Recommended QT Interval Values
The normal corrected QT interval (QTc) is less than 450 ms in men and less than 460 ms in women, with values at or above these thresholds considered prolonged and requiring clinical evaluation. 1, 2
Gender-Specific Normal Values
- Men: QTc <450 ms is normal; QTc ≥450 ms is prolonged 1, 2, 3
- Women: QTc <460 ms is normal; QTc ≥460 ms is prolonged 1, 2, 3
- Gender differences emerge during adolescence (ages 12-16 years), with women having QTc intervals approximately 8-15 ms longer than men 1, 2
- This gender difference diminishes after age 40 and essentially disappears in older adults 1
Short QT Interval Thresholds
- Both genders: QTc ≤390 ms is considered abnormally short 1, 2, 3
- For short QT syndrome diagnosis, uncorrected QT values of 330 ms (310 ms in children) or QTc between 360-380 ms have been proposed 1, 2
- Symptomatic short QT syndrome patients typically have QT ≤320 ms (QTc ≤340 ms) 1
Pediatric Values
- In children younger than 12 years, the 98th percentile limit for QTc is approximately 450 ms without gender differentiation 1, 2
- Gender differences of approximately 8 ms appear in the 12-16 year age group 2
Risk Stratification for Prolonged QTc
The clinical significance of QT prolongation follows a severity gradient:
- QTc 450-480 ms (men) or 460-480 ms (women): "Grey zone" requiring detailed assessment including family history, symptoms (syncope), and consideration of transient causes 1
- QTc >480 ms: Increased arrhythmic risk requiring investigation 1, 2
- QTc ≥500 ms: Strongly suggestive of long QT syndrome regardless of family history or symptoms, with significantly elevated risk of Torsades de Pointes 1, 4
- QTc increase >60 ms from baseline: Clinically significant change associated with increased Torsades de Pointes risk 2, 3
Critical Measurement Considerations
Lead selection and measurement technique significantly impact QT values:
- Measure QT in leads II, V3, or V5; use the longest value obtained 1, 2
- If the longest QT differs by >40 ms from adjacent leads, the measurement may be erroneous and should be rechecked 1
- When T and U waves are superimposed, measure in leads without U waves (often aVR or aVL), or use the tangent method on the steepest T-wave downslope 1, 5
Heart rate correction is essential but has important limitations:
- Bazett's formula (QTc = QT/√RR) overcorrects at heart rates >90 bpm and undercorrects at <50 bpm 2, 5, 6
- Linear regression functions or Fridericia's formula (QTc = QT/∛RR) are more accurate, especially at extreme heart rates 1, 2, 3
- QT correction is unreliable at heart rates <40 bpm or >120 bpm 1, 2
- Do not attempt QT correction with large RR variability (e.g., atrial fibrillation) 1, 3
Common Clinical Pitfalls to Avoid
- Relying on automated measurements without visual validation: Computer algorithms frequently misidentify the T-wave end, particularly with T-U fusion 1, 2, 5
- Ignoring gender differences: Using a single cutoff (e.g., 440 ms) will overdiagnose prolongation in women and underdiagnose it in men 1, 2
- Not accounting for QRS duration: Ventricular conduction delays prolong QT; consider using JT interval (QT minus QRS duration) in bundle branch blocks 1, 3
- Overlooking reversible causes: Hypokalemia, hypomagnesemia, hypocalcemia, and QT-prolonging medications are common acquired causes that must be excluded before diagnosing congenital long QT syndrome 1, 2
- Misinterpreting athlete ECGs: Athletes have longer uncorrected QT intervals due to bradycardia, but their QTc should remain within normal limits 1, 2
Special Population: Athletes
- Athletes typically have longer uncorrected QT intervals due to training-induced bradycardia 1
- Their QTc values should still fall within normal limits, though often toward the upper range 1
- Athletes with QTc 440-500 ms (men) or 460-500 ms (women) require comprehensive evaluation including family history, exercise testing, and consideration of deconditioning to differentiate physiologic adaptation from pathologic long QT syndrome 1