Do you have:
Treatment is by synchronised DC cardioversion. The defibrillator may have difficulty recognising the QRS complexes (as they are of varying morphology) and consequently fail to discharge. If this is the case, administer an unsynchronised shock.
Rectify the QT prolongation by giving IV Magnesium, stopping any causative drugs and addressing any other causes of a prolonged QT such as hypokalaemia.