From the Guidelines
A QT interval is considered long when it exceeds 500 ms, posing a significant risk for dangerous arrhythmias, as stated in the most recent and highest quality study 1. This measurement is often corrected for heart rate (QTc) since the QT interval naturally shortens with faster heart rates. The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization on an electrocardiogram (ECG). Some key points to consider when evaluating QT intervals include:
- QTc intervals >450 ms in men and >460 ms in women are suggested as a guideline for the upper limit of normal on baseline ECG evaluation 1.
- QTc prolongation >500 ms and a change from baseline of >60 ms are considered to be of particular concern because torsades de pointes rarely occurs when QTc is <500 ms 1.
- Long QT syndrome can be congenital or acquired, with the latter commonly caused by medications like certain antiarrhythmics, antibiotics, antipsychotics, and antihistamines.
- Other factors that can prolong QT include electrolyte abnormalities (particularly low potassium, magnesium, or calcium), heart disease, and hypothyroidism.
- Prolonged QT intervals are concerning because they increase the risk of developing torsades de pointes, a potentially fatal ventricular arrhythmia that can lead to sudden cardiac death. It is essential to assess the QT interval and associated risk factors for QT prolongation before and during treatment, and to monitor ECG and electrolyte levels during treatment 1.
From the FDA Drug Label
In this clinical trial Sotalol AF treatment was not initiated if the QT interval was greater than 450 msec and during therapy the dose was reduced or discontinued if the QT interval was ≥520 msec Table 7 below relates the incidence of Torsade de Pointes to on-therapy QTc and change in QTc from baseline. On-Therapy QTc Interval(msec) Incidence of Torsade dePointes less than 5001.3% 500 to 5253.4% 525 to 5505.6%
- 8%
A QTc interval of more than 500 msec is considered to be at a higher risk for Torsade de Pointes, with the risk increasing as the QTc interval prolongs, especially above 550 msec 2, 2.
From the Research
Definition of Long QT Interval
A long QT interval is generally considered to be a QTc interval of 500 milliseconds or more, or a 15% or greater increase from baseline 3.
Factors Influencing QT Interval Prolongation
Several factors can influence QT interval prolongation, including:
- Dosage of certain medications, such as anti-arrhythmic drugs 4
- Coexisting medical conditions, such as hypokalemia or heart failure 5
- Electrolyte imbalances 4
- Age and sex of the patient 5
- Use of certain drugs, such as amiodarone, sotalol, and dofetilide 4, 3
Management of Long QT Syndrome
Management of long QT syndrome typically involves:
- Reducing the dosage of medications that can cause QT prolongation or discontinuing them altogether 4
- Employing selective beta blockers, such as propranolol or nadolol 6
- Close monitoring of the QT interval in patients receiving anti-arrhythmic therapy 4
- Consideration of patient-specific risk factors for long QT syndrome, including age, sex, and electrolyte imbalances 5