Formula & Calculator
Corrected QT Interval (Bazett's Formula)
Adjusts a heart's QT interval (from an ECG) for heart rate, since the raw QT interval naturally shortens as heart rate increases.
Interpretation
QTc = QT / √(RR). Corrects QT interval for heart rate. Used to assess risk of arrhythmias (long QT syndrome). Essential in cardiology and drug safety.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| QTc | Corrected QT interval | s |
| QT | Measured QT interval | s |
| RR | RR interval (time between heartbeats) | s |
What it means
The QT interval on the electrocardiogram (ECG) varies with heart rate. Bazett’s formula corrects it to a standard heart rate of 60 bpm. QTc = QT / √(RR), where RR is the interval between R waves (in seconds). Prolonged QTc (typically >440 ms in men, >460 ms in women) is associated with increased risk of torsade de pointes, a life‑threatening arrhythmia. It is used in cardiology, in medication monitoring (many drugs prolong QT), and in assessing syncope. Understanding QTc is essential for clinicians and for drug development to ensure safety.
Worked example
Corrected QT Interval (Bazett) – Two Detailed Examples
Real‑World| Parameter | Value |
|---|---|
| QT (s) | 0.4 |
| RR (s) | 1.0 |
| Parameter | Value |
|---|---|
| QT | 0.38 |
| RR | 0.8 |
Common mistakes
- Bazett’s formula: QTc = QT / √(RR), where RR is the interval in seconds.
- QT: The measured QT interval – in seconds or milliseconds.
- RR: The interval between consecutive R waves – in seconds (or use heart rate: RR = 60/HR).
- Units: Be consistent – if QT in ms, RR in seconds.
- Correction: Bazett overcorrects at high heart rates – other formulas (Fridericia) may be preferred.
Applications
The corrected QT interval (QTc) using Bazett's formula (QTc = QT / √RR) adjusts the measured QT interval for heart rate, allowing standardised assessment of ventricular repolarisation. An abnormally prolonged QTc (>450 ms in men, >470 ms in women) is a risk factor for torsade de pointes, a life‑threatening arrhythmia. Clinicians, especially cardiologists and emergency physicians, use QTc to evaluate patients with syncope, palpitations, or on medications that may prolong QT. By calculating QTc, healthcare providers can identify those at risk and adjust therapy. This formula is essential in cardiac monitoring and pharmacovigilance.
- Cardiac risk assessment in patients with syncope or arrhythmias
- Monitoring of drug safety (QT‑prolonging medications)
- Diagnosis of congenital long QT syndrome
- Pre‑operative cardiac evaluation
- Research on cardiac electrophysiology
Frequently Asked Questions
It adjusts the measured QT interval (from ECG) for heart rate: QTc = QT / √(RR), where RR is the interval between R waves (in seconds).
The raw QT interval shortens as heart rate increases. Correction allows comparison of QT values across different heart rates.
Normal QTc is ≤ 440 ms in men and ≤ 460 ms in women. Prolonged QTc (> 500 ms) is a risk factor for arrhythmias.
A patient has QT = 400 ms, RR = 800 ms (heart rate ~75 bpm). QTc = 400 / √0.800 = 400 / 0.894 = 447 ms.
- It over‑corrects at fast heart rates and under‑corrects at slow heart rates.
- It is less accurate than Fridericia's formula (QTc = QT / RR^(1/3)) for extremes.
- Applying Bazett's correction at very fast or very slow heart rates, where it is known to be less accurate.
- Using RR in milliseconds instead of seconds (should be in seconds).
- Not ensuring the QT measurement is accurate (e.g., correct lead).
It increases the risk of torsade de pointes, a life‑threatening arrhythmia. Certain drugs can prolong QTc.
Fridericia's formula is QTc = QT / RR^(1/3). It is more accurate at extreme heart rates and is now preferred in many settings.
QT is the measured interval; QTc is the corrected version. Only QTc should be compared across patients.
It is measured from one R wave to the next on the ECG. It is the inverse of heart rate (in seconds).