Antimalarials, azithromycin & QT prolongation:

The American Heart Association, American College of Cardiology, and Heart Rhythm Society have issued recommendations on using hydroxychloroquine and azithromycin to treat COVID-19 in patients with preexisting cardiovascular disease. Complications may include severe arrhythmia, polymorphic ventricular tachycardia, long QT syndrome, and increased risk for sudden death. One recommendation: the drugs should be withheld in patients with baseline QT prolongation. Additionally, the effects of these two drugs in combination on QT prolongation and arrhythmia have not been studied.

https://www.ahajournals.org/doi/pdf/10.1161/CIRCULATIONAHA.120.047521