The prevalence of QT prolongation in a population of patients with substance use disorders

Introduction and Aims Drug induced QT prolongation occurs in patients with substance use disorders from prescription medications that prolong the QT, such as methadone. Knowing the prevalence of QT prolongation in this population is important for prescribers. This study aimed to investigate the prevalence of QT prolongation in patients with current substance use disorders. Design and Methods We undertook a retrospective review of electrocardiograms (ECG) from patients with substance use disorders from an urban general hospital with a large drug and alcohol service and toxicology unit. ECGs were taken from patients seen by the alcohol and drug unit over three years. The QT interval was measured manually on each ECG and defined as abnormal if above the line on the QT nomogram. The QT was also heart rate corrected using Fridericia's formula (QTcF) to investigate associated factors. Results Nine of 446 (2.0%; 95% confidence interval 1.0–3.9%) patients had an ECG with a prolonged QT interval. Three were prescribed methadone for opiate dependence (80, 90 and 125 mg daily), one also with hypokalemia; one prescribed escitalopram with hypokalaemia/hypomagnesaemia; three more with hypokalaemia alone. Only two patients had a prolonged QT with no identifiable cause. There was no association between QTcF and sex (P = 0.34), but there was a statistically significant association with age (Pearson R = 0.19, 95% confidence interval 0.10–0.28, P