Bazett formula (QTc) on ECG

What it looks like on ECG

The Bazett formula is not a separate ECG pattern, but a calculation applied to the standard QT interval. On the tracing, find the lead where the T wave is clearly demarcated from the baseline and the U wave — usually lead II or V5, but choose the lead where the end of the T wave is best visualized; if leads disagree, use the maximum reliably measured QT duration. Measure:

• QT — from the onset of the QRS complex (first deviation from the baseline) to the end of the T wave (return to baseline or intersection point with the tangent to the steepest part of the descending limb of T);
• RR — in regular rhythm, the distance between adjacent R waves preceding the measured complex, in seconds; in sinus arrhythmia or atrial fibrillation, a single cycle is not enough — measure and average QT and RR over several cycles or use a validated correction method for AF.

Bazett formula:

QTc = QT / √RR

Example: QT = 0.40 s, RR = 0.80 s (HR 75 bpm). √0.80 ≈ 0.894. QTc = 0.40 / 0.894 ≈ 0.447 s (447 ms) — within normal limits.

At HR 100 bpm (RR = 0.60 s) and the same QT = 0.40 s: √0.60 ≈ 0.775, QTc ≈ 0.516 s (516 ms) — the formula may overestimate QTc, especially during tachycardia; the 100 bpm threshold here is conventional, the error builds up gradually rather than appearing abruptly at a single heart rate value.

Many modern electrocardiographs automatically calculate QTc using Bazett's formula, but the automated QT is frequently overestimated or underestimated in the presence of low-amplitude T waves, fusion with U waves, or artifacts — manual verification in 1–2 leads is mandatory for any clinically significant result.

Mechanism

The QT interval reflects the total duration of ventricular depolarization and repolarization. QT duration is physiologically dependent on heart rate rhythm: during tachycardia, cardiomyocyte action potential shortens, and during bradycardia, it lengthens. To compare QT among different patients and at different heart rates, it is normalized to the duration of the cardiac cycle (RR). Bazett proposed dividing QT by the square root of RR — an empirical, non-physiologically derived relationship obtained from a small sample in the early 20th century. The square root of RR underestimates QT shortening at high heart rates and overestimates QT shortening at low heart rates, causing the formula to systematically distort QTc at the extremes of the heart rate range.

Normal Variants

Normal QTc values according to Bazett (common variants from various sources):

• men: up to 0.44–0.45 s (440–450 ms);
• women: up to 0.45–0.46 s (450–460 ms);
• borderline values: 0.45–0.46 s in men, 0.46–0.47 s in women.

These figures are population reference limits in general ECG practice, not diagnostic criteria for long QT syndrome. A QTc >450 ms in men and >460 ms in women is generally considered prolonged; the 450–480 ms range requires confirmation and clinical evaluation, rather than a definitive 'normal/pathology' interpretation based on a single number.

Diagnosis of congenital long QT syndrome relies on repeat measurements, clinical criteria (e.g., Schwartz score), personal and family history of syncope and sudden death, and genetic testing. For diagnosing congenital LQTS, a QTc ≥480 ms on repeat ECGs, after excluding secondary causes, is a strong diagnostic criterion; the diagnosis is established taking into account the Schwartz score, history, and family data, rather than a single measurement. A QTc of 460–479 ms is typically considered mainly in the presence of additional clinical features (symptoms, family history, genetics).

Discrepancies in figures between sources (Goldberger, Surawicz/Knilans, AHA/ACCF/HRS 2009 guidelines) are due to different studied populations and rounding; in practice, thresholds around 450 ms (men) and 460 ms (women) serve as the upper limit of normal, with caution warranted already in the 450–480 ms range depending on the clinical situation.

Pathological changes / Method limitations

The Bazett formula is least accurate precisely where QTc measurement is most needed:

• tachycardia — overestimates QTc, which may create a false impression of long QT syndrome;
• bradycardia — underestimates prolongation, potentially missing a truly prolonged QT. Bazett's formula is especially prone to overestimating QTc during tachycardia and underestimating it during bradycardia; the further the HR is from approximately 60–70 bpm, the more cautiously the result should be interpreted — the error accumulates gradually rather than appearing abruptly at a single heart rate cutoff, and its magnitude also depends on the QT measurement method and the individual QT-HR relationship in a given patient;
• atrial fibrillation and irregular rhythm — RR varies from cycle to cycle, calculation based on a single interval is unreliable, and averaging over several cycles is recommended;
• wide QRS (bundle branch block, ventricular pacing, pre-excitation) — QT prolongation is partly due to depolarization itself (wide QRS) rather than repolarization, and a simple change in the heart rate correction formula does not solve this problem. In this situation, QTc cannot be interpreted in a standard manner: it is preferable to evaluate the JT/JTc interval (from the J point to the end of T, minus the QRS duration) or use special QT correction methods adjusted for QRS width, keeping in mind the limitations and lower validation of these approaches compared to standard QTc with a narrow QRS.

In tachy- and bradycardia (without wide QRS), Fridericia's formula (cube root of RR) or other formulas (Framingham, Hodges) are preferred as they distort the result less at the extremes of the heart rate range.

Clinical significance and management

QTc is used to assess the risk of ventricular arrhythmias such as torsades de pointes, diagnose long and short QT syndromes, monitor therapy with QT-prolonging drugs (antiarrhythmics, certain psychotropics, and antibiotics), and evaluate electrolyte disturbances (hypocalcemia, hypokalemia, hypomagnesemia).

The physician should on the ECG: measure QT in the lead with the clearest T-wave boundaries, calculate QTc using Bazett's formula at a heart rate close to normal, use Fridericia's formula additionally in marked tachycardia or bradycardia, evaluate JT/JTc or specialized correction methods instead of standard QTc in wide QRS. The result must be evaluated considering sex and clinical context (medications, electrolytes, family history of syncope and sudden death). A markedly prolonged or shortened QTc is an indication to rule out secondary causes and consult a cardiologist/arrhythmologist.

Common interpretation errors

• including the U wave in the QT measurement when it fuses with T, which overestimates the value;
• calculating QTc with Bazett's formula during marked tachycardia without correction — resulting in false prolongation;
• measuring QT in a lead with a low-amplitude or biphasic T wave, where the end of the wave is difficult to determine;
• using RR from an irregular rhythm (AF) without averaging multiple cycles;
• standard interpretation of QTc in wide QRS (bundle branch block, ventricular pacing) without accounting for the depolarization contribution — in this case, JT/JTc or specialized correction methods are needed, rather than just a different HR-correction formula.

Frequently asked questions

Related terms

More in the section "Criteria, Formulas, Algorithms"

Entire section

Sources

• Goldberger's Clinical Electrocardiography, current edition
• Surawicz, Knilans «Chou's Electrocardiography in Clinical Practice»
• AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
• Orlov, V.N. «Electrocardiography Guide»

Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.