23-18: QT Interval Abnormalities

ECG Findings

• Normal QTc interval is gender based: < 440 ms in men or < 460 ms in women.

• The QTc interval is prolonged if it is > 440 ms in men or > 460 ms in women.

• The QTc interval is abnormally short if < 350 ms.

Pearls

1. The QT interval will increase with bradycardia and decrease with tachycardia; thus, it is important to use the corrected QT interval (QTc = QT/√ R-R interval) for heart rates other than 60 (in which the QTc = QT).

2. In most cases, QT prolongation is acquired, either due to medications or electrolyte abnormalities. In more rare instances, the QT prolongation is caused by a congenital defect.

3. Prolongation of the QT interval predisposes the heart to the R-on-T phenomenon, which causes torsades de pointes. The risk of torsades de pointes tends to be highest when the QTc is > 500 ms.

4. Short QT can be caused by hypercalcemia, congenital short QT syndrome, or digoxin effect. A short QT predisposes the patient to atrial fibrillation, ventricular tachycardia, and ventricular fibrillation.