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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.

FIGURE 23.18A

Prolonged QT Interval. (ECG contributor: James V. Ritchie, MD.)

FIGURE 23.18B

QT Interval Prolongation. QT of 440 ms, QTc of 498 (double arrow). Note the QT interval is measured from the beginning of the QRS complex to the termination point of the T wave.

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