A heart attack is different from a cardiac arrest.

A heart attack originates as a blood flow (circulatory) problem, where insufficient blood reaches the heart muscle. The heart will usually keep beating and the person will usually be conscious, breathing and likely to have symptoms such as chest pain or tightness. Call triple zero (000) and await further instructions.
A cardiac arrest originates as an electrical problem and the heart will stop beating. The person will usually be unconscious and not breathing. Call triple zero (000). You will need to start chest compressions and/or find a defibrillator as soon as possible.