@garry, the QT interval is a measurement made on an electrocardiogram used to assess some of the electrical properties of the heart. QT prolongation happens when the heart muscle takes longer than normal to recharge between beats. It is an electrical disturbance which can be seen on an electrocardiogram (ECG). Excessive QT prolongation can trigger tachycardia - a heart rate over 100 beats per minute.

@dulwich may be able to explain more and his experience with QTc prolongation and azithromycin.