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Chapter: Biology of Disease: Disorders of the Cardiovascular System

Myocardial Infarction

Plasma Enzymes in Myocardial Infarction, Treatment, Coronary Bypass Surgery.

MYOCARDIAL INFARCTION

An infarction is the death of a section of tissue because its blood supply has been cut off; an infarct is the segment of tissue affected. In general, if one artery is blocked neighboring arteries with communicating branches can compensate and tissue death is limited. Infarction occurs in places where small arteries do not communicate with one another, such as in the kidney, or where all the arteries together supply only enough blood for the whole organ, such as in the brain; or where alternative arteries are also blocked and cannot take over. The latter is what happens to the coronary vessels in many middle-aged hearts, particularly in the developed world. Thus myocardial infarction almost always occurs in patients with atheroma in the coronary arteries resulting from sudden coronary thrombosis, usually at the site of a fissure or rupture of the surface of an atheromatous plaque. There may be hemorrhage into the plaque with local coronary spasms. Irreparable damage can begin after only 20 min of occlusion. After about six h, the site of infarction of the myocardium is pale and swollen and after 24 h necrotic tissue appears deep red owing to the hemorrhage. Subsequently an inflammatory reaction develops and the infarcted tissue turns gray in color.

Myocardial infarction is the commonest cause of death in the UK but surprisingly was hardly known before 1910. Patients present with severe intermittent chest pain that is similar in character to the angina that can occur on exertion, but usually occurring at rest and lasting several hours. Sometimes, however, the pain is less severe and may be mistaken for indigestion. The episodes of pain may become more frequent, but about 20% of patients have no pain. If there is pain, the onset is usually, but not always, sudden. The patient may feel restless and there is often sweating, nausea and vomiting. The most recognizable pain is in the middle of the chest that may spread to the back, jaw or left arm. The condition, once recognized, is a medical emergency. Half of the deaths occur in the first three to four h after the symptoms begin, so the sooner treatment begins the better the chances of survival.

PLASMA ENZYMES IN MYOCARDIAL INFARCTION

The diagnosis of myocardial infarction is usually made on the basis of the clinical symptoms and ECG findings, and is confirmed by the characteristic changes in plasma enzyme activities. The enzyme activities that are of the greatest value are creatine kinase (CK), lactate dehydrogenase (LDH) and aspartate transaminase (AST, previously known as GOT, glutamate oxaloacetate transaminase). Plasma enzyme activities are increased in about 95% of cases of myocardial infarction and sometimes increase to high levels. The degree of increase gives a rough estimation of the size of the infarct but is of little prognostic value. A second and subsequent rise after their return to normal may indicate extension of the damage. All tend to show normal serum activities until at least four h after the onset of chest pain due to the infarction and so blood samples should not be taken until after this time. If the initial serum CK activity is approximately normal, a second blood sample should be taken four to six h later. An increase in plasma CK activity supports the diagnosis of an infarction. The sequence of changes in plasma AST activity after a myocardial infarction are similar to those for CK but the increases are significantly less.

 

TREATMENT

Usually the patient is given an aspirin to chew, which should improve the chances of survival by reducing the clot in a coronary artery. AA-blocker may also be given to slow the heart rate and reduce its workload. Oxygen may be given through a facemask to deliver more oxygen to the heart. Blood clots in an artery can often be cleared by intravenous thrombolytic therapy. The indication for thrombolytic treatment is usually based on the clinical presentation and the ECG picture rather than on the activities of plasma enzymes. Treatment must be given within 6 h of the start of the heart attack to be effective. After 6 h it is likely that some of the damage will be permanent and the patient could be compromized and some may die. Most patients who survive for a few days after the attack can expect a full recovery but about 10% will die within a year. The majority of deaths occur in the next three to four months in patients who continue to have angina, arrhythmias and subsequent heart failure.

CORONARY BYPASS SURGERY

In individuals who have angina and coronary arterial disease that is not too widespread, coronary bypass surgery is a possible treatment that improves exercise tolerance, reduces symptoms and decreases the number of drugs that are needed. Bypass surgery involves grafting arteries or veins taken from the leg to take blood from the aorta past the obstructed region, replacing the role of the coronary arteries in supplying blood to the heart muscle. Such a graft often works well for up to 10 years or more.

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