In 10 years of EMS work, I've NEVER seen a medic, EMT firefighter, first responder, nurse, or physician carry a set of handcuffs. If the patient is that violent, the cops are already there (or on their way). The only restraints on most ambulances are soft restraints: cloth and foam things. Occasionally "hard" restraints, leather, are used, but they don't use cuff keys.