Normal QT Interval Duration
The normal QT interval duration varies with heart rate, age, and gender, but when corrected for heart rate (QTc), the upper normal limits are ≤450 ms in adult men and ≤460 ms in adult women. 1, 2
Understanding the QT Interval
The QT interval represents the time from the beginning of ventricular depolarization (start of QRS complex) to the end of ventricular repolarization (end of T wave). 1 Because this interval changes significantly with heart rate, it must be corrected (QTc) to provide meaningful clinical interpretation. 2
Normal Values by Population
Adults
- Men: Normal QTc ≤450 ms; prolonged if ≥450 ms 1, 2
- Women: Normal QTc ≤460 ms; prolonged if ≥460 ms 1, 2
- Short QT: ≤390 ms in both genders 1, 2
The gender difference of 12-15 ms in younger adults and 6-10 ms in older adults emerges during adolescence when testosterone shortens the QT interval in males. 1, 2 This difference becomes negligible after age 40 and essentially disappears in elderly populations. 1, 2
Neonates and Children
- Neonates (day 4 of life): Mean QTc 400±20 ms, with upper normal limit of 440 ms (97.5th percentile) 1
- Children <12 years: Upper limit approximately 450 ms regardless of gender 1, 2
- Adolescents (12-16 years): Gender difference of 8 ms begins to appear 1, 2
A physiological QTc prolongation occurs by the second month of life (mean 410 ms), followed by progressive decline to first-week values by six months. 1
Measurement Technique
Measure the QT interval in leads II, V5, or V6, using the longest value obtained. 1, 3 The measurement extends from the earliest onset of the QRS complex to the point where the descending limb of the T wave intersects the isoelectric line. 1
Critical Measurement Points
- Use the TP segment (not PQ segment) as the isoelectric line in neonates and infants 1
- In fast heart rates where the P wave may superimpose on the T wave, extrapolate the T wave end by drawing a tangent to its downslope 1
- Select leads with T-wave amplitude ≥2 mm and well-defined T-wave end 2
- Avoid leads with prominent U waves; if unavoidable, use aVR or aVL where U waves are typically absent 3
Rate Correction Methods
Bazett's formula (QTc = QT/√RR) remains the clinical standard despite significant limitations, particularly at extreme heart rates. 1, 4 Fridericia's formula (QTc = QT/∛RR) performs better, especially at non-normal heart rates. 2, 3, 4
When NOT to Correct
Do not attempt QT correction when RR interval variability is large, such as in atrial fibrillation or frequent ectopy. 1, 2 Correction requires stable sinus rhythm without sudden RR interval changes. 1
Clinical Significance Thresholds
Risk Stratification
- QTc >500 ms: High risk for Torsades de Pointes and life-threatening arrhythmias 2, 5, 6
- QTc 440-470 ms: "Grey zone" with overlap between affected and unaffected individuals 5
- Increase >60 ms from baseline: Clinically significant change requiring intervention 2, 5
FDA Severity Levels
The FDA recommends reporting three severity thresholds when evaluating QT-prolonging drug effects: >350 ms, >480 ms, and >500 ms. 1, 2
Special Considerations
In ventricular conduction defects (bundle branch blocks), the QT interval artificially lengthens and requires adjustment for QRS duration. 1, 3 This can be accomplished by using the JT interval (QT minus QRS duration) with JT-specific normal standards. 1
Athletes may have longer uncorrected QT intervals due to bradycardia, but their QTc values should remain within normal limits. 2, 3
By definition, 2.5% of normal individuals will have a QTc exceeding the upper normal limit (440 ms in neonates, 450 ms in adult men, 460 ms in adult women), so isolated borderline prolongation requires clinical context for interpretation. 1