Normal QT Interval Range
The normal QT interval on a standard 12-lead ECG is less than 460 ms in women and 450 ms or less in men (when corrected for heart rate), with values of 390 ms or shorter considered short QT intervals. 1
Gender-Specific Thresholds
The QT interval must be interpreted with gender-specific criteria, as physiologically significant differences exist between men and women:
- Women: Normal QT interval is less than 460 ms (corrected for heart rate) 1
- Men: Normal QT interval is 450 ms or less (corrected for heart rate) 1
- Both genders: QT intervals of 390 ms or shorter are considered abnormally short 1
The gender difference ranges from 6-10 ms in older adults and 12-15 ms in younger adults, appearing during adolescence when testosterone shortens the rate-adjusted QT in boys but not in girls. 1 This difference becomes negligible after age 40 and essentially disappears in older populations. 1
Critical Measurement Considerations
Heart Rate Correction Requirements
Linear regression functions should be used for rate correction rather than Bazett's formula, as Bazett's formula leaves substantial correlation with heart rate and produces false QT prolongations, particularly at high heart rates. 1 The method used for rate correction must be identified in ECG analysis reports. 1
Do not attempt rate correction when: 1
- RR interval variability is large (as in atrial fibrillation)
- The end of the T wave cannot be reliably identified
Measurement Technique
The QT interval should be measured in lead II by convention, as U waves are less prominent in this lead and the end of the T wave in lead II corresponds to the end of significant repolarization in other leads. 2
Age-Specific Considerations
Children younger than 12 years: The 98th percentile limit for rate-adjusted QT is approximately 450 ms, with an 8 ms gender difference appearing in the 12-16 year age group. 1
Conduction Abnormalities
When ventricular conduction defects are present, the QT interval prolongs and requires adjustment for QRS duration. 1 This is best accomplished by:
- Incorporating QRS duration and RR interval as covariates into the adjustment formula, or
- Using the JT interval (QT duration minus QRS duration) with JT-specific normal standards 1
FDA Drug Safety Thresholds
For drug-induced QT prolongation assessment, the FDA recommends reporting three severity levels of rate-corrected QT: 1
- Greater than 450 ms
- Greater than 480 ms
- Greater than 500 ms
Changes from baseline of greater than 30 ms and greater than 60 ms should also be reported in serial ECG monitoring. 1
Common Pitfalls
Bazett's formula overcorrects at high heart rates and should not be used during exercise testing, where it incorrectly identifies QT prolongation in normal individuals. 3 During ramp exercise, the QT-heart rate relationship follows a straight line (QT = 0.459 - 12.3 × 10⁻⁴ × HR). 3
Sinus arrhythmia complicates interpretation: In patients with sinus arrhythmia, the QT interval following the shortest RR interval provides more diagnostic value than standard correction formulas. 2