What is a heart attack?
A heart attack (a myocardial infarction, or MI) is the death of heart muscle, caused by a blockage in one of the coronary arteries. A heart attack occurs when a blood clot forms at the site of an atherosclerotic plaque in a coronary artery. The clot blocks the artery, and blood flow stops. The heart muscle being supplied by that artery immediately becomes starved for oxygen (i.e., the muscle becomes ischemic), and if blood flow is not restored within a few hours, the heart muscle dies. (Click here for a pictorial representation of how blood clots, and how a blood clot can cause the death of heart muscle.)What are the consequences of a heart attack?
When a portion of heart muscle dies, several bad things can happen. If the amount of damage is large enough, the patient can develop immediate heart failure and shock. (When doctors refer to “shock,” they are talking about extremely low blood pressure, caused by the heart’s inability to pump sufficient blood to the body’s tissues.) When heart failure occurs immediately after a heart attack, there is an extremely high risk of early death.If the amount of heart damage is only mild or moderate, heart failure does not occur – at least, not right away. However, in the effort to heal itself, the heart goes through a period of “remodeling,” in which the heart enlarges and changes shape. This remodeling eventually leads to a decrease in cardiac pumping efficiency, and can lead to a more gradual onset of heart failure months or years after the heart attack. To a large degree, the outcome of a heart attack depends on the amount of heart muscle that dies. This, in turn, is related to which coronary artery is blocked (click here for a description of the coronary arteries), and to where in the artery the blockage occurs. (A blockage near the origin of an artery will affect more heart muscle than a blockage farther down the artery.)
During the first few hours of an acute heart attack, a transient form of electrical instability occurs that makes the heart prone to develop ventricular tachycardia and ventricular fibrillation - frequently lethal heart arrhythmias. Furthermore, during the healing process, the scar tissue that replaces the dead heart muscle can cause a permanent electrical instability. Thus, sudden death is common both during acute heart attacks and among survivors of heart attacks, and accounts for approximately 50% of all deaths due to coronary artery disease.
Why are the first few hours critical?
The first 3 – 6 hours after the onset of the heart attack are utterly critical. First, most of the lethal arrhythmias seen with acute heart attacks occur during the first few hours. If these arrhythmias occur while the victim is under medical attention, they can virtually always be stopped in time to prevent a catastrophe.Further, if the artery can be opened within the first few hours after the blockage occurs, much of the dying heart muscle can be saved, much of the permanent heart damage can be avoided, and the patient’s risk of death or permanent disability can be greatly diminished. But if treatment is delayed beyond 6 hours, the amount of heart muscle that can still be saved drops off significantly.
What this means, of course, is that if a patient having a heart attack tries to “ride it out” at home for a while, hoping the symptoms are due to indigestion or muscle strain, or if the medical personnel delay instituting therapy for any reason, the delay can lead to devastating and permanent results.
