Normal QT Interval Values
The normal QT interval corrected for heart rate (QTc) is <450 ms in men and <460 ms in women after puberty, with values ≥470 ms in men and ≥480 ms in women considered definitively prolonged. 1
Sex and Age-Specific Normal Values
The QTc interval shows critical sex and age variations that must be considered:
Adults (Post-Puberty)
- Men: Normal <450 ms; borderline 450-469 ms; prolonged ≥470 ms
- Women: Normal <460 ms; borderline 460-479 ms; prolonged ≥480 ms
- Both sexes: QTc ≥500 ms significantly increases risk of torsades de pointes 1
The 2023 Canadian guidelines provide the most current consensus, noting that the 99th percentile is 470 ms for men and 480 ms for women 1. These values align with the 2009 AHA/ACCF/HRS recommendations that established 450 ms for men and 460 ms for women as upper normal limits 2.
Children and Adolescents
- Pre-puberty: QTc <440 ms is normal for both sexes
- Post-puberty (age 12-14 years): Girls develop longer QTc values than boys, requiring sex-specific thresholds
- Infants: QTc <460 ms during first year of life 3
The gender difference emerges during adolescence due to hormonal changes, with testosterone shortening the QTc in boys while girls maintain longer intervals 2.
Short QT Intervals
A QTc ≤390 ms is considered abnormally short 2. However, the clinical significance varies:
- QTc <350 ms warrants investigation for short QT syndrome
- QTc <330 ms requires exclusion of short QT syndrome 3
Critical Measurement Considerations
Heart Rate Correction
Linear regression formulas are preferred over Bazett's formula for QT-rate correction, though Bazett's remains most commonly used in clinical practice 2, 1. Do not attempt rate correction with irregular rhythms (e.g., atrial fibrillation) or when the T-wave end is unreliable 2.
Measurement Technique
Use the tangent method in leads II or V5: draw a tangent at the maximum downslope of the T wave to where it intersects the baseline 1. Automated measurements are frequently inaccurate with irregular rhythms, wide QRS complexes, or abnormal T waves 1.
Common Pitfalls
Overlap with pathology: There is significant overlap between the upper limit of normal QTc distribution and the lower limit in patients with long QT syndrome 1
Post-cardiac arrest: QTc can be prolonged for several days after resuscitation due to cooling and myocardial injury—serial ECGs are required to avoid overdiagnosis 1
Context matters: QTc prolongation alone is insufficient for diagnosis of long QT syndrome without clinical correlation (unexplained syncope, seizures, family history of sudden death) 1
Population variations: Recent data from 79,743 subjects showed the 2nd-98th percentile range for QTc-Bazett was 361-457 ms, with marked age and sex variations 4