myocardial infarction

ABBR: MI The loss of living heart muscle as a result of coronary artery occlusion.
SYN: SEE: cardiac infarction

Descriptive text is not available for this image

MYOCARDIAL INFARCTION Myocardial infarction as seen on an electrocardiogram SEE: advanced cardiac life support ; SEE: atherosclerosis; SEE: cardiac arrest ; SEE: sudden death

INCIDENCE
Acute MI affects about 800,000 Americans each year and approx. 700,000 die of heart disease each year.

CAUSES
In outpatients, MI or its related syndromes (acute coronary syndrome or unstable angina) usually occurs when an atheromatous plaque in a coronary artery ruptures, and the resulting clot obstructs the injured blood vessel. Perfusion of the muscular tissue that lies downstream from the blocked artery is lost. If blood flow is not restored within a few hours, the heart muscle dies.

RISK FACTORS
Proven risk factors for MI are tobacco use, diabetes mellitus, abnormally high cholesterol levels, elevated levels of high-sensitivity C-reactive protein, high blood pressure, male gender, advanced age, obesity, physical inactivity, chronic kidney disease, or a family history of MI at an early age.

SYMPTOMS
Classic symptoms of MI in men are a gradual onset of pain or pressure, felt most intensely in the center of the chest, radiating into the neck, jaw, shoulders, or arms, and lasting more than a half hour. Pain typically is dull or heavy rather than sharp or stabbing, and often is associated with difficult breathing, nausea, vomiting, and profuse sweating. Clinical presentations, however, vary considerably, and distinct presentations are seen in women and older adults, in whom, e.g., unexplained breathlessness is often the primary symptom. Many patients may mistake their symptoms for indigestion, intestinal gas, or muscular aches. About a third of all MIs are clinically silent, and almost half present with atypical symptoms. Often patients suffering MI have had angina pectoris for several weeks before and simply did not recognize it.

DIAGNOSIS
A compatible history associated with elevated blood levels of cardiac muscle enzymes (cardiac troponins) establishes the diagnosis. An ST-segment elevation of more than 1 mm above baseline in at least two contiguous precordial leads (or two adjacent limb leads) supports the diagnosis. MIs with this presentation are known as ST-segment elevation MI (STEMI). This finding usually indicates significant muscle damage in the infarct area, a poorer prognosis, and a higher incidence of complications (such as arrhythmias, heart failure, or cardiogenic shock) than in a non-ST-segment elevation MI.

DIFFERENTIAL DIAGNOSIS
The differential diagnosis of chest pain must always be carefully considered because other serious illnesses, such as pulmonary embolism, pericarditis, aortic dissection, esophageal rupture, acute cholecystitis, esophagitis, pancreatitis, or splenic rupture may mimic MI.

TREATMENT
MI is a medical emergency; diagnosis and treatment should not be delayed. People who experience symptoms suggestive of MI should be instructed to call 911 immediately and chew and swallow aspirin. Oxygen is administered at 4 L/min as soon as it is available. History is gathered throughout the first few minutes after admission even as a 12-lead ECG is being done and blood taken for biomarkers. Cardiac troponins may not become elevated until 4 or more hr after symptoms begin. If the patient is hypotensive or in cardiogenic shock, right-sided ECG leads are assessed for a right ventricular (RV) infarct. An intravenous access is established along with continuous cardiac monitoring, and medications (which may include chewed aspirin [162 to 325 mg], heparins, or other medications to inhibit platelet aggregation, nitroglycerin [given under the tongue, sprayed or IV], IV morphine, and beta-blockers) are administered as prescribed. Pain is assessed on a 1 to 10 intensity scale, and morphine 2 to 10 mg administered IV, with incremental doses of 2 to 8 mg every 5 to 15 min until relief is obtained. Beta-blockers (such as metoprolol or atenolol) decrease myocardial oxygen demand, helping to limit the amount of heart muscle damaged. An IV beta-blocker should be given if the patient is hypertensive or has a tachyarrhythmia provided there are no contraindications. Patients with STEMI who arrive at the hospital within 6 hr of the onset of symptoms are treated with fibrinolytic therapy or percutaneous coronary intervention (PCI). The goal for administration of fibrinolytic therapy is 30 min postsymptom onset (door-to-needle); for PCI 90 min (door-to-balloon inflation). Absolute contraindications of fibrinolytic therapy include previous intracranial hemorrhage or ischemic stroke within 3 months (intracranial malignancy), active bleeding, or bleeding disorders (except menses), significant closed head or facial trauma within 3 months (known structural cerebral vascular lesions), and suspected aortic dissection. Reperfusion is the immediate goal and is usually best accomplished with balloon angioplasty and endovascular stent placement, but emergency coronary bypass surgery may be needed in cases when PCI fails. An angiotensin-converting enzyme (ACE) inhibitor is administered within 24 hr of a STEMI to suppress the renin-angiotensin-aldosterone system and prevent excess fluid retention. ACE inhibitors also prevent conversion of angiotensin I to angiotensin II (a potent vasoconstrictor), reducing afterload to help prevent heart failure.

In MI complicated by pulmonary edema, diuretics are administered, and dobutamine infusions may be necessary to increase cardiac output. Strict glucose control (maintaining blood sugars below 150 mg/dL, and preferably in the normal range) reduces mortality in acute MI. Hypotension and circulatory collapse frequently occur in patients with significant RV infarctions, and fluid challenge is administered to optimize RV preload. If this is unsuccessful, the patient with an RV infarct will require inotropic support, correction of bradycardia, and measures to achieve atrioventricular synchrony (such as cardioversion for atrial fibrillation). In patients with ventricular arrhythmias, defibrillation, or cardioversion, lidocaine, vasopressin, or amiodarone infusions, or other drugs, may be necessary. Anemic patients (hematocrit less than 30 or those actively bleeding) benefit from blood (packed red cell) transfusions.

With contemporary care, about 95% of patients with acute MI who arrive at the hospital in time will survive. These patients are referred to nutrition therapists to learn how to use low-fat, low-cholesterol diets, and to cardiac rehabilitation programs for exercise training, tobacco cessation, and psychosocial support. Most patients who have survived an MI will be treated for specified periods with antiplatelet medications (aspirin plus P2Y12 receptor antagonists), beta-blocking drugs, ACE inhibitors, statins, and/or nitrates.

IMPACT ON HEALTH
The probability of dying from MI is related to the patient's general health, whether arrhythmias such as ventricular fibrillation or ventricular tachycardia occur, and how rapidly the patient seeks medical attention and receives appropriate therapies (such as thrombolytic drugs, angioplasty, antiplatelet drugs, beta-blockers, and intensive electrocardiographic monitoring).

PATIENT CARE
Acute Care: All diagnostic and treatment procedures are explained briefly to the patient upon admission in order to reduce stress and anxiety. Continuous electrocardiographic monitoring is used to identify changes in heart rhythm, rate, and conduction. Location, radiation, quality, severity, and frequency of chest pain are documented and relieved with IV morphine. Bleeding is the most common complication of antiplatelet, anticoagulant, and fibrinolytic therapies. The complete blood count, prothrombin time, and activated partial thromboplastin time are monitored at daily intervals. IV sites are assessed for evidence of bleeding. Fluid balance and pulmonary status are closely monitored for signs of fluid retention and overload. Breath sounds are auscultated for crackles (the crackles, if caused by atelectasis, may resolve by having the patient cough; the crackles, if they do not resolve by coughing, may indicate pulmonary edema. Heart sounds are auscultated for S3 or S4 gallops or new heart murmurs. Patient care and other activities should be organized to allow for periods of uninterrupted rest. Stool softeners are prescribed to prevent straining during defecation, which can cause vagal stimulation and slow the heart rate. Antithrombotic stockings help prevent venostasis and deep vein thrombosis. Emotional support is provided to decrease stress and anxiety. Adjustment disorders and depression are often experienced by MI patients, and the patient and family are assisted to deal with these feelings. Stress tests, coronary angiography, cardiac imaging procedures, reperfusion techniques, and other interventions are explained. The patient receives assistance in coping with changes in health status and self-image.

Ambulatory Care: Cardiac rehabilitation begins as soon as the patient is physiologically stable. The goal of cardiac rehabilitation is to have the patient establish a healthy lifestyle that minimizes the risk of another MI. Ambulation is slowly increased, and a low-level treadmill test may be ordered before discharge to determine exercise tolerance and the risk of future heart attacks. Patients are instructed not only to measure their pulse but also to assess their response to exercise in terms of fatigue, ease of breathing, and perceived workload. After the patient is discharged, his or her exercise is slowly increased, first while being monitored closely by supervised cardiac rehabilitation, and then more independently. The patient also receives information about a low saturated fat, low cholesterol, low calorie diet, such as the DASH eating plan (Dietary Approaches to Stop Hypertension), resumption of sexual activity, work, and other activities. The patient is instructed about desired and adverse effects of all medications: aspirin therapy is usually prescribed as ongoing antiplatelet therapy (with or without clopidogrel), but patients should be warned about the risk of bleeding and be advised to avoid products containing ibuprofen, which blocks antiplatelet effects of aspirin. Smoking cessation is an important preventive for future MIs. High blood pressure, obesity, adverse cholesterol levels, and diabetes mellitus also should be carefully managed to help prevent future MIs. Alcohol intake should be limited to 1 drink daily (women), 2 drinks daily (men). Opportunities are created for patients and families to share feelings and receive realistic reassurance about common fears.

There's more to see -- the rest of this topic is available only to subscribers.