What is a QT interval and how might it relate to SIDS?

A QT interval is the amount of time between the earliest ventricular depolarization and the completion of repolarization. Put simply, it is the time it takes for the heart to recharge its battery for its next beat to supply the body with blood. The QT interval is commonly corrected for the subject's heart rate (QTc). An abnormal QTc interval is commonly defined as one that exceeds an upper limit in duration. For Schwartz's study, QTc intervals exceeding 440 msec were considered abnormal. This duration represented the 97.5th percentile of all QTc intervals in his study population. To make this more clear, only 2.5% of infants in the study population had QTc intervals exceeding 440 msec.

The long QT interval may be indicator of problems with the electrical discharges controlling the cardiac muscle. Abnormal discharges in the heart can create irregular beats or even stop the heart. One of the most common theories for the relationship between a prolonged QT interval and SIDS is that the long interval may indicate a propensity for developing ventricular arrhythmia (an abnormal rhythm in the heart). This arrhythmia could most certainly lead to death and in some cases leave no pathological evidence for the cause of death.

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