Electrocardiogram changes and arrhythmias in venlafaxine overdose

Abstract

Aims: To investigate serial electrocardiogram (ECG) parameters, haemodynamic changes and arrhythmias following venlafaxine overdose.

Methods: The study included 369 venlafaxine overdoses in 273 patients presenting to a toxicology unit where an ECG was available. Demographic information, details of ingestion, haemodynamic effects [heart rate and blood pressure (BP)] and complications (arrhythmias and conduction defects) were obtained. ECG parameters (QT, QRS) were measured manually and analysed by visual inspection, including plotting QT-HR pairs on a QT nomogram.

Results: The median ingested dose was 1500 mg [interquartile range (IQR) 600-3000 mg; range 75-13 500 mg). Tachycardia occurred in 54% and mild hypertension (systolic BP >140 mmHg) in 40%. Severe hypertension (systolic BP >180 mmHg) and hypotension (systolic BP <90 mmHg) occurred in 3% and 5%, respectively. No arrhythmias occurred based on continuous telemetry, and conduction defects were found in only seven of 369 admissions; five of these conduction defects were pre-existing abnormalities. In 22 admissions [6%, 95% confidence interval (CI) 4-10] there was an abnormal QT-HR pair, with larger doses being more likely to be associated with an abnormal QT. The median maximum QRS width was 85 ms (IQR 80-90 ms; range 70-145 ms) and the QRS was greater than 120 ms in only 24 admissions (7%, 95% CI 4-10).

Conclusions: Venlafaxine overdose causes only minor abnormalities in the QT and QRS intervals, unlikely to be associated with major arrhythmias, except possibly with large doses.

Figure 1

Plot of QT vs. heart rate (HR) in patients taking venlafaxine overdoses with one ECG for each patient (a). The nomogram line separates HR, QT pairs above the line associated with an increased risk of torsades de pointes compared with those below the line [7]. (b) Shows patients co-ingesting drugs known to cause QT prolongation in filled grey squares. (c) Shows patients taking the large doses (top 10%) in grey crosses and (d) has patients taking the smallest doses (bottom 10%) in grey triangles. (e) Provides a comparison with a control group of patients taking overdoses of drugs that do not affect the QT interval [7] in light grey squares