From the Guidelines
A dangerous QT interval is generally considered to be one that exceeds 500 milliseconds (ms) or shows an increase of more than 60 ms from baseline, as evidenced by the most recent and highest quality study 1. The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization on an electrocardiogram (ECG). When prolonged, it increases the risk of developing torsades de pointes, a life-threatening ventricular arrhythmia that can lead to sudden cardiac death. Certain medications can prolong the QT interval, including some antiarrhythmics, antibiotics, antipsychotics, and antidepressants. Other risk factors include electrolyte abnormalities, heart disease, female gender, advanced age, and genetic predisposition. Key considerations for managing QT interval prolongation include:
- Discontinuing QT-prolonging medications
- Correcting electrolyte imbalances
- Monitoring the patient closely
- Assessing cardiac risk before initiation of treatment, as recommended by 1
- Evaluating the QT interval before initiation of treatment and during titration of dose, as suggested by 1 The danger of QT prolongation lies in its disruption of normal cardiac repolarization, creating vulnerability to dangerous rhythm disturbances in the heart's electrical system. According to 1, 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, and QTc prolongation ≥500 ms and a ΔQT (i.e., change from baseline) of ≥60 ms are considered to be of particular concern. It is essential to prioritize the patient's safety and take immediate action if a prolonged QT interval is identified, as the risk of torsades de pointes rarely occurs when QTc is <500 ms, as noted in 1.
From the FDA Drug Label
The best predictor of this arrhythmia appears to be the length of the QTC interval, and quinidine should be used with extreme care in patients who have preexisting long-QT syndromes, who have histories of torsades de pointes of any cause, or who have previously responded to quinidine (or other drugs that prolong ventricular repolarization) with marked lengthening of the QTC interval If, at any point during administration, the QRS complex widens to 130% of its pre-treatment duration; the QTC interval widens to 130% of its pre-treatment duration and is then longer than 500 ms;
A dangerous QT interval is considered to be a QTC interval longer than 500 ms when it has widened to 130% of its pre-treatment duration 2 2.
- Key factors that increase the risk of a dangerous QT interval include:
- Preexisting long-QT syndromes
- History of torsades de pointes
- Marked lengthening of the QTC interval in response to quinidine or other drugs that prolong ventricular repolarization
- Monitoring should be performed to check for widening of the QTC interval and other signs of toxicity, and the dose of quinidine should be reduced if the QTC interval widens to 130% of its pre-treatment duration and is then longer than 500 ms 2.
From the Research
Definition of a Dangerous QT Interval
A dangerous QT interval is typically defined as a QTc interval of 470-500 ms for males or 480-500 ms for females, or an increase of 60 ms or more from pretreatment values 3. However, it is essential to note that there is no established threshold below which prolongation of the QT interval is considered free of proarrhythmic risk 4.
Risk Factors for a Dangerous QT Interval
Several risk factors can contribute to a dangerous QT interval, including:
- Hypokalemia
- Female sex
- Drug-drug interactions
- Advancing age
- Genetic predisposition
- Hypomagnesemia
- Heart failure
- Bradycardia
- Use of QT-interval-prolonging drugs 3, 5
QT Interval Duration and Risk of Torsades de Pointes
The QT interval duration is a significant determinant of the risk of drug-induced torsades de pointes (TdP) 5. A QTc interval of at least 500 milliseconds is generally considered to correlate with a higher risk of TdP 4. However, the risk of TdP should be assessed in patients who are about to begin taking a QT-prolonging medication, and close monitoring for QTc prolongation is necessary to prevent TdP 3, 6.
Management of a Dangerous QT Interval
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, and electrolytes corrected as needed 3. If the QTc interval is ≥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.