QTc Interval Calculation Methods
The most widely used formula for QT correction is Bazett's formula (QTc = QT/√RR), but linear regression functions are recommended over Bazett's formula for more accurate QT-rate correction. 1, 2
Measurement of QT Interval
- The QT interval should be measured from the beginning of the QRS complex to the end of the T wave 2
- The end of the T wave is best identified using the "Teach-the-Tangent" or "Avoid-the-Tail" method: draw a tangent to the steepest downslope of the T wave and define the intersection of this line with the baseline 1, 2
- Leads II, V3, or V5 are recommended for QT measurement, with the longest value being considered 1, 2
- Discrete U waves that arise after the T wave returns to baseline should not be included in the QT interval measurement 2
QT Correction Formulas
Common Formulas:
- Bazett's formula: QTc = QT/√RR (RR in seconds) - most widely used clinically despite limitations 2, 3
- Fridericia's formula: QTc = QT/∛RR - performs better than Bazett's, especially at high heart rates 1, 3
- Framingham formula: QTc = QT + 0.154 × (1000 - RR) 4, 5
- Hodges formula: QTc = QT + 105 × (1/RR - 1) 4
Limitations of Bazett's Formula:
- Overestimates QTc at heart rates >90 beats/min 1, 6
- Underestimates QTc at heart rates <50 beats/min 1
- Leaves a strong positive residual correlation with heart rate (r≈0.32) 1
Practical Application
- For heart rates <50 beats/min, perform mild aerobic activity to achieve a heart rate closer to 60 beats/min before measuring 1
- For heart rates >90 beats/min, allow additional resting time to achieve a lower heart rate 1
- If sinus arrhythmia is present, use an average QT interval and average RR interval 1
- QT correction should not be attempted when RR interval variability is large (as in atrial fibrillation) or when identification of the T wave end is unreliable 1
- Always visually validate computer-reported QT interval prolongation 1, 2
Normal QTc Values
- Normal QTc: ≤460 ms in women, ≤450 ms in men 1, 2
- Prolonged QTc: >460 ms in women, >450 ms in men 1
- Some guidelines suggest higher thresholds: ≥470 ms in male athletes and ≥480 ms in female athletes 1
- QTc ≥500 ms correlates with higher risk for torsades de pointes 2
- Short QTc: <390 ms in both men and women 1
Common Pitfalls to Avoid
- Failing to use the same lead for serial QT measurements in the same patient 2
- Including U waves in the QT interval measurement when they are discrete 2
- Not accounting for QRS prolongation when measuring QT interval 2
- Relying solely on automated measurements without visual validation 1, 2
- Using Bazett's formula for patients with very fast or slow heart rates 1, 6
Remember that QT interval is modulated by gender, with women typically having longer QTc intervals than men, though this difference diminishes after age 40 1. The gender difference appears during adolescence when the rate-adjusted QT shortens in boys but undergoes little change in girls 1.