Vtach and Vfib are the two shockable rhythms. Those are common in many medical emergencies. Just because the patient doesn't have a feelable pulse doesn't mean they have "flatlined" and don't have a shockable rhythm! You have to put the AED electrodes on their chest to be sure.

If the patient has truly flatlined (asystolic) then shocking them won't bring them back. That only works in the movies.