What is the normal range for the QTc (corrected QT) interval?

Normal Range for QTc Interval

The normal QTc interval is considered to be less than 450 ms in men and less than 460 ms in women, with values of 390 ms or shorter considered abnormally short in both sexes. 1

Gender-Specific Normal Ranges

• QTc values differ between genders, with women having longer QTc intervals than men (mean QTc 433 ms vs 422 ms) 2
• For adult males, a QTc interval less than 450 ms is considered normal 1
• For adult females, a QTc interval less than 460 ms is considered normal 1
• The gender difference appears during adolescence when the rate-adjusted QT shortens in boys (possibly due to testosterone effects) but undergoes little change in girls 1
• This gender difference becomes smaller after age 40 and practically disappears in older men and women 1

Age-Specific Considerations

• In children younger than 12 years, the 98th percentile limit for rate-adjusted QT is approximately 450 ms 1
• The gender difference of 8 ms appears in the 12-16 year age group 1
• In children, QT interval is considered prolonged when QTc exceeds 440 ms in children younger than 8 years and in male adolescents, or 460 ms in girls aged 8 years or older and in children during the first year of life 3

QT Correction Methods

• The QT interval changes with heart rate and must be corrected (QTc) to provide standardized values 4
• Bazett's formula (QTcB = QT/√RR) is the most commonly used but has limitations 1
  • Overcorrects at higher heart rates (>80 bpm)
  • Undercorrects at lower heart rates
• Fridericia's formula (QTcF = QT/∛RR) is more accurate, especially at higher heart rates 1
• Linear regression functions are recommended over Bazett's formula for QT-rate correction 1
• Rate correction should not be attempted when RR interval variability is large (e.g., atrial fibrillation) or when identification of the T-wave end is unreliable 1

Clinical Significance of Abnormal QTc Values

• FDA guidelines recommend reporting three severity levels for rate-corrected QT when considering QT-prolonging effects of drugs 1:

  • 350 ms

  • 480 ms

  • 500 ms

• QTc values of 500 ms or greater are considered highly abnormal and indicative of unequivocal Long QT Syndrome (LQTS), regardless of family history and symptoms 1
• QTc intervals between 440-470 ms are considered a "grey zone" due to considerable overlap between affected and unaffected individuals 1
• QTc intervals >440 ms (males)/460 ms (females) and <500 ms require detailed assessment to achieve a definitive diagnosis 1
• A QTc shorter than 390 ms is considered abnormally short in both men and women 1
• Values of 330 ms (310 ms in children) for the QT and between 360-380 ms for the QTc have been proposed as thresholds for abnormally short QT intervals 1

Measurement Technique

• The QT interval should be measured in leads L2, V3, or V5; the longest value should be considered 1
• For accurate QT measurement, select a lead with well-defined T-wave end 4
• The calculation of the QT interval has inherent limitations due to sinus arrhythmia, slightly widened QRS complexes, and T-U complexes 1
• Corrections for heart rates may be inaccurate at heart rates <40 and >120 bpm 1

Special Considerations

• QT interval prolongs in ventricular conduction defects, requiring adjustment for QRS duration 1
• This can be accomplished by incorporating QRS duration and RR interval as covariates into the QT-adjustment formula or by using the JT interval (QT duration–QRS duration) 1
• High altitude exposure can lengthen a healthy adult's QTc interval by more than 20 ms 5
• Electrolyte abnormalities, particularly hypokalemia, hypomagnesemia, or hypocalcemia, can increase the risk of QTc prolongation 6