The absolute QT interval varies inversely with heart rate. A raw QT of 400ms might be normal at 60 bpm but dangerously prolonged at 100 bpm. Correction (QTc) allows for a standardized assessment across different heart rates.
Section 2
Formula & Logic
Mathematical Formula
QTc = QT / √(RR)
Where RR is in seconds (60 / Heart Rate).
Diagnostic Thresholds
Normal (Males)
≤ 440 ms
Normal (Females)
≤ 460 ms
Borderline (Males)
441 – 470 ms
Borderline (Females)
461 – 480 ms
Prolonged (High Risk)
> 500 ms (Strong TdP risk)
Section 3
Pearls/Pitfalls
The "Bazett Limitation"
Bazett is known to over-correct at HR > 100 bpm (leading to false positives for LQTS) and under-correct at HR < 60 bpm. For tachycardic patients, Fridericia or Framingham formulas are evidence-based alternatives.
Measurement Pearls
Measure in Lead II or V5-V6 where the end of the T-wave is most distinct.
Use the "Tangent Method" (Maximum Slope Method) if the T-wave end is ambiguous.
Exclude U-waves from the measurement unless they are merged with the T-wave.
Perform a repeat ECG to confirm measurement accuracy.
04
Evaluate for underlying bradycardia or heart block.
Alternative Formulas
Fridericia’s Formula (Preferred for Tachycardia)
Framingham Formula (Linear correction)
Hodges Formula
HAS-BLED (Bleeding risk)
Section 5
Evidence Appraisal
Primary Derivation
An analysis of the time-relations of electrocardiograms.
Bazett HC. • Heart.. 1920;One of the first and most enduring formulas in cardiology.
Section 6
Literature
A Century of Correction
In 1920, British physiologist Henry Cuthbert Bazett published his study "An analysis of the time-relations of electrocardiograms." Working in an era before automated ECG interpretation, Bazett sought a simple mathematical way to adjust the QT interval for varying heart rates. Despite modern evidence showing it is less accurate than newer linear models, his square-root formula remains the most widely used and recognized QTc method in clinical practice worldwide.
Henry Cuthbert Bazett (1885–1950)
A pioneer in applied physiology, Bazett was a leader in temperature regulation and aviation medicine. He was known for performing physiological experiments on himself to understand human blood flow and heat loss. His formula for the corrected QT was a side project in his broader study of cardiovascular timing.
Historical Timeline
01
1920 — Henry Bazett publishes the square-root formula.
02
1948 — Fridericia proposes the cube-root formula to fix Bazett’s tachycardic errors.
03
1960s — Bazett’s formula is hardcoded into automated ECG machines.
04
2005 — FDA mandates "Thorough QT" studies for all new drugs.
05
Present — Guidelines maintain Bazett as a common clinical reference despite known flaws.
