What is a normal QT (quinidine time) interval?

Normal QT Interval Values

The normal corrected QT interval (QTc) is less than 450 ms in adult men and less than 460 ms in adult women, with these gender-specific thresholds representing the standard cutoffs recommended by major cardiology societies. 1, 2

Gender and Age-Specific Normal Values

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

Adult thresholds:

• Men: QTc < 450 ms is normal; QTc ≥ 450 ms is prolonged 1, 2
• Women: QTc < 460 ms is normal; QTc ≥ 460 ms is prolonged 1, 2

Pediatric considerations:

• In children younger than 12 years, the upper limit is approximately 450 ms without gender differentiation 2
• Gender differences emerge after puberty (ages 12-16 years), with approximately 8-10 ms difference appearing between males and females 2

Alternative Thresholds and Gray Zones

Some authorities have suggested slightly higher cutoffs, particularly for athletes:

• Male athletes: up to 470 ms may be acceptable 3
• Female athletes: up to 480 ms may be acceptable 3
• The "gray zone" of 440-470 ms in males and 460-480 ms in females requires detailed assessment to distinguish normal variants from pathologic prolongation 3

Athletes commonly have longer uncorrected QT intervals due to lower resting heart rates from athletic training, but their QTc values should remain within normal limits, though often toward the upper range. 3, 2

Short QT Interval

A QTc ≤ 390 ms is considered abnormally short in both men and women. 1, 2

More stringent criteria for short QT syndrome include:

• Uncorrected QT < 330 ms (310 ms in children) 3, 2
• QTc between 360-380 ms 3, 2

Measurement Considerations and Common Pitfalls

Critical measurement principles:

• The QT interval is measured from the beginning of the QRS complex to the end of the T wave 1
• Select a lead with T-wave amplitude of at least 2 mm and well-defined T-wave end (typically lead II, V3, or V5) 1, 2
• Use the longest QT value when measuring multiple leads 2

Correction formula selection matters:

• Bazett's formula (QT/√RR) is most commonly used but has significant limitations, particularly overcorrecting at high heart rates (>80 bpm) and undercorrecting at low heart rates (<60 bpm) 1, 2, 4
• Fridericia's formula (QT/∛RR) is more accurate, especially at higher heart rates, and is recommended by the FDA 1, 2
• QT corrections may be inaccurate at heart rates <40 bpm or >120 bpm 3, 2

Common pitfalls to avoid:

• Not accounting for gender differences when interpreting QTc values, particularly in post-pubertal patients 2
• Relying on automated ECG printouts, which cannot be trusted for accurate QT measurement 5
• Failing to recognize that electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia) can affect QTc values 2
• Not considering that time of day and autonomic tone can influence QT interval measurements 2, 6
• Attempting QT correction when RR interval variability is large (e.g., atrial fibrillation) 1

Clinical Risk Stratification

High-risk thresholds for arrhythmia:

• QTc > 500 ms is associated with increased risk of Torsades de Pointes, a potentially life-threatening ventricular arrhythmia 1, 7, 8
• An increase in QTc > 60 ms from baseline is considered clinically significant 1, 7
• FDA guidelines recommend reporting three severity levels: >350 ms, >480 ms, and >500 ms 1, 2

However, there is no established threshold below which QT prolongation is considered completely free of proarrhythmic risk. 8