How do I interpret a Bazett-corrected QT interval (QTcB) on a standard 12‑lead ECG?

How to Interpret QTcB (Bazett-Corrected QT Interval)

Direct Answer

Do not rely on QTcB (Bazett's formula) as your primary method for QT interval interpretation—use linear regression-based correction formulas instead, and always visually validate any automated QTcB measurement before making clinical decisions. 1

Understanding QTcB and Its Fundamental Limitations

QTcB refers specifically to the QT interval corrected using Bazett's formula (QT ÷ √RR interval in seconds). 1 While this remains the most widely recognized correction method, it has critical mathematical flaws that compromise clinical accuracy:

• Bazett's formula systematically overcorrects at heart rates >90 bpm and undercorrects at heart rates <50 bpm, leading to false-positive prolongation at high rates and missed prolongation at low rates. 2, 1
• The formula retains a strong positive residual correlation with heart rate (r = 0.32), meaning the "corrected" value still varies inappropriately with heart rate. 1
• The American Heart Association, American College of Cardiology, and Heart Rhythm Society issue a Class I guideline recommendation against using Bazett's formula—they explicitly recommend linear regression-based correction functions instead. 1

When QTcB Values Are Particularly Unreliable

Avoid interpreting QTcB in these specific clinical scenarios:

• Heart rates >90 bpm: QTcB will artificially inflate the corrected value, potentially triggering unnecessary interventions. 2, 3 In one study, QTcB overestimated drug-induced QT changes by clinically significant margins at elevated heart rates. 3
• Heart rates <50 bpm: QTcB will underestimate true QT prolongation, potentially missing dangerous prolongation. 2, 1
• Atrial fibrillation with beat-to-beat variability: Bazett's formula shows the most imprecision in AF compared to other formulas. 4
• Large RR interval variability: Rate correction should not be attempted when RR variability is substantial. 2, 1

Practical Approach When You Encounter QTcB

If you must work with a QTcB value (e.g., from an automated ECG report), follow this algorithm:

1. Check the heart rate first: 2

   • If HR is 50-90 bpm, QTcB may be reasonably accurate
   • If HR <50 bpm: Consider having the patient perform mild aerobic activity to raise HR closer to 60 bpm before remeasuring 2
   • If HR >90 bpm: Allow additional resting time to lower HR before remeasuring 2

2. Always visually validate the automated QTcB measurement: 1, 5

   • Measure the QT interval manually from the beginning of the QRS to the end of the T wave 2
   • Use the "tangent method": draw a tangent to the steepest downslope of the T wave where it intersects the baseline 2, 1
   • Measure in leads II, V5, or V6, using the longest value 1

3. Apply appropriate normal values: 2, 1

   • Normal QTcB: ≤450 ms in men, ≤460 ms in women
   • High-risk threshold: ≥500 ms in either sex (associated with torsades de pointes risk) 1, 5

4. Consider recalculating with a superior formula: 1, 6

   • Fridericia's formula (QT ÷ RR^1/3) performs better than Bazett's at extreme heart rates 1, 7
   • Linear regression formulas (Framingham, Hodges) eliminate heart-rate dependence entirely 1, 3
   • In one hospital study, switching from QTcB to Fridericia reduced false-positive prolonged QTc reports by 21% and reduced QTc >500 ms reports by 57%. 6

Critical Pitfalls to Avoid

• Never rely solely on computer-generated QTcB without visual confirmation—automated algorithms vary in accuracy and may include U waves or misidentify T-wave termination. 8, 1, 5
• Do not use different leads for serial QTc measurements in the same patient—QT length varies across the 12 leads, making comparisons invalid. 1, 5
• Do not include discrete U waves that arise after the T wave returns to baseline in your QT measurement. 1
• Account for bundle branch block: If QRS >120 ms, subtract the increased QRS duration from the QT interval or measure the JT interval instead. 1, 5
• Do not intermix correction formulas—Bazett-corrected values cannot be compared with Fridericia- or Framingham-corrected measurements. 8

Special Exception: Long QT Syndrome

In genetically confirmed LQTS type 1 and 2 patients, Bazett's formula remains the preferred method because it was used in the diagnostic and prognostic studies that established QTc thresholds for these populations. 9 In this specific context, Bazett's formula is the only method that eliminates the inverse QT-HR relationship in LQTS patients, and switching formulas would invalidate established diagnostic criteria. 9

Measurement Technique Details

When you must measure and interpret QTcB yourself:

• Identify the end of the T wave using the tangent method on the steepest downslope, not the "tail" of the T wave. 2
• If the T wave is notched, consider the end of the entire complex as the T-wave termination. 1
• Ensure T-wave amplitude is ≥2 mm for reliable measurement. 5
• Use the TP segment as your isoelectric baseline reference. 5
• Measure from the earliest QRS onset to the latest T-wave end across all leads if performing global interval measurement—this will be 30-40 ms longer than single-lead measurements. 8