What is a normal QTc (corrected QT interval) range?

Normal QTc Interval Range

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

QTc Normal Ranges by Gender and Age

• Adult Men: ≤450 ms is normal, 451-480 ms is borderline prolonged, >480 ms is prolonged 1, 2
• Adult Women: ≤460 ms is normal, 461-480 ms is borderline prolonged, >480 ms is prolonged 1, 2
• Children under 12 years: ≤450 ms is considered normal 2
• Adolescents (12-16 years): Gender differences begin to emerge, with values similar to adults 1

High-Risk QTc Values

• QTc >500 ms: Considered high-risk for both men and women, with significantly increased risk of torsades de pointes (TdP) and sudden cardiac death 2, 3
• QTc >500 ms (unexplained): Indicative of unequivocal Long QT Syndrome (LQTS), regardless of family history and symptoms 1
• For every 10 ms increase in QTc: Approximately 5% increase in the risk of arrhythmic events 3

QTc Calculation Methods

The method used to calculate QTc significantly impacts the measured values:

• Bazett's formula (QTc = QT/√RR): Most commonly used but has significant limitations:

  • Leaves a strong positive residual correlation with heart rate
  • Produces artificially prolonged QTc values at higher heart rates
  • Results in wider distribution of QTc values 2, 4

• Preferred methods:

  • Fridericia's formula (QTcF = QT/∛RR): Performs better than Bazett's, especially at heart rates >85 bpm 2, 5
  • Linear regression formulas: Recommended by AHA/ACC as they better adjust for heart rate 2

Special Considerations

• QRS prolongation: In ventricular conduction defects, QT interval naturally prolongs and requires adjustment:

  • Consider using JT interval (QT duration–QRS duration) with appropriate normal standards 1, 2

• Serial ECG comparisons:

  • Use the same recording system and standardized procedures
  • Ideally have the same reader interpret sequential tracings
  • FDA guidelines recommend reporting increases of >30 ms and >60 ms from baseline when evaluating drug effects 1, 2

• Measurement technique:

  • Measure in leads with T-wave amplitude of at least 2 mm (typically leads II, V3, or V5)
  • Measure from beginning of QRS complex to end of T wave
  • Use the longest value (return to baseline) 1, 2
  • Avoid leads with U waves 2

Common Pitfalls

• Using Bazett's formula at extreme heart rates: Can produce false QT prolongations, especially at heart rates >100 bpm or <60 bpm 1, 2, 4
• Attempting rate correction with high RR variability: Not recommended in conditions like atrial fibrillation 2
• Inconsistent lead selection: Using different leads for serial measurements introduces variability 2
• Ignoring gender differences: Using the same cutoff for men and women can lead to misclassification 1, 2
• Overlooking age-related differences: The gender difference in QTc intervals becomes smaller after age 40 and nearly disappears in older adults 2

Remember that QTc prolongation >500 ms significantly increases the risk of torsades de pointes and sudden cardiac death, requiring prompt evaluation and management, especially when considering QT-prolonging medications 3, 6.