Why is monitoring QT interval important with methadone therapy?

• A. Methadone can prolong QT and predispose to torsades de pointes; baseline and periodic EKG recommended.
• B. Methadone shortens QT intervals.
• C. QT interval is not affected by methadone.
• D. Baseline EKG is never necessary.

Answer: (A)

Prolongation of the QT interval with methadone is a real safety concern because it can disrupt the heart’s electrical stability and lead to torsades de pointes, a dangerous form of ventricular tachycardia that can progress to cardiac arrest. The QT interval reflects how long the heart’s repolarization takes, and when methadone lengthens this interval, the risk of this abnormal rhythm increases.

Starting with a baseline ECG helps identify anyone who already has a prolonged QT, congenital long QT syndrome, or other risk factors such as electrolyte disturbances. Repeating an ECG during treatment is important because QT prolongation can develop as the methadone dose increases, with prolonged exposure, or when there are interactions with other drugs that extend the QT, as well as with electrolyte imbalances (low potassium or magnesium). If the QTc is markedly long—for example, exceeding about 500 ms—or if it increases substantially from baseline, clinicians consider reducing the methadone dose, correcting electrolytes, or switching to a different opioid to reduce the risk of torsades de pointes. Avoiding additional QT-prolonging drugs when possible is also a key part of safety while on methadone.