Normal QTc Interval Values
Normal QTc values are <450 ms in men and <460 ms in women, with values ≤390 ms considered short QT in both sexes. 1, 2, 3
Gender-Specific Thresholds
- Men: Normal QTc is <450 ms; prolonged QTc is defined as ≥450 ms 1, 2, 3
- Women: Normal QTc is <460 ms; prolonged QTc is defined as ≥460 ms 1, 2, 3
- Short QT interval: ≤390 ms in both men and women 1, 3
The 10-ms difference between male and female thresholds reflects a physiologic testosterone-mediated shortening of the QT interval in males during adolescence, while females experience minimal change. 1, 3 This gender gap of approximately 12–15 ms in younger adults narrows to 6–10 ms in middle age and essentially disappears after age 70. 1, 4
Age-Related Considerations
- Children <12 years: The 98th percentile limit is approximately 450 ms without gender differentiation 1, 3
- Adolescents (12-16 years): A gender difference of approximately 8 ms emerges during this period 1, 4
- Adults <40 years: Women's QTc is 12–15 ms longer than men's 4
- Adults 40-69 years: The gender gap narrows to 6–10 ms 4
- Adults ≥70 years: Gender differences become negligible 1, 4
Clinical Risk Stratification
High-risk thresholds that warrant immediate clinical action include:
- QTc >500 ms: Markedly increased risk of torsades de pointes; discontinue offending medications immediately 1, 2, 5
- Increase >60 ms from baseline: Clinically significant regardless of absolute QTc value 1, 2, 5
- QTc 480-500 ms (Grade 2): Moderate prolongation requiring aggressive electrolyte correction and increased monitoring 2
- QTc 450-480 ms (Grade 1): Borderline prolongation requiring identification of reversible causes 2
The FDA recommends reporting three severity levels when evaluating QT-prolonging drug effects: >350 ms, >480 ms, and >500 ms. 1, 4
Measurement Technique and Common Pitfalls
Correction formula selection is critical:
- Fridericia's formula (QTc = QT/∛RR) is preferred, especially at heart rates >80 bpm 2, 3, 6
- Bazett's formula (QTc = QT/√RR) systematically overcorrects at heart rates >80-90 bpm and undercorrects at heart rates <60 bpm, producing falsely elevated QTc values 1, 3, 7, 6
- Linear regression formulas (Framingham, Hodges) effectively remove heart-rate dependence and are superior to both Bazett's and Fridericia's formulas 3, 7
Critical measurement caveats:
- Measure QT from the onset of QRS to the end of the T wave in leads II, V3, or V5 where the T wave is most distinct 1, 3
- Identify the T-wave end by drawing a tangent to the steepest downslope and marking its intersection with the baseline 2, 3
- Exclude discrete U waves that appear after the T wave returns to baseline 3
- QT correction is unreliable with large RR-interval variability (e.g., atrial fibrillation) 1, 4
- In bundle-branch block, adjust for prolonged QRS duration or use the JT interval (QT minus QRS) 1, 4
Automated ECG measurements should never be used alone—visual verification is essential because many devices default to Bazett's formula, which can overestimate QTc at higher heart rates. 3 Using Bazett's formula with a 440-ms threshold would incorrectly flag 30% of normal ECGs as abnormal. 7