A patient on methadone maintenance therapy for opioid use disorder presents with syncope. ECG shows QTc prolongation to 520 ms. Which property of methadone explains this adverse effect?

• A Methadone blocks the rapid delayed rectifier potassium current (IKr) in cardiomyocytes ✓
• B Methadone enhances vagal tone through mu receptor action on the SA node
• C Methadone causes hypokalemia through renal potassium wasting
• D Methadone is a calcium channel blocker causing bradycardia

Explanation

Methadone blocks the hERG potassium channel, inhibiting the rapid delayed rectifier potassium current (IKr) in cardiomyocytes. This prolongs the QT interval and increases risk of torsades de pointes, especially at higher doses. ECG monitoring is recommended for patients on methadone. This is a distinct adverse effect unrelated to its opioid receptor activity and is not shared by all opioids.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.