A prolonged QTc is rarely due to one cause. Low potassium, low magnesium and low calcium all lengthen it, and several QT-prolonging drugs together can have an additive effect even when none is a problem alone.
Correct the electrolytes first, then repeat the ECG. Review the full medication list, including over-the-counter and herbal products and antiemetics such as ondansetron. Make sure the QT has been corrected for heart rate and, if the rate is fast or irregular, measured carefully by hand rather than accepted from the machine.
- Check potassium, magnesium and calcium and correct them before judging a drug.
- List every QT-prolonging medicine, including antiemetics and antibiotics.
- Repeat the ECG after correction rather than acting on a single reading.
- Measure the QT by hand when the rate is fast or irregular.
Why it matters
Stopping a useful drug for an artefactual or electrolyte-driven QTc is an avoidable harm.
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