Normal QTc Interval Values
A normal QTc interval is less than 450 ms in men and less than 460 ms in women, with values ≤390 ms considered abnormally short in both sexes. 1
Gender-Specific Normal Ranges
The definition of normal QTc intervals must account for gender differences that emerge during adolescence:
- Men: Normal QTc < 450 ms 1, 2
- Women: Normal QTc < 460 ms 1, 2
- Short QT interval: ≤390 ms in both men and women 1
The gender difference ranges from 12-15 ms in younger adults and decreases to 6-10 ms in older age groups, becoming negligible after age 40. 1 This difference appears during adolescence when testosterone causes QT shortening in boys while girls experience minimal change. 1
Pediatric Considerations
For children younger than 12 years, the 98th percentile limit is approximately 450 ms without gender differentiation. 1 The gender difference of approximately 8 ms emerges in the 12-16 year age group. 1
Critical Thresholds for Risk Stratification
Beyond simple normal/abnormal cutoffs, specific QTc values carry different clinical implications:
- QTc > 500 ms: Associated with significantly increased risk of Torsades de Pointes, a potentially life-threatening ventricular arrhythmia 2, 3
- QTc 480-500 ms: Moderate risk requiring careful monitoring 1, 2
- QTc increase > 60 ms from baseline: Clinically significant change warranting intervention 2
The FDA recommends reporting three severity levels when evaluating QT-prolonging effects: >350 ms, >480 ms, and >500 ms. 1
Measurement Method Matters
The correction formula used significantly impacts whether a QTc appears normal or abnormal. Bazett's formula (QTc = QT/√RR), while most commonly used, overcorrects at heart rates >80 bpm and undercorrects at rates <60 bpm. 2, 3, 4 This can result in falsely prolonged QTc values, particularly problematic in clinical practice. 5, 6
Linear regression functions or Fridericia's formula (QTc = QT/∛RR) provide more accurate rate correction and are recommended over Bazett's formula. 1, 2, 7 Using Bazett's formula with a 440 ms threshold would incorrectly flag 30% of normal ECGs as abnormal, compared to <2% with other formulae. 6
Common Pitfalls to Avoid
- Ignoring gender differences: Applying a single cutoff to both sexes will misclassify women with borderline values 2, 7
- Misinterpreting athlete ECGs: Athletes often have longer uncorrected QT intervals due to lower resting heart rates, but their QTc should remain within normal limits 2
- Using Bazett's formula at extreme heart rates: QT correction becomes inaccurate at heart rates <40 bpm or >120 bpm 2
- Not adjusting for bundle branch blocks: Ventricular conduction defects prolong the QT interval and require adjustment for QRS duration 1
- Attempting correction with irregular rhythms: Rate correction should not be attempted when RR interval variability is large, such as in atrial fibrillation 1, 7
Practical Clinical Application
When evaluating QTc intervals, measure QT in leads II, V3, or V5, selecting the longest value. 2 Ensure the T-wave amplitude is at least 2 mm with a well-defined end point. 7 For serial comparisons, use the same ECG recorder and reader to minimize variability. 2
Electrolyte abnormalities (particularly hypokalemia, hypomagnesemia, and hypocalcemia) can affect QTc values and should be corrected before attributing prolongation solely to other causes. 2