What is the method for calculating the corrected QT (QTc) interval?

From the Guidelines

To calculate QTc, use Bazett's formula: QTc = QT/√(RR), where RR is the interval between two consecutive R waves measured in seconds, as it is the most commonly used and accepted method, despite its limitations at extreme heart rates 1. When calculating QTc, it is essential to first measure the QT interval on an ECG from the beginning of the Q wave to the end of the T wave.

• Measure the QT interval in seconds
• Determine the RR interval in seconds
• Apply Bazett's formula: QTc = QT/√(RR) Alternative formulas, such as Fridericia's (QTc = QT/∛(RR)), may be more accurate at extreme heart rates, but Bazett's formula remains the standard for clinical use 1. Many modern ECG machines automatically calculate QTc, but it is crucial for physicians to be able to measure the QT interval manually, especially in cases where the ECG is not normal 1. Normal QTc values are generally considered to be <450ms for men and <460ms for women, with values >500ms indicating an increased risk of dangerous arrhythmias, such as Torsades de Pointes 1. QTc calculation is vital for monitoring medication effects and conditions that may prolong the QT interval, potentially leading to life-threatening arrhythmias 1.

From the Research

Calculating QTc Interval

To calculate the QTc interval, various formulae can be used to adjust for the variation in QT interval with heart rate. The most commonly used formula is not specified in the provided studies, but the importance of calculating QTc interval is emphasized in several studies 2, 3.

Factors Affecting QTc Interval

Several factors can affect the QTc interval, including:

• Patient-related risk factors such as female sex, age >65 years, and uncorrected electrolyte disturbances 2
• The potential risk and degree of QT prolongation associated with a proposed drug 2
• Co-prescribed medicines that could increase the risk of QT prolongation 2
• Hypokalemia, hypomagnesemia, heart failure, bradycardia, and genetic predisposition 3

Normal QTc Intervals

Normal QTc intervals are typically:

• <450 ms for men 2
• <460 ms for women 2

Risk of Arrhythmic Events

For every 10 ms increase in QTc interval, there is a ~5% increase in the risk of arrhythmic events 2.

Monitoring QTc Interval

Close monitoring of QTc interval is necessary to prevent Torsades de Pointes (TdP), a potentially life-threatening cardiac arrhythmia 3. When the QTc interval is:

• 470-500 ms for males, or 480-500 ms for females, or the QTc interval increases 60 ms or more from pretreatment values, dose reduction or discontinuation of the offending drug should be considered 3
• ≥500 ms, the offending drug should be discontinued, and continuous EKG telemetry monitoring should be performed, or the 12-lead EKG should be repeated every 2-4 hours, until the QT interval has normalized 3