Normal QTc Interval Range for Adults
The normal QTc interval is <450 ms in men and <460 ms in women, with values ≥450 ms in men and ≥460 ms in women considered prolonged, and QTc ≤390 ms in both sexes considered short. 1
Gender-Specific Normal Values
- Adult men: Normal QTc is <450 ms; prolonged QTc is ≥450 ms 1, 2
- Adult women: Normal QTc is <460 ms; prolonged QTc is ≥460 ms 1, 2
- Short QT interval: ≤390 ms in both men and women 1, 2
Understanding Gender Differences
- Women have longer QTc intervals than men, with differences of 12-15 ms in younger adults and 6-10 ms in older age groups 1
- This gender difference emerges during adolescence when testosterone causes QT shortening in males 1
- The gender difference becomes minimal after age 40 and practically disappears in older adults 1
Critical Thresholds for Clinical Decision-Making
- QTc >500 ms: Highly abnormal with significantly increased risk of torsades de pointes, a potentially life-threatening arrhythmia 2, 3
- QTc 480-500 ms: Represents the 99th percentile threshold (480 ms for women, 470 ms for men) and warrants heightened monitoring 3
- QTc increase >60 ms from baseline: Clinically significant change that requires intervention regardless of absolute value 2, 3
Important Measurement Considerations
- Linear regression functions are strongly preferred over Bazett's formula for QT rate correction, as Bazett's systematically overcorrects at heart rates >85 bpm, producing falsely prolonged values 1, 4, 5
- Fridericia's formula (QTc = QT/∛RR) provides more accurate rate correction, especially at higher heart rates 2, 3, 5
- Using Bazett's formula can result in 30% of normal ECGs being incorrectly labeled as abnormal when using a 440 ms threshold 4
- The correction formula used should always be identified in ECG reports 1
Common Pitfalls to Avoid
- Do not attempt QT correction when RR interval variability is large (e.g., atrial fibrillation) or when the T-wave end is unreliable 1
- Adjust for QRS duration in ventricular conduction defects, as bundle branch blocks artificially prolong the QT interval due to delayed depolarization, not repolarization abnormality 1, 3
- Recognize that automated ECG algorithms often use Bazett's formula, which can lead to overdiagnosis of QT prolongation, particularly at higher heart rates 5, 6
- Select appropriate leads for measurement: Choose leads with T-wave amplitude ≥2 mm and well-defined T-wave end 2
Age-Specific Considerations
- In children <12 years, the 98th percentile limit is approximately 450 ms without gender differentiation 1
- Gender differences of approximately 8 ms appear in the 12-16 year age group 1
- More refined age- and sex-specific criteria exist, with upper normal limits increasing slightly with age in both sexes 7