Qt prolongation does not mean you have arrhythmias. You don't get tachycardia or atrial flutter or necessarily sympthoms. Clinically if it means there is a risk if the QT interval gets to long that you will have a fatal arrhythmia - torsades de pointes. So you need to know how bad the Qt prolongation is. In the paper 2 patients had serious side effects - one with QT prolongation over 500ms and the second with a first-degree atrioventricular block. Another maybe had a left bundle branch (not great, but not fatal). They resolved by stopping HCQ treatment.