Normal QT Interval on EKG
The normal QT interval must be corrected for heart rate (QTc), with normal values defined as less than 450 ms in men and less than 460 ms in women. 1, 2
Gender-Specific Normal Values
The American Heart Association and American College of Cardiology Foundation provide clear thresholds for normal QTc intervals:
- Men: Normal QTc < 450 ms 1, 2
- Women: Normal QTc < 460 ms 1, 2
- Short QT interval: ≤ 390 ms in both men and women 3, 1, 2
Gender differences emerge during adolescence (12-16 years), with women having QTc intervals approximately 8-15 ms longer than men in younger adults. 3, 1 This difference diminishes after age 40 and practically disappears in older adults. 3, 2
Pediatric Considerations
In children younger than 12 years, the 98th percentile limit for rate-adjusted QT is approximately 450 ms without gender differentiation. 3, 1 The gender difference of approximately 8 ms appears in the 12-16 year age group. 1
Rate Correction Methods
Linear regression functions or Fridericia's formula should be used instead of Bazett's formula for QT-rate correction. 3, 2 This is critical because:
- Bazett's formula overcorrects at heart rates > 80-85 bpm and undercorrects at heart rates < 60 bpm 1, 4
- Fridericia's formula (QTc = QT/∛RR) is more accurate, especially at higher heart rates 1, 2, 4
- The method used for rate correction should be identified in ECG analysis reports 3
Research comparing 10,303 normal ECGs found that Bazett's formula produced significantly wider QTc distributions, with 30% of normal ECGs incorrectly classified as abnormal using a 440 ms threshold. 4 In contrast, Fridericia, Framingham, and Hodges formulae had similar distributions with only 2% false positives. 4
Measurement Technique
For accurate QT measurement:
- Measure in leads II, V3, or V5, selecting the longest value 1
- Choose a lead with T-wave amplitude of at least 2 mm and well-defined T-wave end 2
- Measure from the beginning of the QRS complex to the end of the T wave 2
When NOT to Correct QT
Rate correction should not be attempted when RR interval variability is large (as occurs with atrial fibrillation) or when identification of the end of the T wave is unreliable. 3, 2 QT interval corrections may be inaccurate at heart rates < 40 bpm or > 120 bpm. 1
Special Adjustments Required
QRS duration adjustment: The QT interval prolongs in ventricular conduction defects, requiring adjustment for QRS duration by incorporating it as a covariate or using the JT interval (QT duration minus QRS duration). 3, 2
Clinical Severity Levels
The FDA recommends reporting three severity levels for rate-corrected QT: 3, 1, 2
- Level 1: > 450 ms (men) or > 460 ms (women)
- Level 2: > 480 ms
- Level 3: > 500 ms
A QTc > 500 ms or an increase > 60 ms from baseline is associated with increased risk of Torsades de Pointes. 1, 2
Common Pitfalls to Avoid
- Not accounting for gender differences when interpreting QTc values, particularly in younger adults 1, 2
- Using Bazett's formula at heart rates outside 60-80 bpm, which produces systematic errors 1, 4
- Failing to recognize that athletes often have longer uncorrected QT intervals due to lower resting heart rates, though their QTc should be within normal limits 1, 2
- Overlooking electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia) that can affect QTc values 1
- Attempting QT correction in atrial fibrillation or other conditions with significant RR variability 3, 2