Question 1

Bazett or Fridericia — which formula should I use?

Accepted Answer

Bazett is the historical default and is what most monitors display, but it overcorrects at fast heart rates and undercorrects at slow ones, so it is least reliable at the rate extremes. Fridericia (and the Framingham correction) stay much more rate-independent, and drug-safety review under ICH E14 / FDA treats Fridericia as the preferred correction. A practical rule: near a heart rate of 60 bpm the formulas agree (at exactly 60 bpm QTc = QT) and differences are modest across about 60–100 bpm; outside that range, favor Fridericia. This calculator shows both so you can compare them directly.

Question 2

Why is Bazett inaccurate at very high or very low heart rates?

Accepted Answer

Bazett corrects with the square root of the RR interval, which over-adjusts when RR is short (fast heart rate) and under-adjusts when RR is long (slow heart rate). The result is that Bazett tends to overestimate QTc during tachycardia and underestimate it during bradycardia. Because both errors are largest exactly where QTc decisions matter most, a Bazett value near a threshold should be rechecked with Fridericia. In one study, automatic Bazett-corrected QT detected only about half of truly prolonged cases, while Fridericia identified essentially all of them.

Question 3

What does a QTc above 500 ms mean?

Accepted Answer

A QTc of 500 ms or more is markedly prolonged and is the threshold most consistently associated with a high risk of torsades de pointes — a polymorphic ventricular tachycardia that can be fatal — regardless of sex. Risk rises roughly 5–7% for every additional 10 ms above normal. A value this high is a flag to look for and correct reversible contributors (electrolytes, contributing drug classes, bradycardia) and to monitor closely; this calculator interprets the number but does not direct management.

Question 4

Why are the prolongation thresholds different for men and women?

Accepted Answer

Women have a slightly longer baseline QTc than men after puberty, so sex-specific cutoffs avoid over-calling prolongation in women and under-calling it in men. Using Goldenberg et al. (2006), QTc is prolonged above ~450 ms in men and ~470 ms in women, with a borderline band just below each. Exact cutoffs vary by source (the AHA/ACCF 2009 99th-percentile values are higher), but the ≥500 ms high-risk line is the same for both sexes.

Question 5

How do I measure QT without including the U wave?

Accepted Answer

Measure from the start of the QRS complex to the end of the T wave, ideally in lead II or V5. Use the tangent method: draw a tangent along the steepest part of the T wave's downslope and take the QT endpoint where that line crosses the baseline. A U wave (a small deflection after the T wave) should be excluded — unless a large U wave (>1 mm) is fused to the T wave, in which case it is included. As a quick check, a normal QT is less than half the preceding RR interval.

Question 6

What causes QT prolongation?

Accepted Answer

Common contributors are electrolyte abnormalities (low potassium, magnesium, or calcium), congenital long QT syndrome, bradycardia, and several drug classes — including certain antiarrhythmics, some antibiotics (macrolides, fluoroquinolones), antipsychotics, and antiemetics. The CredibleMeds.org registry maintains the reference list of QT-prolonging drugs by risk category. Identifying and correcting reversible causes, especially electrolytes, comes before attributing prolongation to other factors.

Category	Men (ms)	Women (ms)
Normal	≤ 430	≤ 450
Borderline	431 – 450	451 – 470
Prolonged	> 450	> 470
Markedly prolonged	≥ 500 (either sex) — strongly associated with torsades de pointes

QTc Calculator (Bazett, Fridericia)

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