Automatic:

Designed for use by laypersons (ideally with basic training on how to use the AED but the units will give visual and audible instructions allowing practically anyone to use them successfully), these units will issue shocks as and when needed automatically.

Semi-Automatic:

Designed for use by healthcare professionals who are familiar with and can understand ECG (electrocardiogram) readings, these units will detect when a shock is required and alert the user who will then make the decision on whether or not the shock should be issued. Semi-automatic units also have a pacemaker function for occasions where the victims heart rate is too slow (bradycardia).