Heart attacks in horses are very rare, however, they can occur. In people, heart attacks are often caused by blockage of the blood vessels that nourish the heart muscle itself. This causes a portion of the heart muscle to be starved of oxygen and can trigger a heart attack. This has not been found to occur in horses.
In horses, the most common cause of a heart attack is aortic rupture. In affected horses, there is an area within the wall of the aorta that is weak. This weakness (also known as an aneurysm) predisposes the aortic wall to rupture. If it does rupture, the horse rapidly bleeds to death internally. Usually, the only external signs that this is happening are sudden collapse and pale mucous membranes. In human medicine, people with known aortic aneurysms are taken to surgery and the aortic wall is preemptively re-enforced. Sadly, in equine veterinary medicine, there is currently no way to detect aortic aneurysms or to effectively repair them.