Normal QTc Interval in Adults
The normal QTc interval is less than 450 ms in men and less than 460 ms in women, with values ≤390 ms considered short QT in both sexes. 1
Gender-Specific Normal Ranges
The QTc interval demonstrates important gender differences that must be considered when interpreting values:
- Men: Normal QTc <450 ms 1, 2
- Women: Normal QTc <460 ms 1, 2
- Both sexes: QTc ≤390 ms is considered a short QT interval 1, 3
The gender difference of 12-15 ms in younger adults originates during adolescence when testosterone causes QT shortening in males, while females experience minimal change. 1, 4 This difference decreases to 6-10 ms in older age groups and practically disappears after age 40. 1, 3
Thresholds for Prolonged QTc
The AHA/ACCF/HRS guidelines define prolonged QTc as ≥450 ms in men and ≥460 ms in women (representing the 95th percentile). 1, 2
More stringent criteria exist for higher-risk situations:
- 99th percentile thresholds: 470 ms for men, 480 ms for women 2
- High-risk threshold: QTc >500 ms is associated with significantly increased risk of torsades de pointes 2, 4
FDA Drug Safety Monitoring Levels
The FDA recommends three severity levels for monitoring QT-prolonging drug effects: 1, 4
- Level 1: >450 ms (or >460 ms for women)
- Level 2: >480 ms
- Level 3: >500 ms
Critical Measurement Considerations
The correction formula used significantly impacts QTc values and clinical interpretation. Bazett's formula (most commonly used) systematically overcorrects at heart rates >80-85 bpm, producing falsely prolonged values. 2, 5 Linear regression functions or Fridericia's formula provide more accurate rate correction, particularly at higher heart rates. 1, 2
When NOT to Calculate QTc
Do not attempt QT correction when: 1, 3
- RR interval variability is large (e.g., atrial fibrillation)
- The end of the T wave cannot be reliably identified
Bundle Branch Block Adjustment
The QT interval prolongs in ventricular conduction defects due to delayed depolarization, requiring adjustment for QRS duration using the JT interval (QT duration minus QRS duration) with JT-specific normal standards. 1, 2
Clinically Significant Changes from Baseline
When monitoring serial ECGs: 2, 4, 3
- Increase >30 ms from baseline: Warrants monitoring
- Increase >60 ms from baseline: Clinically significant and increases torsades de pointes risk
Age-Related Variations
While the primary guidelines focus on adult thresholds, pediatric values differ: 1
- Children <12 years: 98th percentile approximately 450 ms without gender differentiation
- Ages 12-16: Gender difference of 8 ms begins to appear