an AED is an automated external defibrillator. (read here) it's a crucial piece of equipment used in Cardio-Pulmonary Resuscitation, & in addition to being readily available at all medical facilities, the American Heart Association recommends that public places like office complexes, shopping malls & sports arenas be equipped with these devices.
although any man-on-the-street can perform CPR, the AED should only be used by those trained in CPR (read here) which explains the warning label & the key in the glass casing.
although this mall is to be applauded for installing AEDs for emergency use, the success of this operation is dependent on many interconnected processes, eg.
1) the public must be aware of its location;
2) it must be regularly serviced/maintained to preserve its function;
3) there must be a mass training in CPR for the public before the use of AEDs can be effective;
4) there must be an rapid-respond emergency team available within minutes of such an an emergency;
if all these procedures were in place, you'd wonder if the tragedy at a recent marathon could have been prevented. read Zewt's take here.
it's all academic now, but don't bet on such an incident NOT repeating in the near future.
PS. the AED in this post is located at the basement level of Wisma Atria & i'm told there are many like this positioned in offices/malls along orchard road. iPhone users in s'pore can download the free app "AED nearby" to locate the nearest device.