From the Guidelines
The normal QTc interval in females is less than 460 milliseconds, as suggested by the most recent guidelines. Women generally have slightly longer QTc intervals than men due to hormonal influences, particularly estrogen, which can prolong cardiac repolarization 1. A QTc interval is considered prolonged in women when it exceeds 460 milliseconds, which may indicate an increased risk of dangerous arrhythmias like Torsades de Pointes.
Key Considerations
- The QTc interval represents the heart's electrical recovery time corrected for heart rate, and accurate measurement requires proper ECG technique and appropriate correction formulas such as Bazett's or Fridericia's.
- Regular monitoring of the QTc interval is particularly important in women taking QT-prolonging medications or those with congenital long QT syndrome.
- QTc prolongation of more than 500 ms and a change from baseline of more than 60 ms are considered to be of particular concern because torsades de pointes rarely occurs when QTc is less than 500 ms 1.
Clinical Implications
- The measurement of QTc interval is important in clinical practice when prescribing medications that might prolong QT interval, evaluating cardiac health, or investigating symptoms like syncope or palpitations.
- The European Society of Cardiology suggests that QTc intervals more than 450 ms in men and more than 460 ms in women are considered abnormal and may require further evaluation or monitoring 1.
From the Research
Normal QTc Interval in Females
The normal QTc interval in females can vary based on age.
- According to the study 2, the criteria for prolonged QTc were:
- Age < 40 years, women 440 ms
- Age 40 to 69, women 450 ms
- Age ≥ 70 years, women 460 ms
- Another study 3 found that approximately one-third of pediatric patients exhibited QTc values of ≥440 milliseconds.
- A study on pregnant and non-pregnant women 4 found that the QT interval was longer in pregnant women compared to non-pregnant women, but still within the normal range.
- It is essential to use age- and sex-specific criteria for evaluation of QT prolongation, as upper normal limits for QTc vary substantially by age and sex 2.
Factors Affecting QTc Interval
Several factors can affect the QTc interval, including:
- Age: QTc shortens in men during adolescence and becomes 13 ms shorter than in women at age 20-29 years 2
- Sex: Sex difference in QTc originates from shortened QT in adolescent males 2
- Pregnancy: QT interval may increase during pregnancy 4
- Heart rate: QTc interval may shorten or normalize during effort with an increased heart rate 5