A person having a heart attack is awake and may complain of a vice like squashing pain in the central chest. The pain may also radiate out into the arms, shoulders, neck, jaw and back. They might look a pale grey colour. Their pulse could be irregular, and they might feel short of breath, dizzy, nauseous, and have an impending sense of doom.
A sudden cardiac arrest (SCA) occurs when the heart has stopped. This could be caused by a heart attack, but it could also have other causes, like heart disease, drowning, electrocution, recreational drugs or other poisons.
The heart of a person in cardiac arrest has stopped beating altogether, or stopped beating in an efficient, organised way. Blood is no longer pumped through the body and oxygen is no longer supplied to the brain and cells. The person becomes unconscious and lifeless, though may have agonal gasps or seizure-like activity.