What is a normal QTc (corrected QT) interval?

Normal QTc Interval Values

A normal QTc interval is ≤450 ms in men and ≤460 ms in women, with values ≤390 ms considered abnormally short in both genders. 1, 2

Gender-Specific Normal Ranges

The QTc interval demonstrates important gender differences that should be considered when interpreting ECGs:

• Adult males: Normal QTc ≤450 ms
• Adult females: Normal QTc ≤460 ms
• Children under 12 years: Normal QTc ≤450 ms 1

These gender differences emerge during adolescence and diminish with age, becoming smaller after age 40 (6-10 ms) and nearly disappearing in older adults 1.

Classification of QTc Intervals

QTc intervals can be categorized as follows:

• Normal:

  • Men: ≤450 ms
  • Women: ≤460 ms

• Borderline prolonged:

  • Men: 451-480 ms
  • Women: 461-480 ms

• Abnormally prolonged:

  • Both genders: >480 ms

• High-risk prolongation:

  • Both genders: >500 ms 1

• Abnormally short:

  • Both genders: ≤390 ms 2, 1

QT Correction Formulas

The choice of correction formula significantly impacts QTc values and clinical interpretation:

• Bazett's formula (QTcB = QT/√RR) is commonly used but overcorrects at high heart rates and undercorrects at low heart rates 3

• Linear regression formulas (e.g., Hodges and Framingham) are recommended over Bazett's formula as they more effectively remove rate dependence 1

• Fridericia's formula (QTcF = QT/∛RR) performs better than Bazett's, especially at heart rates >85 bpm 1, 4

A study comparing correction formulas found that Hodges formula had the least correlation with heart rate (correlation coefficient 0.11) compared to Bazett (0.33), Fridericia (0.24), and Framingham (0.26) 3.

Clinical Pitfalls and Considerations

1. Overdiagnosis risk: Using Bazett's formula can lead to overdiagnosis of QT prolongation. One study found that implementing an alternative correction algorithm would reduce the number of patients considered to have prolonged QTc by 65.2% 5.

2. First-time ECGs after syncope: QTc values may be transiently prolonged after syncopal episodes. One study found that approximately one-third of pediatric patients in an emergency department setting had QTc values ≥440 ms with significant normalization on follow-up 6.

3. Measurement technique: For accurate assessment:

   • Use leads with T-wave amplitude of at least 2 mm
   • Measure from beginning of QRS complex to end of T wave
   • Use the same lead consistently for serial measurements
   • Avoid leads with U waves 1

4. QRS duration adjustment: In patients with ventricular conduction defects, the QT interval naturally prolongs. Consider using the JT interval (QT duration–QRS duration) with appropriate normal standards 2.

5. Serial ECG comparisons: When comparing sequential ECGs, use the same recording system, standardized acquisition procedures, and ideally the same reader to minimize variability 2.

FDA Thresholds for Drug-Induced QT Prolongation

The FDA recommends reporting three severity levels when considering possible QT-prolonging effects of drugs:

• QTc >350 ms
• QTc >480 ms
• QTc >500 ms 2

For serial ECG comparisons in drug studies, increases of >30 ms and >60 ms from baseline should be reported 2.