Most ECG machines correct the QT interval with Bazett's formula, which overcorrects at heart rates above about 90 to 100 beats per minute and can report a prolonged QTc that is not really there. Fridericia's correction, which uses the cube root of the RR interval, is less distorted by heart rate and is the one most drug-safety studies use.
In a tachycardic, anxious or febrile patient, a machine-reported long QTc should be rechecked with Fridericia or repeated once the heart rate settles before it stops a needed drug.
- Recalculate a machine-reported long QTc with Fridericia when the heart rate is above about 90.
- Repeat the ECG once the heart rate settles if the result would change a prescribing decision.
- Measure the QT in the lead with the longest clear interval, often II or V5.
- Check potassium, magnesium and the drug list whenever QTc is genuinely prolonged.
Why it matters
A Bazett artefact can deny a patient a drug they need.
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