Normal QTc Interval Values

A normal corrected QT (QTc) interval is less than 450 ms in men and less than 460 ms in women. 1, 2

Gender-Specific Normal QTc Values

The American College of Cardiology recommends that normal QTc intervals are:
- Men: Less than 450 ms 1, 2
- Women: Less than 460 ms 1, 2
These gender differences emerge after puberty, with the gender difference of approximately 8-10 ms appearing in the 12-16 year age group 1
In children younger than 12 years, the 98th percentile limit for rate-adjusted QT is approximately 450 ms, without gender differentiation 1

The QT interval must be corrected for heart rate to provide standardized values (QTc) 2
Bazett's formula (QTc = QT/√RR) is the most commonly used but has limitations:
- Overcorrects at higher heart rates (>80 bpm)
- Undercorrects at lower heart rates (<60 bpm) 1, 2, 3
Fridericia's formula (QTc = QT/∛RR) is more accurate, especially at higher heart rates, and is recommended by the FDA 2, 4
For accurate measurement, QT should be measured in leads II, V3, or V5, with the longest value considered 1

QTc intervals are considered abnormally prolonged when:
- Men: ≥450 ms 1, 2
- Women: ≥460 ms 1, 2
A QTc interval ≤390 ms is considered a short QT interval in both men and women 1
For short QT syndrome, values of 330 ms (310 ms in children) for uncorrected QT and between 360-380 ms for QTc have been proposed 1

QTc intervals can be categorized by severity 2:
- 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 2, 5
FDA guidelines recommend reporting three severity levels: >350 ms, >480 ms, and >500 ms 1

The calculation of QT interval has inherent limitations due to:
- Sinus arrhythmia
- Widened QRS complexes
- T-U complexes 1
QT interval corrections may be inaccurate at heart rates <40 bpm or >120 bpm 1
For serial comparisons, it's recommended to use the same ECG recorder type and have a single reader responsible for measurements 1
QT interval prolongs in ventricular conduction defects, requiring adjustment for QRS duration 1

Using Bazett's formula for heart rates outside 60-100 bpm can lead to inaccurate QTc values 3, 4
Not accounting for gender differences when interpreting QTc values 1, 2
Failing to consider that athletes often have longer uncorrected QT intervals due to lower resting heart rates, though their QTc is usually within normal limits 1
Not recognizing that time of day can influence QT interval when comparing sequential tracings 1
Overlooking that electrolyte abnormalities (particularly hypokalemia, hypomagnesemia, and hypocalcemia) can affect QTc values 5, 6