Prolonged QT / QTc
Prolonged QT means delayed ventricular repolarization. On ECG, the QT interval is measured from the start of the QRS complex to the end of the T wave. Because QT changes with heart rate, clinical interpretation usually uses the corrected QT interval, or QTc.
Common QTc cutoffs
| Group | Prolonged QTc |
|---|---|
| Adult men | > 450 ms |
| Adult women | > 470 ms |
| Children | > 460 ms |
| High-risk range, any age | >= 500 ms |
Nursing-school version
- QTc > 450 ms in adult males = prolonged.
- QTc > 470 ms in adult females = prolonged.
- QTc > 460 ms in children = prolonged.
- QTc >= 500 ms is markedly prolonged and carries higher risk for dangerous ventricular dysrhythmias, especially torsades de pointes.
Why it matters
A prolonged QT increases risk for:
- torsades de pointes;
- ventricular tachycardia;
- syncope;
- sudden cardiac death.
Common causes
- hypokalemia;
- hypomagnesemia;
- hypocalcemia;
- congenital long-QT syndrome;
- myocardial ischemia;
- bradycardia;
- medications, especially some antiarrhythmics, macrolide or fluoroquinolone antibiotics, antipsychotics, antidepressants, antiemetics such as ondansetron, and methadone.
Class-ready wording
Prolonged QT is a lengthened QTc interval, reflecting delayed ventricular repolarization. It is generally defined as QTc greater than 450 ms in adult men, greater than 470 ms in adult women, and greater than 460 ms in children. A QTc of 500 ms or more is especially concerning because it increases the risk for torsades de pointes and other ventricular dysrhythmias.