Normal QT 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, 2
Gender-Specific Normal Values
The QT interval must be corrected for heart rate (QTc) to provide standardized values, and these values differ by gender after puberty:
- Men: Normal QTc <450 ms; prolonged ≥450 ms 1, 2, 3
- Women: Normal QTc <460 ms; prolonged ≥460 ms 1, 2, 3
- Short QT: ≤390 ms in both men and women 1, 2
The gender difference emerges during adolescence (12-16 years), with women having QTc values approximately 8-15 ms longer than men in younger adults, narrowing to 6-10 ms in older age groups. 1, 3 This difference practically disappears after age 40. 1, 3
Pediatric Values
- Children <12 years: The 98th percentile limit is approximately 450 ms without gender differentiation 1, 2
- Gender differences appear in the 12-16 year age group with approximately 8 ms separation 2
Critical Measurement Considerations
Heart Rate Correction Methods
Use Fridericia's formula (QTc = QT/∛RR) rather than Bazett's formula, especially at heart rates >80 bpm. 1, 3 Bazett's formula significantly overcorrects at higher heart rates and undercorrects at lower heart rates (<60 bpm), leading to false prolongations in up to 30% of normal ECGs when using traditional cutoffs. 1, 4
Linear regression functions are recommended over Bazett's formula by the American Heart Association and American College of Cardiology. 1, 3
Measurement Technique
- Measure QT in leads II, V3, or V5, using the longest value 2
- The beginning of the QT interval is the first deflection from the isoelectric line after the P-wave 1
- The end of the T-wave is identified by the intercept between the steepest tangent at the descending part of the T-wave and the isoelectric line 1
- Do not attempt rate correction when RR interval variability is large (e.g., atrial fibrillation) 1, 3
- Corrections are inaccurate at heart rates <40 bpm or >120 bpm 1, 2
Risk Stratification Thresholds
FDA Severity Levels
A QTc >500 ms carries the highest risk for torsades de pointes, regardless of family history or symptoms. 1, 5 An increase of >60 ms from baseline is clinically significant and warrants immediate attention. 2, 5
Common Pitfalls to Avoid
The "Gray Zone" Problem (440-470 ms in men, 460-480 ms in women)
QTc values in this range represent considerable overlap between affected individuals and controls. 1 These require detailed assessment including:
- Careful family history with cascade ECG screening of family members 1
- 24-hour ECG monitoring for serial evaluation 1
- Exercise testing to assess QT shortening during effort 1
Emergency Department Context
First-time ECGs obtained after syncope must be interpreted with extreme caution. 6 In pediatric ED patients, approximately one-third exhibit QTc ≥440 ms, but follow-up ECGs show significant normalization with decreases of 33±43 ms. 6 Only pursue long QT syndrome diagnosis if QTc remains elevated on repeat testing in a calm, non-acute setting.
Athlete Considerations
Athletes have longer uncorrected QT intervals due to lower resting heart rates from athletic training, but their QTc values should be within normal limits, typically toward the upper limit. 1, 2 Do not diagnose long QT syndrome based solely on a single borderline value in an athlete.
Bundle Branch Block
QT interval prolongs in ventricular conduction defects due to delayed depolarization, not repolarization abnormality. 5 Adjustment for QRS duration is necessary, either by incorporating QRS duration as a covariate or using the JT interval (QT duration minus QRS duration). 1, 3
Transient Causes of QT Prolongation
Before diagnosing congenital long QT syndrome, exclude: