Billhook
Its very common that people get Heart Attack and Cardiac Arrest muddled up (and indeed heart attack and stroke, strokes being something that affects the brain).
A heart attack or myocardial infarction (MI) is an occlusion of the coronary arteries which are vessels that supply the heart muscle tissue with its own blood supply. This loss of blood to a part of the heart causes death of the muscles tissue in part of the heart affected. The person having an MI will usually be alert, breathing and have a pulse, but they will have chest pain, often described as crushing or heavy, which may radiate to the neck, jaw or arm(s). They will usually look grey and clammy. This is a 999 call immediately job and if there is a community defib nearby, send someone to get it as they could go into Cardiac Arrest at any moment and having the defib there before this happens could make a difference.
A cardiac arrest is basically no or insufficient output from the heart. It aint pumping properly and could be in one of several rhythms (eg Asytole, PEA, VF or VT). They will be unconscious, not breathing and have no pulse and look very pale. 999 and get on that chest asap once you establish they are not breathing. Don't worry about getting it wrong - if they aren't in cardiac arrest, the minute you get on their chest, they'll soon come round albeit with a few broken ribs, but better that than do nothing. If you have a 999 call handler on the phone, they will stay on the phone and talk you through CPR so have phone on speaker if you can, or just think Nelly the Elephant. And if theres more than one of you.... swap over chest compressions every 2 mins between you (if the others are trained or feel happy to do so). Doing cpr right is knackering and the longer you do it, the less effective your compressions will be, no matter how big and hard you think you are. Swap every two mins (5 cycles of 30 compressions).
Your instructor was right about it not being like on TV where people rush in with two big paddles, shock them and they sit up. Out of hospital arrests have a very low success rate, less than1 in 10, though that will include many where you wouldn't expect to get them back like 80 year olds. Good cpr can make big difference though with younger people who usually will have a healthier heart and more likely to make a good recovery. This very much depends on the reason for the arrest in the first place though.....
J