What causes sudden cardiac arrest?

Blood clot, atheroma: Hi. A sudden heart attack (MI) is caused when a lesion in an artery that projects into the lumen (called an atheroma, and made of inflammatory cells, cholesterol, and junk = "hardening of the arteries") has a surface exposed to the blood cells that promotes clotting. The clot (thrombus) forms and blocks the artery; no more blood gets thru. That part of the heart gets no more oxygen, and dies = MI.

Long list: Sudden cardiac death had several causes: Coronary disease or heart attack may cause sudden cardiac death, but also sudden death can occur in patients with cardiac abnormality of the heart muscle that can be detected on echo cardiogram and sometime undetected on cardiac echo, electrical genetic anomaly may cause sudden death