From the Guidelines
The normal QTcB limits are 0.46 second in women and 0.45 second in men, with a QTcB of 0.50 second or more considered to correlate with a higher risk for torsades de pointes in either sex. When interpreting QTcB intervals, it is essential to consider the potential risks associated with QT prolongation, including the development of life-threatening arrhythmias such as torsades de pointes 1. The QTcB interval is calculated using Bazett's formula, which divides the QT interval by the square root of the R-R interval measured in seconds 1. Although the adequacy of Bazett's formula has been questioned, it remains the most commonly used correction formula in clinical practice 1. Key factors to consider when monitoring QTcB intervals include the potential impact of medications, such as antiarrhythmics, antibiotics, antipsychotics, and antidepressants, which can prolong the QT interval 1. Additionally, electrolyte abnormalities like hypokalemia, hypomagnesemia, and hypocalcemia can also contribute to QT prolongation 1. Regular ECG monitoring is crucial when starting medications known to affect QT intervals, particularly in patients with other risk factors for QT prolongation 1. It is also important to note that a normal QTcB interval does not necessarily preclude the risk of torsades de pointes, as individual cases may vary 1. In clinical practice, a comprehensive approach to QTcB interval monitoring should include consideration of these factors to minimize the risk of life-threatening arrhythmias.
From the Research
QTcB Limits for Men and Women
The QTcB (QT interval corrected by Bazett's formula) limits for men and women are as follows:
- Normal QTc intervals are typically <450 ms for men and <460 ms for women 2
- Prolonged QTcB is defined as >450 ms for men and >470 ms for women 3
- Severely prolonged QTcB is defined as >500 ms for both men and women 3
- Age- and sex-specific criteria for QT prolongation are:
- Age < 40 years: men 430 ms, women 440 ms
- Age 40 to 69: men 440 ms, women 450 ms
- Age ≥ 70 years: men 455 ms, women 460 ms 4
- QTcB intervals greater than 470 ms for females and greater than 450 ms for males are considered prolonged 5