From the FDA Drug Label
Methadone should be administered with particular caution to patients already at risk for development of prolonged QT interval (e.g., cardiac hypertrophy, concomitant diuretic use, hypokalemia, hypomagnesemia). Careful monitoring is recommended when using methadone in patients with a history of cardiac conduction disease, those taking medications affecting cardiac conduction, and in other cases where history or physical exam suggest an increased risk of dysrhythmia
Methadone is not explicitly contraindicated in first-degree atrioventricular (AV) block. However, caution is advised when using methadone in patients with a history of cardiac conduction disease.
- The FDA label recommends careful monitoring in such cases.
- The label does not provide a clear contraindication for first-degree AV block, but it does highlight the potential risks of methadone in patients with cardiac conduction abnormalities 1.
From the Research
Methadone is not absolutely contraindicated in patients with first-degree AV block, but caution is warranted. Patients with pre-existing conduction abnormalities, including first-degree AV block, should be monitored carefully when starting methadone therapy or increasing doses. Methadone can prolong the QT interval and potentially worsen existing cardiac conduction abnormalities, as seen in patients with conduction disturbances who undergo general anesthesia 2. The risk increases with higher doses (typically above 100 mg daily), electrolyte abnormalities (particularly hypokalemia and hypomagnesemia), and when combined with other QT-prolonging medications.
Key Considerations
- Before initiating methadone in patients with first-degree AV block, a baseline ECG should be obtained, with follow-up ECGs recommended after dose increases, as conduction abnormalities can occur during stress testing 3.
- Electrolytes should be monitored and maintained within normal ranges.
- The mechanism behind methadone's cardiac effects involves blockade of the hERG potassium channel, which affects cardiac repolarization.
- If methadone is deemed necessary for a patient with first-degree AV block, start with lower doses, increase gradually, and maintain vigilant cardiac monitoring throughout treatment, considering the potential for pseudopacemaker syndrome in marked first-degree AV block 4.
- Recent reports suggest that first-degree atrioventricular block is not benign, and biventricular pacing may be used according to accepted guidelines for third-degree AV block, especially in patients with a very long PR interval ≥0.3 s 5.
Monitoring and Management
- Patients with first-degree AV block should be closely monitored for signs of cardiac conduction abnormalities, such as dizziness or syncope.
- The use of methadone in patients with first-degree AV block requires careful consideration of the potential risks and benefits, and alternative treatments should be considered if possible.
- In patients undergoing cardiac resynchronization therapy (CRT), first-degree AV block can cause electrical desynchronization, and AV junctional ablation may be necessary in some cases 5.