From the Guidelines
The normal QTc interval for females is typically considered to be less than 460 milliseconds. Women generally have a slightly longer QTc interval than men, whose normal value is less than 450 milliseconds, as suggested by 1. This gender difference emerges after puberty due to hormonal influences, particularly estrogen, which can affect cardiac repolarization. The QTc interval represents the heart's electrical recovery time corrected for heart rate, and it's an important measurement when assessing risk for certain arrhythmias.
Key Considerations
- A prolonged QTc interval (above 460 ms in women) may indicate an increased risk for potentially dangerous heart rhythm disturbances, such as Torsades de Pointes, as noted in 1 and 1.
- When interpreting QTc values, it's essential to consider that certain medications, electrolyte abnormalities (especially low potassium, magnesium, or calcium), and some medical conditions can prolong the QTc interval.
- Regular monitoring of the QTc interval is particularly important when starting medications known to affect cardiac repolarization.
Clinical Implications
- The QTc interval should be measured from the beginning of the QRS complex to the end of the T wave, as described in 1.
- The most commonly used QT correction formula in clinical practice is the one introduced by Bazett, QTC = QT interval divided by the square root of the R-R interval measured in seconds, although its adequacy has been questioned, as discussed in 1 and 1.
- A QTC of 0.50 second in either sex has been shown to correlate with a higher risk for torsades de pointes, as reported in 1. Some key points to consider when evaluating QTc intervals include:
- The upper normal limits for QT adjusted for rate as a linear function of RR in small groups of children stratified by age, gender, and heart rate have been published, as noted in 1.
- The limits for prolonged and short QT established in reasonably large groups of children have been reported only for Bazett’s formula, as discussed in 1.
- FDA guidelines for industry recommend that 3 severity levels for rate-corrected QT be reported when considering possible QT-prolonging effects of drugs: longer than 350 ms, longer than 480 ms, and longer than 500 ms, as mentioned in 1.
From the Research
Normal QTC Interval for Females
The normal QTC interval for females can vary based on age. According to the study 2, the criteria for prolonged QTC were established as follows:
- Age < 40 years: 440 ms
- Age 40 to 69: 450 ms
- Age ≥ 70 years: 460 ms
Comparison with Other Studies
Other studies provide additional insights into QTC intervals:
- A study 3 found that approximately one-third of pediatric patients exhibited QTC values of ≥440 milliseconds.
- A study 4 found that 11.6% of patients had a QTC > 460 msec, and the proportion of women with a QTC > 470 msec was 3.6%.
- A study 5 predicted that the abnormal value of the iCEBc in females is 5.315, but did not provide a direct QTC value.
Key Points
- The normal QTC interval for females varies by age.
- Age- and sex-specific criteria should be used for evaluation of QT prolongation.
- The upper normal limits for QTC vary substantially by age and sex.