From the Guidelines
The normal QTc interval in females is considered to be less than 450 ms, as indicated by the most recent and highest quality study 1. This recommendation is based on the study published in the Journal of the American College of Cardiology in 2017, which provides the most up-to-date and reliable information on QTc intervals.
Key Points to Consider
- The QTc interval is an important measure of cardiac repolarization, and its prolongation can increase the risk of dangerous arrhythmias like Torsades de Pointes.
- The Fridericia formula is recommended by the FDA for heart rate correction, and it is used to calculate the QTc interval.
- A QTc interval of 450 ms or less is considered normal in females, while a QTc interval of 451-480 ms is considered grade 1 QTc prolongation, 481-500 ms is considered grade 2, and greater than 501 ms is considered grade 3.
- Certain medications, electrolyte abnormalities, and congenital conditions can prolong the QTc interval, and regular monitoring is recommended for patients on QT-prolonging medications or those with conditions that may affect cardiac repolarization.
Clinical Implications
- When interpreting ECG results, it is essential to consider the potential effects of medications, electrolyte abnormalities, and congenital conditions on the QTc interval.
- If a prolonged QTc is detected, addressing underlying causes and potentially adjusting medications may be necessary to reduce the risk of cardiac events.
- The use of automated QT interval measuring methods is recommended only if the ECG is otherwise normal, and manual measurement is still essential in clinical practice, as indicated by the study published in the European Heart Journal in 2014 1.
From the Research
Normal QTc Interval in Females
The normal QTc interval in females can be determined based on various studies.
- According to 2, normal QTc intervals are typically <460 ms for women.
- A study by 3 suggests that when the QTc interval is 480-500 ms for females, dose reduction or discontinuation of the offending drug should be considered.
- Another study by 4 determined the upper normal limits of corrected QTc by excluding the top 2% from the global distribution charts, which resulted in an upper normal limit of 460 ms for females using the Fridericia formula.
- A study published in 5 found that the mean corrected QT interval (QTc) in pregnant and non-pregnant women were within the normal range, but the QTc was longer in pregnant women compared to non-pregnant women.
- Age- and sex-specific criteria for QT prolongation were established in a study by 6, which suggested that the upper normal limits for QTc in women vary substantially by age:
- Age < 40 years: 440 ms
- Age 40 to 69: 450 ms
- Age ≥ 70 years: 460 ms