Heart attacks (blockage of the coronary arteries) often cause an arrhythmia that renders the heart beat useless to provide necessary blood pressure. That’s why you drop. The heart fibrillates instead of beating properly. The AED, once the pads are attached, reads what’s going on and knows to shock only when appropriate. The machine talks you through everything. Some can tell you if chest compressions are being done adequately. They can handle children too but some require pediatric pads separately, others use the same paddles.