Ventricular tachycardia with structural heart disease or coronary heart disease as I understand can be dangerous, also V-tach related to ARVD, Brugada and long qt syndromes, also HCM can
be dangerous. rapid ventricular response in WPW syndrome with a-fib can
on rare occasions be dangerous also. Also any incessant tachycardia that last more than 12 hours a day can lead to cardiomyopathy, It is my understanding this can take months to years to manifest itself depending.