Long QT syndrome is a genetic abnormality that can lead to VT and cardiac arrest. The diagnosis is usually made by a 12 lead ECG, but an exercise test may be required to identify those with latent long QT syndrome manifest as a lack of QT shortening during exercise. Beta-blockade remains the mainstay of treatment, especially in the type l and ll subtypes, but symptomatic patients despite beta-blockade may require defibrillator implantation or sympathectomy. Long QT type 3 patients are at particular risk as their first presentation may be sudden cardiac death, and prophylactic implantation of an ICD is recommended.
http://www.londonarrhythmiacentre.co.uk/diagnosis-ventricular-ventricular-tachycardia.html