Normal QTc Interval Values
The normal QTc interval is <450 ms for men and <460 ms for women, with values ≤390 ms considered short QT in both sexes. 1, 2
Gender-Specific Normal Ranges
The American College of Cardiology and American Heart Association establish clear thresholds that differ by sex:
- Men: QTc <450 ms is normal; values ≥450 ms indicate prolongation 1, 2, 3
- Women: QTc <460 ms is normal; values ≥460 ms indicate prolongation 1, 2, 3
- Both sexes: QTc ≤390 ms defines short QT syndrome 1, 2
Age-Related Considerations
The gender difference in QTc values emerges during adolescence and varies with age:
- Young adults (age <40): The gender difference is 12-15 ms, with QT shortening in adolescent males while remaining unchanged in females 1, 2
- Middle-aged adults (40-69 years): Gender differences persist but narrow to 6-10 ms 2
- Older adults (≥70 years): Gender differences become minimal and practically disappear 2
Critical Measurement Considerations
Correction Formula Selection
Linear regression functions or Fridericia's formula should be used instead of Bazett's formula, which systematically overcorrects at heart rates >80-90 bpm and produces falsely prolonged QTc values. 1, 2, 3
- Bazett's formula (QTc = QT/√RR) is most commonly used but has significant limitations, particularly at higher heart rates 2, 4
- Fridericia's formula (QTc = QT/∛RR) provides more accurate rate correction, especially at heart rates >80-90 bpm 2, 3
- Using Fridericia instead of Bazett reduces false-positive prolonged QTc diagnoses by 65.2% and severely prolonged QTc by 79.6% 5
Measurement Technique
- Select a lead with T-wave amplitude ≥2 mm and well-defined T-wave end 2
- Do not attempt QT correction when RR interval variability is large (e.g., atrial fibrillation) 2
- Adjust for QRS duration in ventricular conduction defects 2
Clinical Risk Stratification
The FDA and cardiology societies recommend three severity levels for clinical decision-making:
- Level 1 (Prolonged): >450 ms in men or >460 ms in women 1, 3
- Level 2 (Moderately severe): >480 ms in either sex 2, 3
- Level 3 (Severely prolonged): >500 ms, associated with significantly increased risk of torsades de pointes 1, 2, 3
Baseline Change Thresholds
- QTc increase >60 ms from baseline: Clinically significant and increases torsades de pointes risk 1, 3
- QTc increase >30 ms from baseline: Warrants monitoring 3
Common Pitfalls to Avoid
- Bazett overcorrection: At heart rates >85 bpm, Bazett's formula produces false QT prolongations; recalculate using Fridericia before making clinical decisions 3, 4
- Ignoring gender differences: Women have inherently longer QTc intervals and are more prone to malignant arrhythmias in long QT syndrome 1
- Missing dynamic changes: A single ECG may not capture the full QTc range; 24-hour Holter monitoring reveals that LQTS type 1 patients show more QTc prolongation during daytime (
3 PM) while type 2 patients show more at night (3 AM) 6 - Bundle branch blocks: These increase QT interval due to delayed depolarization, not repolarization abnormality, requiring adjustment 3