Bazett's formula overcorrects at higher heart rates and undercorrects at low ones. In an anxious, depressed or unwell patient with a heart rate over 90, Bazett can manufacture a prolonged QTc that is not there — and lead to an effective drug being withheld. Fridericia (QT divided by the cube root of RR) is more accurate across the usual range and is the correction most drug-safety trials use, including the BMJ antidepressant analysis in this edition.
- Check which correction the ECG machine reports — many default to Bazett
- Recalculate with Fridericia when the heart rate is above about 90 or below 60
- Measure QT manually in lead II or V5 when the automated value is borderline
- Look at the trend in the same patient rather than a single value against a threshold
Why it matters
A Bazett artefact can deny a patient an antidepressant or antiemetic they need.
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