Variability of QT Interval Measurements in Opioid‐Dependent Patients on Methadone

Background: Methadone can prolong the QT interval, a precursor to ventricular dysrhythmias. Measurement of the QT interval can vary with intrinsic and extrinsic factors. Our objective was to assess the agreement and repeatability between different QT interval measurements. Methods: We recorded ECGs daily for five consecutive days from adults receiving a stable, single daily dose of methadone for opioid dependence. We compared manually measured QT intervals and the calculated QTc intervals to automated measurements. Results: We obtained 123 ECGs from 26 patients receiving a mean methadone dose of 71 mg/day (range 5 to 240 mg). The manually measured QT intervals from leads II and V5 were similar and consistent (95% limits of agreement ‐46 to 59 ms), as were the corresponding manual QTc. However, the automated QT reported by the ECG was less consistent (95% limits of agreement ‐85 to 65 ms), especially at higher QT intervals and at methadone doses over 100 mg/day. Only one of the two subjects at greatest risk for torsades de pointes was identified based on the automated QT reading, while six other patients had at least one automated QTc over 480 ms that could not be confirmed on manual measurement. Conclusions: Accurate identification of QT prolongation in methadone patients is difficult because of intrinsic patient variability in QT length and accuracy of automated measurements. When contemplating ECG screening for this population, these challenges are best overcome with manual and serial QT measurements, especially at doses exceeding 100 mg/day, and interpretation using a QT nomogram to determine risk of ventricular dysrhythmia.