What is a normal QT (QT interval) interval?

Normal QT Interval Values

A normal corrected QT interval (QTc) is less than 450 ms in men and less than 460 ms in women, according to the American College of Cardiology. 1

Gender-Specific Normal Values

The QT interval must be corrected for heart rate to provide standardized values, as the uncorrected QT varies inversely with heart rate. 1, 2

For adult men:

• Normal QTc: <450 ms 1, 2
• Some sources suggest <430 ms as the stricter normal threshold 3

For adult women:

• Normal QTc: <460 ms 1, 2
• Some sources suggest <450 ms as an alternative threshold 3

Gender differences emerge after puberty (ages 12-16 years), with women having QTc intervals approximately 8-10 ms longer than men. 1 In children younger than 12 years, the 98th percentile limit is approximately 450 ms without gender differentiation. 1

Lower Limit of Normal (Short QT)

A QTc ≤390 ms is considered abnormally short in both men and women. 1, 2 More stringent criteria for short QT syndrome include values of 330 ms (310 ms in children) for uncorrected QT and 360-380 ms for QTc. 4, 1

QT Correction Methods

The choice of correction formula significantly impacts the calculated QTc value:

• Bazett's formula (QTc = QT/√RR) is most commonly used but overcorrects at heart rates >80 bpm and undercorrects at heart rates <60 bpm 1, 3, 5
• Fridericia's formula (QTc = QT/∛RR) is more accurate, especially at higher heart rates, and is recommended by the FDA 1, 2, 3
• Hodges formula shows the least correlation with heart rate (correlation coefficient 0.11 vs 0.33 for Bazett) 5

Clinical Risk Stratification

QTc prolongation severity grades: 1, 3

• Grade 1: 450-480 ms
• Grade 2: 481-500 ms
• Grade 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, 3, 6

Measurement Considerations and Common Pitfalls

Critical measurement points:

• Measure QT in leads II, V3, or V5, using the longest value 1
• Select leads with T-wave amplitude ≥2 mm and well-defined T-wave end 2, 3
• QT corrections are inaccurate at heart rates <40 bpm or >120 bpm 4, 1

Common pitfalls to avoid:

• Not accounting for gender differences after puberty when interpreting QTc values 1, 2
• Relying on automated ECG measurements without manual verification, as automated readings are unreliable 7
• Using Bazett's formula at extreme heart rates, which leads to 30% of normal ECGs being misclassified as abnormal when using a 440 ms threshold 5
• Failing to recognize that athletes have longer uncorrected QT intervals due to bradycardia, though their QTc remains within normal limits 4, 1, 2
• Not considering electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia) that can prolong QTc 1, 8
• Overlooking QT-prolonging medications as the most frequent cause of acquired long QT syndrome 4, 8

Special Populations

Athletes: QT intervals are longer due to lower resting heart rates from athletic training, but QTc values should remain within normal limits, typically toward the upper limit. 4, 2 Values between 440-500 ms in male athletes and 460-500 ms in female athletes represent a "grey zone" requiring detailed assessment. 4

Age considerations: Gender differences in QTc become smaller after age 40 and practically disappear in older adults. 2