What is the difference between cardiac arrest and mi?

Arrest requires CPR. Myocardial infarction (mi) occurs when there is an obstruction of a blood vessel that supplies part of the heart and this leads to damage to the heart muscle. Mi might lead to death and can cause cardiac arrest if not treated but not always. Cardiac arrest means that the heart is no longer pumping blood, can be caused by a variety of things and always leads to death if not treated promptly.
Degree... Mi is when the heart muscle is not getting the blood supply it needs. The part of the heart not receiving enough blood is at risk to die. Cardiac arrest is when the heart stops beating entirely. One of the causes of cardiac arrest is mi or myocardial infarction. Both are life-threatening and require an immediate 911 call.
Life or death. Cardiac arrest is when the heart stops, if not immediately reversed, it's death. Myocardial infarction is when the blood supply to part of the heart is interrupted, leading to serious damage to the heart. The damage can be so severe that it leads to cardiac arrest.