What is a normal QT (QT interval) interval?

Normal QT Interval Values

The normal QT interval, when corrected for heart rate (QTc), is less than 450 ms in men and less than 460 ms in women, with values above these thresholds considered prolonged and potentially indicating increased risk of arrhythmias. 1, 2

Gender-Specific Normal Values

• In adult men, the normal QTc interval is less than 450 ms 1, 2
• In adult women, the normal QTc interval is less than 460 ms 1, 2
• Some experts suggest slightly different thresholds, with normal values being less than 430 ms for men and less than 450 ms for women 3
• Gender differences in QTc intervals emerge after puberty, with a difference of approximately 8-10 ms appearing in the 12-16 year age group 1
• In children younger than 12 years, the normal QTc is less than 440 ms, without gender differentiation 1

QT Correction Methods

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

Abnormal QTc Values

• QTc intervals are considered abnormally prolonged when they are ≥450 ms in men and ≥460 ms in women 1, 2
• The American College of Cardiology categorizes QTc prolongation 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 1, 2
• A QTc interval ≤390 ms is considered a short QT interval in both men and women 2
• 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 4

Special Considerations

• Athletes often have longer uncorrected QT intervals due to lower resting heart rates, but their QTc values should be within normal limits 4, 2
• QT interval prolongs in ventricular conduction defects and requires adjustment for QRS duration 2
• QTc increases with increasing age, making assessment in older people especially important 5
• The age-related increase in QTc is more evident in men than women 5
• QT correction calculations may be inaccurate at heart rates <40 bpm or >120 bpm 4

Clinical Implications

• Recording of QTc intervals beyond the normal cut-off values should raise suspicion of either acquired or congenital long-QT syndrome (LQTS) 4
• Common causes of acquired LQTS include QT-prolonging drugs, bradycardia, metabolic changes, and electrolyte disorders 4
• When an abnormally prolonged QTc interval is identified, potential transient causes should be carefully evaluated 4
• A QTc value of 500 ms, otherwise unexplained, is indicative of unequivocal LQTS, regardless of family history and symptoms 4
• QTc intervals between 440-500 ms in males and 460-500 ms in females represent a "grey zone" requiring detailed assessment 4

Measurement Pitfalls to Avoid

• Not accounting for gender differences when interpreting QTc values 1
• Using Bazett's formula at heart rates <40 or >120 bpm 4
• Failing to recognize that electrolyte abnormalities (particularly hypokalemia, hypomagnesemia, and hypocalcemia) can affect QTc values 1
• Not selecting a lead with well-defined T-wave end for measurement 2
• Overlooking that time of day can influence QT interval when comparing sequential tracings 1