You can use a defibrillator if there are no medics online, on the server. Otherwise you can not use them. If you are a SUR+ in the NHS you have the ability to revive people at Kavala hospital so long as the medic is more than 1km away from the hospital.
Off Duty Surgeons and Reviving
There are some restrictions to reviving while off duty as a medic.
You must be a civilian.
You must have the person to be revived outside the Kavala hospital building.
You must have defibs.
There must not be a medic within 1km of the body.
You must be a Surgeon in the medics.
If this criteria is met the person can be revived and you can be on your way!