Prolonged QT interval
A QT interval that is long for the heart rate at which it was measured. Repolarization time shortens as the rate rises, so the interval only means anything when it is judged against the rate at the time.
Key ECG features
- ✓QT long for the prevailing heart rate
- ✓Measured from QRS onset to the end of the T wave
- •Rate recorded alongside the measurement
- ±A U wave or following P may blur the endpoint
✓ required · • typical · ± may be present
ECG characteristics
The stretch from the onset of the QRS to the end of the T wave occupies more of the cardiac cycle than the prevailing rate would lead you to expect. The end of the T wave is the difficult part: take it where the downslope returns to baseline, avoid complexes where a following P wave or a U wave blurs the endpoint, and be cautious when the ST segment is itself displaced, because then the baseline the T wave returns to has moved as well.
This illustration is schematic. Dashed line is a normal complex. The grid is not drawn to scale.
Common causes
Low potassium, low calcium and low magnesium are the common reversible causes. Also hypothermia, central nervous system disease, marked bradycardia, and an effect of several antiarrhythmic drug classes as well as some non-cardiac ones. A congenital long QT syndrome is described but is rare in veterinary patients.
Why it matters
Prolonged repolarization widens the window in which an ectopic beat can trigger polymorphic ventricular tachycardia, so this is a genuine warning rather than a bookkeeping detail. Correct the electrolytes, review the medication list for agents that prolong repolarization, and recheck.
Differentiate from
- ST segment deviationA displaced ST segment can hide where the T wave ends and shifts the baseline it is read against, so the QT measured across it may look long when the repolarization time is not.
- Peaked T waveUseful as a contrast rather than a look-alike: the hyperkalemic T wave peaks and the QT tends to shorten, which is the opposite direction.