My sister has developed long qt syndrome. what is it exactly?

See below: Long qt syndrome (lqts) is a heart rhythm disorder that can potentially cause rapid heartbeats which may trigger a sudden fainting spell or seizure and in some cases, death. You can be born with a genetic mutation that puts you at risk of long qt syndrome. In addition, certain medications and medical conditions may cause this, however it is treatable.

Long QT Syndrome.: EKG records electrical events in the heart, measured as 'intervals'. The QT interval records the time it takes for the electrical tissues to 'recharge' in preparation for the next heartbeat. In long QT syndrome, prolongation of this interval occurs, to where it may allow abnormal electrical activity to occur, causing an irregular heart rhythm, which may need medication or even a shock to correct.