Myocardial Infarction, Acute

© Reed Group

Risk: Risk factors for atherosclerotic plaque formation which cannot be modified include age over 60 (plaques take time to develop), male gender, and family history. Risk factors that can be modified include obesity, smoking, diabetes mellitus, elevated lipid levels (hypercholesterolemia, hypertriglyceridemia), uncontrolled hypertension, and sedentary lifestyle.

Among individuals between 40 and 70 years of age, more men than women will have atherosclerosis and MIs; risk for men and women becomes equal after age 70. (Fenton). Although MI is most common in individuals older than age 45 years, the risk in younger individuals increases for cocaine users, insulin-dependent diabetics, those with hypercholesterolemia, smokers, and those with a positive family history of atherosclerosis. Those with a first-degree male relative younger than 45 years of age with MI, or with a first-degree female relative younger than 55 years of age with MI also have a heightened risk (Fenton).

Incidence and Prevalence: In the US, about 1.5 million MIs per year result in 500,000 to 700,000 deaths (Fenton). An MI occurs every 29 seconds, with one death each minute. In the US, incidence of MI is 600 per 100,000 individuals per year (Fenton). Incidence of MI in developed countries is similar to the US with the exception of France and the Mediterranean region where the incidence is lower; some researchers believe that this is due to a diet rich in monounsaturated fats and wine consumption with meals. Cardiovascular disease is the leading cause of death worldwide, causing 12 million deaths per year (Garas).

History: A thorough history is essential in making the diagnosis of MI, since MI can be “silent” or go unrecognized. Patient care always takes priority, but it is important to obtain a history of any risk factors including cardiac history, diabetes mellitus, elevated lipid levels (i.e., hypercholesterolemia, hypertriglyceridemia), uncontrolled hypertension, smoking, and sedentary lifestyle, as well as family history. The circumstances surrounding onset should be delineated; time since onset is especially important since chest discomfort that lasts longer than 30 minutes suggests MI. It is also important to ask about aggravating and alleviating factors. Individuals may delay seeking treatment because “the pain went away when I rested.” Pain accompanying MI is often localized in the anterior chest and may be described as tightness, pressure, heaviness, or a squeezing sensation. Pain may radiate into the arm (more commonly the left arm), neck, jaw, or stomach area. It is very important to ask about shortness of breath (dyspnea) and the circumstances under which this occurs (i.e., with exercise, with any movement, all the time). Dyspnea may be the only symptom of MI in diabetics and the elderly. There may also be cough and wheezing. Nausea with or without vomiting and abdominal pain may be present, particularly with inferior or posterior wall involvement. The individual may be lightheaded with or without loss of consciousness (syncope), anxious, sweaty (diaphoretic), weak, or fatigued. It is especially important to consider the diagnosis of MI in individuals who are more likely to have atypical presentations including diabetics, the elderly, and women.

Physical exam: On physical examination, the individual may be lying quietly in bed and appear pale and sweaty. Heart rate may be increased (tachycardia), and the pulse may be irregular due to dysrhythmias. Additional sounds not heard in a healthy heart may also be present (third and/or fourth heart sounds). Blood pressure may be increased (hypertension) or decreased (hypotension). Some individuals may have a low-grade fever. If congestive heart failure (CHF) is present, there may be abnormal lung sounds (rales), swelling in the lower extremities (edema), and distention of the neck veins (elevated jugular venous pressure).

Tests: Commonly ordered blood tests include a complete blood count (CBC) to exclude anemia and infection; serial cardiac enzymes (troponin, creatine kinase, and creatine kinase-MB) to follow damage to heart muscle over time; serum electrolyte levels; and C-reactive protein (CRP), a marker of inflammation. An electrocardiogram (ECG) shows changes in the electrical patterns of the heart and can help to localize the damaged area. However, only about half of all individuals with MI will show changes on their ECG.

Imaging studies can help to rule out alternative diagnoses or complications as well as localize the damaged area of heart muscle. Chest x-ray can reveal pulmonary edema resulting from heart failure. Echocardiography is helpful in evaluating wall motion abnormalities and ventricular function and also can show complications from an MI such as valvular insufficiency or pericardial effusion. Transesophageal echocardiography is used to distinguish MI from aortic dissection. Radioisotope scans reveal areas of healthy tissue and are especially helpful in individuals with an atypical presentation. High-resolution CT scans allow direct visualization of the interior (lumen) of coronary arteries as well as any plaque in the coronary walls.