Myocardial Infarction (Heart Attack)

Myocardial Infarction is a vascular (relating to blood vessels) disease of the heart characterized by changes to the myocardium (heart muscle) due to inadequate or interrupted blood flow to the heart muscle. It is most commonly caused by coronary insufficiency resulting from atherosclerosis of the coronary arteries. In this event, a coronary artery may become blocked by a blood clot. Blockage of the coronary artery deprives the heart muscle of blood and oxygen causing injury to the heart muscle. From then on, the heart muscle dies unless the coronary artery is unblocked or the dying heart muscle receives blood and oxygen from elsewhere by surgical intervention. Chest pain and chest pressure sensation are usually felt at the onset of injury to the heart muscle. If blood flow to the heart muscle is not restored within up to about 40 minutes, the heart muscle irreversibly dies until completely dead and scar tissue replaces the dead heart muscle.

Coronary arteries are the blood vessels responsible for taking blood to all the tissues of the heart, excluding certain layers of the pericardium (protective sac that encloses the heart and functions to prevent the sudden overfilling of the heart). They originate from the aorta, which is the major artery that receives oxygenated blood from the heart and allows the blood to flow to all parts of the body, and are commonly two in number (right and left). The branches of the coronary arteries are referred to as ‘end arteries’ because the area in which they are distributed is not supplied by any other branch of another large artery. In this way, the death of tissue of a particular area of the heart muscle can be appreciated more. When a branch of a coronary artery is blocked, loss of blood supply to the area results in dead tissue in a matter of hours. The most common site of atherosclerosis, leading to a heart attack, is the anterior interventricular branch of the left coronary artery, which supplies the left and middle aspects of the heart in front.

Causes

The most common cause of myocardial infarction is the narrowing of blood vessels of the heart due to atheromatous (referring to atherosclerosis) plaques. Plaque rupture can result in a blood clot and further blockage of the coronary arteries, along with varying degrees of vasospasm (narrowing of the artery).

Besides atherosclerotic causes, the inflammation of the coronary arteries, a stab wound to the heart, a blood clot forming elsewhere in the body (for instance, in a heart chamber), cocaine abuse which can cause spasms of the coronary arteries, use of amphetamines, complications from heart surgery, coronary artery vasospasm, chest trauma and some other rare heart problems may cause the onset of a heart attack.

Risk Factors

Similar to other cardiovascular diseases, the risk factors for myocardial infarction are smoking, hypertension, high cholesterol level, lack of exercise, obesity, a poor diet and excess alcohol intake. Having diabetes can also contribute to its occurrence. Ethnicity, gender (being male), and having a family history of heart disease or a stroke that occurred in a father or brother aged below 55 or in a mother or sister below 65.

Diagnosis

ECG
An electrocardiogram (ECG) or heart tracing, which measures the electrical activity of the whole heart, is used to point out changes to the normal pattern of a heart tracing, indicating the occurrence of a heart attack. Among the abnormal patterns for myocardial infarction are ‘pathological Q waves’, ST elevation (when the infarction spans the entire thickness of the heart wall) and possibly T wave inversions. Note that it is possible to have had a heart attack and not be detected by an ECG but this usually occurs if a lot of time has passed since the heart attack or it could have been a very mild heart attack.

Blood Tests
A blood test that measures certain substances, which may be associated with heart muscle cells, can also be used to confirm a myocardial infarction. A substance, troponin, is present in heart muscle cells and, in the event of damage to heart muscle, it is released into the bloodstream at a level above normal. Creatinine kinase may also be released into the bloodstream in response to damage to heart muscle. These substances are usually at their highest level in the blood stream within 1-2 days of having a heart attack but soon decrease in level after a few days up to a couple weeks.

Other tests may also be used to verify the diagnosis such as an echocardiogram (ultrasound of the heart), etc.

Severity

A substantial number of people die from myocardial infarctions each year. Most of the time, a heart attack may go unnoticed or is not paid special attention to because it may have been minor. Once you’ve had a heart attack, it does not usually end there. You may seem fine for the time being, until a couple of years later when your heart might have suffered more damage. Note that many people do recover from a myocardial infarction and have no complications, which may be associated with a change from bad lifestyle habits. The larger the myocardial infarction, the more life-threatening it is and the more complications may result. Some possible complications are heart failure, as the heart muscle becomes unable to pump an adequate amount of blood to the rest of the body, and abnormal heart rhythms, e.g ventricular fibrillation, which can result in collapse and sudden death.

Treatment

Treatment may vary depending on the situation. The possibilities include but are not limited to:
1. Restoration of Blood Flow
Emergency angioplasty – a tiny wire with a balloon is carefully guided to the blocked coronary artery and is used to open up the artery wide enough for adequate blood to pass through.

An injection of a thrombolytic (clot-busting) drug, e.g. streptokinase.

2. Beta-blocker Drug
Beta-blockers block the action of certain hormones which can increase the rate and force of contractions of the heart. They also help to prevent abnormal heart rhythms from developing.

After having a myocardial infarction, normally it is advised to take regular medication for the rest of your life. To prevent further myocardial infarction and to help prevent complications, the following may be prescribed:

Aspirin – helps to prevent blood clots from forming.

A beta-blocker – (mentioned previously)

An Angiotensin Converting Enzyme (ACE) inhibitor – has a number of actions that prove to be protective to the heart.

A statin drug – can lower cholesterol levels in the blood (see post on high cholesterol).