A 30-year-old heroin-dependent patient is started on methadone maintenance therapy. After 2 weeks at 80 mg/day, he is brought unconscious with a QTc of 560 ms on ECG. What is the most important electrolyte/investigation to check urgently?

• A Serum sodium and urine osmolality
• B Serum calcium and phosphate
• C Serum ammonia and liver enzymes
• D Serum potassium and magnesium ✓

Explanation

Methadone is well-known to prolong the QTc interval by blocking cardiac hERG (IKr) potassium channels. The risk is dose-dependent and markedly increased by hypokalemia and hypomagnesemia, which also reduce cardiac repolarization reserve. A QTc >500 ms indicates high risk for Torsades de Pointes (TdP). Serum K+ and Mg2+ must be checked and corrected urgently. Baseline ECG before starting methadone and periodic monitoring are recommended. Drug interactions (e.g., azithromycin, fluconazole, ciprofloxacin) further prolong QTc.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.