The QT interval is a measure of the time between the start of the Q wave and the end of the T wave in the hearts electrical cycle. A lengthened QT interval is a marker for the potential of torsade des pointes, which is a type of abnormal heart rhythm that can lead to a sudden death.
Long QT-syndrome and short QT-syndrome are inherited causes of an abnormal QT interval.
The QT interval can also be affected by adverse drug reactions. Many drugs can increase the QT interval. Macrolides, fluoroquinolones and plaquenil are drugs that can affect the QT interval and might be of relevance to Lyme disease patients.