentials, a marker of structural substrate for reentrant ventricular tachyarrhythmias, is not as farfetched as it may seem. Although in several autopsy studies the myocardium of patients with congenital long QT syndrome was normal both grossly and microscopically, there are reports of neural degeneration and fibrosis within the conduction system that could explain the late potentials. But before pursuing further speculations along these lines it is necessary to take a closer look at the family reported on by Tobe et al. (I), the sudden death, the QT intervals and the signal-averaged ECG. The reported family. Of 112 family members, 15 reportedly "died suddenly at young age during sleep." Figure 7 of the article (1) depicts only 8 of these persons, including I member who died at the age of 54 and another at the age of 70 years. Informed consent for examination was obtained in only 28 of the 97 presumably living consanguineous family members. Of these, two required pacemaker implantation for sinus arrhythmias and in one subject the QRS duration exceeded 120 ms. Table 2 (1) shows that among the 22 persons in the study group, 3 had a QRS duration of 110 ms, and I a QRS duration of 120 ms. It appears therefore that QT prolongation was not the sole cardiac abnormality in about 25% of the examined family members and that not all deaths in the family occurred at a young age. Sudden death. A long QT interval with an abnormal T wave was recorded before death in one of the victims. No information about the ECG of the other 14 subjects who died suddenly is available, and no sudden death occurred during the follow-up of the studied cohort. Thus, except for one