The QT interval throughout the first 6 months of life: a prospective study.

A prospective electrocardiographic study was designed to establish baseline values for electrocardiographic measurements, with specific reference to the QT interval during infancy, and to test the "QT hypothesis" for the sudden infant death syndrome (SIDS). In this ongoing study, ECGs are recorded on the fourth day of life and in the second, fourth and sixth months. The state of health at 1 year is ascertained by a phone call. So far, 4205 newborns have been enrolled. The mean QTc (QT interval corrected for heart rate) was 397 +/- 18 msec (+/- SD) at the fourth day, 409 +/- 15 msec (p less than 0.0001) at the second month, and 406 +/- 15 msec at the fourth month; by the sixth month, it returned to 400 +/- 14 msec. In 88 newborns, the QTc increased by over 40 msec at the second month. Among the 2000 infants checked at 1 year, there have been three sudden and unexpected deaths. The QTc of one of the victims at the fourth day was 563 msec, which exceeded the mean by more than 9 standard deviations, while the QTc of the other SIDS victims exceeded the mean by more than 2 and 3 standard deviations. These results are consistent with the "QT hypothesis," but more data are necessary before any conclusion on the potential relationship between QT interval prolongation and SIDS can be drawn. This study provides definitive waking normal values for QT interval in infancy and indicates that the QT interval lengthens physiologically and temporarily during the first months of life. In some infants, this lengthening may transiently impair cardiac electrical stability.