QTc vs QTcB: Understanding the Difference
QTcB is simply QTc calculated using Bazett's formula (QT/√RR), while QTc is a general term referring to any heart rate-corrected QT interval that can be calculated using multiple different formulas. 1, 2
Key Definitions
QTc (Corrected QT interval): A generic term for the QT interval adjusted for heart rate, which can be calculated using various correction formulas 1
QTcB (Bazett's corrected QT): Specifically refers to QTc calculated using Bazett's formula: QT interval divided by the square root of the RR interval (in seconds) 1, 2, 3
Why This Distinction Matters Clinically
The American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society explicitly recommend that linear regression functions rather than Bazett's formula be used for QT-rate correction, and that the method used for rate correction be identified in ECG analysis reports. 1
Critical Limitations of Bazett's Formula (QTcB)
Bazett's formula leaves a strong positive residual correlation (r=0.32) with heart rate, meaning it systematically overcorrects at fast heart rates (>90 bpm) and undercorrects at slow heart rates (<50 bpm) 1, 3, 4
The adjusted QT values may be substantially in error, particularly when the heart rate is high 1
Despite being the most widely used formula since 1920, Bazett's formula was derived from only 39 young subjects and has significant mathematical flaws 1
In a study of 44,566 ECG reports, QTcB identified 37.4% as prolonged, but 21% of these corrected to normal when using the Fridericia formula instead, representing a clinically significant false-positive rate 5
When QTcB Still Has Clinical Utility
Paradoxically, despite its mathematical limitations, Bazett's formula demonstrated the highest sensitivity (23.07%) for cardiac mortality prediction in a large cohort study 6. This suggests that while QTcB may overcorrect and appear falsely prolonged, this overcorrection may actually capture higher-risk patients more effectively in some clinical contexts 6.
Alternative Correction Formulas
More recently introduced formulas for QT adjustment as linear or power functions of RR or heart rate effectively remove the rate dependence of the adjusted QT, and they are clearly preferable to both Bazett's and Fridericia's formulas. 1
Fridericia's Formula (QTcFri)
- Uses the cube root of RR: QT/∛RR 1
- Leaves a negative correlation (r=-0.26 to -0.32) with heart rate, but performs better than Bazett's at extreme heart rates 1, 4
- Demonstrated the smallest slopes of QTc/RR analysis in males and was identified as the best mathematical formula 6
- Many modern ECG systems (including Epic EHR) now auto-populate QTcFri instead of QTcB 5
Other Superior Formulas
- Hodges, Karjalainen-Nomogram, Sagie-Framingham, and Rautaharju formulas all show less heart rate dependence than Bazett's 1, 4
- Linear regression functions are recommended as the gold standard 1, 3
Practical Clinical Recommendations
For Routine Clinical Practice
Always visually validate QT-interval prolongation reported by a computer algorithm, regardless of which formula is used 1, 2, 7. Automated measurements can be inaccurate, and cardiac monitors do not have algorithms to measure QT intervals 7.
When Measuring QT Intervals
- Measure in leads II, V5, and V6, using the longest value 2
- Use the tangent method: draw a tangent to the steepest downslope of the T wave and define the intersection with the baseline 2, 3
- Do not attempt rate correction when RR interval variability is large (as in atrial fibrillation) or when the end of the T wave is unreliable 1, 3
For Extreme Heart Rates
- For heart rates <50 bpm, perform mild aerobic activity to achieve a heart rate closer to 60 bpm before measuring 3
- For heart rates >90 bpm, allow additional resting time to achieve a lower heart rate 3
- Bazett's formula should be avoided at these extremes 3, 8
Normal Values (Regardless of Formula Used)
As practical clinical limits, a QTc ≥460 ms in women and ≥450 ms in men should be considered prolonged, and QTc ≤390 ms should be considered short 1, 2, 3. However, QTc >500 ms represents markedly elevated risk for torsades de pointes regardless of sex 7.
Common Clinical Pitfalls
- Failing to use the same lead for serial QT measurements in the same patient (QT length varies across the 12 leads) 2, 7
- Not knowing which formula your ECG machine uses - many machines use Bazett's without clearly identifying it on the printout, which can cause unnecessary concern 8, 5
- Including discrete U waves that arise after the T wave has returned to baseline in the QT measurement 2
- Not accounting for QRS prolongation - in bundle branch block, either subtract the increased QRS length or measure the JT interval instead 2