What it is
The QT interval shortens as heart rate rises, so a raw QT cannot be compared against a fixed threshold. A corrected QT (QTc) normalises the measured QT to a heart rate of 60 bpm, letting you screen for long-QT syndrome (LQTS) and drug-induced repolarisation delay. In children — who run faster baseline heart rates than adults — the choice of correction formula materially changes the answer.
The formulas
Both use QT and the RR interval (in seconds; RR = 60 ÷ heart rate):
$$ QTc_{Bazett} = \frac{QT}{\sqrt{RR}} \qquad QTc_{Fridericia} = \frac{QT}{\sqrt[3]{RR}} $$
Bazett applies a square-root correction; Fridericia a cube-root. At a heart rate of exactly 60 (RR = 1 s) the two agree. Away from 60 they diverge — and the divergence is the whole point.
When to use it
Use a QTc on any child with syncope, a family history of sudden death or LQTS, before and during QT-prolonging drugs (macrolides, ondansetron, antipsychotics, methadone), in electrolyte disturbance, and in poisoning. Measure QT in lead II or V5 over 3–5 beats; avoid measuring during sinus arrhythmia swings.
Worked example
QT = 0.40 s, heart rate 100 bpm, so RR = 0.60 s.
$$ QTc_{Bazett} = \frac{0.40}{\sqrt{0.60}} = 0.516\ s\ (516\ ms) $$
$$ QTc_{Fridericia} = \frac{0.40}{\sqrt[3]{0.60}} = 0.474\ s\ (474\ ms) $$
Bazett flags a markedly prolonged QTc; Fridericia is only borderline. The 42 ms gap is entirely an artefact of the faster heart rate.
Pitfalls and caveats
- Bazett over-corrects at high heart rates and under-corrects at low ones. In children, who tachycardia readily (crying, fever, postural provocation), this generates a high false-positive rate for LQTS. The mean QTc difference between Bazett and Fridericia is roughly 25 ms.
- Thresholds. A widely used paediatric screening cut-off is QTc > 460 ms (≈ 2 SD above the mean in infants and young children); values > 480 ms are clearly prolonged, and > 500 ms carries higher arrhythmic risk. Borderline values warrant repeat ECGs and specialist review rather than a single label.
- Fridericia is the better screener in paediatrics because it is far less heart-rate dependent; prefer it (or Hodges) when the rate is well above 60.
- Tachycardia, sinus arrhythmia, wide QRS (use a QT-corrected approach), and U-wave contamination all distort the measurement. Never act on a single automated machine read — confirm by hand.
- A normal QTc does not exclude concealed LQTS; clinical context and genetics still matter.
Run it: Pediatric QTc Calculator
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.