ECG rhythms Conduction, ST and QT
Prolonged QT interval ECG
Prolonged QT interval. The T wave ends late: the corrected QT is well above the normal limit.
A 30-second recording of the heart model, played in real time. Run live starts the engine itself with this rhythm.
Computed by the TracingsMD heart model (Junkers engine): simulated for education, not a patient recording and not for diagnosis.
Measured on this recording
| Ventricular rate | 64/min |
|---|---|
| PR | 158 ms |
| QRS | 84 ms |
| QT | 481 ms |
| QTc (Bazett) | 497 ms |
| QTc (Fridericia) | 492 ms |
What it is
Delayed ventricular repolarization: the QT interval, corrected for heart rate (QTc), is prolonged — 450 ms or more in men and 460 ms or more in women. It predisposes to torsades de pointes.
How to read this strip
| Rate | 60–100/min. |
|---|---|
| Rhythm | Regular sinus. |
| P waves | Normal. |
| PR | Normal. |
| QRS | Narrow. |
| QT | Prolonged: the QTc is above the AHA cut-off for prolonged QT (460 ms or more in women, 450 ms or more in men), and here close to 500 ms. |
Causes
- Drugs: class IA (procainamide, quinidine) and class III antiarrhythmics (sotalol, dofetilide, ibutilide; amiodarone rarely causes torsades), macrolide and fluoroquinolone antibiotics, antipsychotics, methadone, ondansetron.
- Low potassium, magnesium or calcium.
- Congenital long QT syndromes.
- Bradycardia and pauses.
- Hypothermia, increased intracranial pressure, myocardial ischemia.
What to consider
- Measure to the end of the T wave in the lead where it is longest, excluding a separate U wave, and correct for rate; Bazett's formula overcorrects at fast and undercorrects at slow heart rates, and Fridericia or linear formulas are more reliable.
- A QTc above 500 ms, or a rise of more than 60 ms from baseline, signals a high risk of torsades.
- Review every QT-prolonging drug and correct potassium and magnesium.
- A long QT without an acquired cause, especially with unexplained syncope or a family history of sudden death, suggests congenital long QT syndrome and warrants evaluation by a cardiologist or electrophysiologist.
Key point
No QTc makes torsades certain, but above 500 ms the risk is about two to three times higher. In patients with risk factors, check the QTc before and after starting a QT-prolonging drug.
Not to be confused with
- First-degree AV block — The PR interval is normal. The long interval is from QRS onset to the end of the T wave.
- Peaked T waves of hyperkalemia — Hyperkalemia gives tall, narrow T waves. Here the problem is the long time to the end of the T wave.
- Normal ECG — Measure the QT and correct it for rate; it is clearly prolonged.