Acute Myocardial Infarction

Myocardial infarction—a heart attack, in common parlance—affects at least three-quarters of a million Americans a year. It is an acute event in the course of atherosclerotic coronary-artery disease; after the initial attack of chest pain it takes approximately six weeks for the body to repair the damage to the heart muscle by forming a firm scar. Present standards of care require that the early treatment be administered in the intensive-care unit of a hospital (or in a coronary-care unit if available), followed by further care at a routine hospital facility. The average hospital stay is one week, with wide variations depending on the type of myocardial infarction and the possibility of complications.

The term "infarction" means damage to a tissue in the body caused by depriving it of blood supply. Myocardial infarction results from myocardial ischemia that has lasted too long for the affected tissue to recover and has become irreversible, thereby producing necrosis (death associated with softening) of a portion of the heart muscle. Myocardial infarction is almost always caused by an intracoronary accident (described earlier in this chapter) and in most cases involves formation of a intracoronary thrombus occluding the coronary artery at the point of the accident (usually rupture of a plaque). The size of the infarction—in other words, the extent of the damage to the myocardium—depends on the size of the occluded coronary artery, the location of the occlusion, and the availability of protective collateral circulation. If irreversible ischemia affects a large portion of the muscle of the left ventricle, the patient may not survive; hence the first few hours after the initial chest pain are critical, and mortality in that period is high. However, ventricular fibrillation leading to cardiac arrest may also develop in patients with smaller infarctions; they can be successfully resuscitated and may completely recover.

The outlook for patients suffering a myocardial infarction who reach the hospital alive is favorable. Two studies dealing with early treatment, based on some 26,000 cases, showed that the survival rate for patients admitted to a hospital with myocardial infarction is

greater than 87 percent when they are treated conventionally and 90 percent when thrombolytic drugs are administered.

Diagnosis of Myocardial Infarction . A patient's description of an attack of chest pain and associated symptoms is often a sufficient basis for suspecting acute myocardial infarction. Further medical evaluation in the emergency unit, or later in the hospital, may confirm the diagnosis. Myocardial infarction can be classified by its size and location. The most extensive infarction is a transmural infarction , involving all layers of the wall of the left ventricle. It is also referred to as a Q-wave infarction because of certain electrocardiographic abnormalities. It can affect the front wall of the ventricle (anterior infarction) or the lower back wall (inferior infarction). This type of infarction causes the most serious damage and leads to the most complications. Subendocardial infarction affects the inner layer of the left ventricular muscle. Infarction of the right ventricle is less common than that of the left ventricle and as a rule represents an extension of an inferior infarction of the left ventricle. Myocardial infarction may also be less well defined, involving smaller sections of the myocardium, sometimes in more than one area. Furthermore, there are instances in which myocardial infarction can only be suspected, not proven.

The characteristic feature of the initial attack of myocardial infarction is chest pain, usually in the center of the chest behind the breastbone or across the upper portion of the front of the chest. Pain may radiate to one or both arms and the neck. The severity of the pain varies, but it develops without provocation and persists, usually unrelieved by nitroglycerin. About one-half of patients having their first myocardial infarction experience chest pain for the first time during that attack. In others it may be preceded by stable or unstable angina pectoris of varying duration. Other symptoms may accompany or follow the attack of chest pain—dyspnea, nausea with or without vomiting, pallor with cold perspiration, faintness or dizziness, and collapse.

Classic attacks of myocardial infarction are usually easy to identify, not only for a physician but also for the person stricken. There are many instances, however, when an attack may prove difficult to diagnose. Chest pain may be in unusual locations or so slight as to be dismissed by the patient. Occasionally pain is altogether absent;

in that case the patient may merely experience shortness of breath and sudden weakness or may collapse without warning. The reaction of someone experiencing a myocardial infarction runs from shock and alarm at recognizing the onset of a serious illness to dismissal of the pain as indigestion.

The course and outcome of acute myocardial infarction relates to many factors, the most important of which is the size of the infarct. Irreversible damage to a large portion of the left ventricle, the main pumping chamber, may be incompatible with survival. If the damage involves 40 percent or more of the left ventricular musculature, the outcome is invariably fatal: cardiac arrest soon follows, and efforts to resuscitate, even when immediate, are unsuccessful. Myocardial infarcts involving less than 40 percent of the muscle vary in their effect on the cardiac function according to the size and location of the infarct (those affecting the anterior wall are most serious); possible underlying heart disease, such as damage to the heart from hypertension or previous coronary episodes; the general condition of the patient and the presence of other diseases, such as diabetes, kidney disease, or lung disease; the presence of disease in other coronary arteries than the one occluded; and the development of complications.

The relationship between the size of the infarction and the effect on cardiac function, as modified by the various secondary influences, can be presented (in an admittedly oversimplified manner) as follows:

massive myocardial infarction® sudden death

very large infarction® cardiogenic shock

large infarction® left ventricular failure

small to medium infarction® no impairment of functions

This relationship describes the initial response of the heart to the sudden damaging effect of ischemia. The overall course is further determined by the secondary sequelae of the infarction and its possible complications. Yet the initial response to the attack is critical: about one-third of all deaths resulting from myocardial infarction occur immediately after the onset or within a few hours, often before the patient reaches a hospital. Out-of-hospital resuscitation of patients

can save only those with modest damage to the myocardium, that is, who have developed primary ventricular fibrillation.

Myocardial infarction produces a variety of secondary effects or complications. They most commonly develop within 48 hours of the attack and may or may not respond favorably to treatment. It should be reiterated, however, that close to 90 percent of patients admitted to the hospital survive the attack. Most enter the hospital after chest pain and associated symptoms have subsided and may feel well throughout the hospital stay. Yet their prognosis may be affected by one or more of the immediate sequelae:

Cardiogenic shock . This may develop with the initial attack, may come on gradually, or may strike suddenly later. Shock developing after a day or two of improvement is usually caused by a major complication of the infarction. Cardiogenic shock is associated with high mortality.

Arrhythmias . Ventricular arrhythmias are very common and are usually inconsequential if limited to premature beats. Monitoring of cardiac rhythm permits immediate intervention if more-serious ventricular arrhythmias develop. Atrial arrhythmias are less common and as a rule respond well to treatment.

Heart failure . This may be present at admission to the hospital or may develop later. Left ventricular failure requires immediate therapy but usually can be contained. Unresponsive heart failure, particularly if affecting both ventricles, is an unfavorable sign and often signifies an extension of the infarction to the right ventricle.

Hypotension . The patient's blood pressure is usually lower than normal following a heart attack; however, in some patients blood pressure falls below an acceptable level (but not low enough to cause shock). Appropriate treatment can rectify the problem.

Major complications of myocardial infarction include the following:

Extension of myocardial infarction . Occasionally a new attack of chest pain develops after one or two days without pain. New electrocardiographic abnormalities may show that the infarct has increased in size.

Angina pectoris . The patient may continue having attacks of chest pain after the initial attack has subsided, sometimes indicating disease in the nonoccluded branches.

Rupture of the heart . A transmural infarction may soften the infarcted muscle to the point that an opening develops connecting the ventricular chamber with the pericardial space. This complication is usually fatal, although in rare instances immediate surgery may save the patient's life.

Rupture of the ventricular septum . If the softened portion of the cardiac muscle affects the septum rather than the outside wall of the left ventricle, an opening between the left and right ventricles develops. In consequence, blood is shunted from the high-pressure left ventricle to the low-pressure right ventricle, greatly increasing the workload of the heart. Depending on the size of the opening, the result may be death, cardiogenic shock, or mild-to-moderate heart failure. Usually there is time to arrange for corrective surgery in more-serious cases. In milder cases surgery may be deferred until after recovery from the myocardial infarction.

Acute mitral regurgitation . Myocardial infarction occasionally damages one of the two papillary muscles in the left ventricle anchoring the mitral valve through the attached chordae. Such damage may produce incompetence of the mitral valve, leading to an overloading of the circulation. The effects of this complication are similar to those of rupture of the ventricular septum, and emergency surgery is often required.

Emboli in the systemic circulation . During the early stages of myocardial infarction clots may develop inside the left ventricle at the point where infarction may have damaged the endocardium. Portions of thrombi may detach themselves from the wall of the ventricle and travel in the bloodstream, producing emboli, which in turn may lead to stroke.

Pericarditis . Inflammation of the pericardium sometimes develops in the course of myocardial infarction. It is usually a benign complication and does not influence the course of the attack.

Heart block . Damage to the conducting system of the heart often produces varying degrees of conduction disturbance. This may be a transient phenomenon requiring no intervention. Often,

however, insertion of an electronic pacemaker is needed, either temporarily or permanently.

The diagnosis of myocardial infarction is based on three component findings—the initial attack of chest pain, a sequence of electrocardiographic changes, and the results of a blood test to determine the level of serum enzymes. The initial diagnostic evaluation is usually performed in an emergency unit. Great weight has to be placed on the patient's description of the attack since the physician's examination often contributes little to the diagnosis. The initial electrocardiogram may establish a tentative diagnosis of myocardial infarction, but full confirmation depends on two or more serially performed tests. Early testing for cardiac enzymes in the blood serum cannot contribute to the diagnosis because the characteristic rise in enzymes occurs several hours after the attack (peaking 12 to 24 hours later). Diagnostic difficulty may arise if the attack is atypical and the electrocardiographic changes are noncharacteristic or delayed. Since early treatment in an intensive-care unit is essential, patients suspected of suffering myocardial infarction are usually admitted to a hospital facility. The subsequent evaluation of doubtful cases, usually completed within 24 to 48 hours of hospitalization, can distinguish those patients whose chest pain is caused by abnormalities other than those of the heart or who are experiencing severe unstable angina without damage to the heart muscle.

The establishment of a diagnosis of myocardial infarction is only the first step in the diagnostic assessment of the problem. It is necessary to evaluate the damage to the heart and its consequences for the circulation and to recognize, or even anticipate, any change so as to provide appropriate treatment. The patient's heart rate and rhythm are continuously displayed on the cardiac monitor, the blood pressure is frequently checked, and a physical examination is performed periodically. In patients who are medically stable and feel well, observation and routine care may be all that is required. But further diagnostic procedures are available if indicated. A chest X-ray may show the presence of left ventricular failure. Echocardiographic examination or nuclide ventriculography permits an evaluation of the function of the damaged left ventricle, can provide information regarding the size and location of dead muscle tissue,

and can detect clots in the left ventricle. More elaborate tests may be needed if the patient's recovery is marred by continuing or delayed circulatory problems. Overt heart failure or shock may call for continuous monitoring of cardiac dynamics by means of flow-directed cardiac catheter. A complete cardiac catheterization and angiographic study may become necessary if a more severe complication is suspected.

Management of Myocardial Infarction . Acute myocardial infarction is a self-limited, self-healing disease of the heart that happens also to be a stage in coronary-artery disease. The goal of treatment is to facilitate the healing process, contain the damage to the heart, and protect the patient, if possible, from the sequelae and complications. In many cases the success of therapy is difficult to determine. The success of remedial interventions, such as treatment of heart failure or shock or surgical correction of some major complications, may be judged by the patient's response. But since most therapeutic endeavors are prophylactic, involving attempts to reduce the size of the infarct, prevent ventricular fibrillation, or avert major complications, their effectiveness in individual cases is not known. Evaluation of the success of therapy requires studies comparing results in large samples of treated and untreated patients. Interpretation of data from such intervention trials is often difficult, and the results are occasionally contradictory.

Acute myocardial infarction is in the majority of cases a benign disease. Many patients, even those suffering a large (transmural) myocardial infarction, feel well once the initial attack of chest pain has subsided and make an uneventful recovery. It is even possible to recover without any treatment. For example, occasionally an electrocardiogram taken during a routine checkup of a patient unaware of any heart problem shows unmistakable evidence that he or she has at some time suffered a major myocardial infarction. The initial attack in such a patient may have been milder than usual and have been overlooked or dismissed as an attack of indigestion; consequently the patient continued engaging in normal activity when he or she should have been treated in a hospital.

Nevertheless, the variability of the course of myocardial infarction, together with the possibility of major complications, makes it essential that once the condition is recognized appropriate management

be instituted. Treatment is usually initiated in an emergency unit or even during transport of the patient to the hospital. As soon as the probability of myocardial infarction has been established, the patient is moved to the coronary-care unit, first developed in the early 1960s. The expert care in this unit includes electrocardiographic monitoring of the patient's heart rhythm and hemodynamic monitoring in the event of circulatory failure.

Since the risk of serious arrhythmias is highest at the earliest stage of myocardial infarction, monitoring of cardiac rhythm is started as early as possible, in most cases in the ambulance transporting the patient to an emergency unit. Many ambulances are equipped with a transmitter to send electrocardiographic signals to the emergency unit, from where a physician can authorize and direct an ambulance attendant to administer drugs or use defibrillators.

The latest advance in the treatment of myocardial infarction is the use of thrombolytic drugs to dissolve the clot responsible for the infarction. The attack of chest pain usually indicates when a coronary artery became occluded. Death of heart muscle resulting from the occlusion occurs too rapidly to expect that dissolving the clot will prevent myocardial infarction. The basis for thrombolytic therapy rests in the hope that within the first few hours after the occlusion some myocardial cells are still capable of reviving if the blood supply is reestablished, thereby reducing the damage to the heart. Obviously, the earlier the drug is administered, the more likely the patient is to benefit from the treatment. Though this treatment is widely used, a small risk of serious hemorrhage leading to stroke or excessive blood loss calls for caution in administering it to certain patients. An alternative approach to early removal of coronary occlusion is interventional therapy (PTCA) performed immediately after diagnosis. Benefits of thrombolytic treatment have been demonstrated by two major studies, involving observations of thousands of patients, which have shown significant reduction of mortality from myocardial infarction. Benefits from PTCA have not yet been clearly demonstrated; hence its use is still considered experimental. Both approaches are subject to controversy and lively debate among experts regarding which patients are most likely to benefit from such intervention.

Following the initial intervention and after the patient has been moved to the coronary-care unit, management is guided by the

patient's condition. Those who feel well and show no significant abnormalities may be transferred within a day or two to a less intensive monitoring facility, then to a regular hospital room, and are candidates for early discharge. Patients who have arrhythmias, unstable blood pressure, or other abnormalities may require longer treatment in each type of facility. Dangerous manifestations, such as heart failure or shock, require intensive therapy, usually guided by hemodynamic monitoring by means of flow-directed cardiac catheters. Cardiogenic shock may require the use of an intraaortic balloon pump.

If chest pain continues beyond the first 24 hours or reappears along with evidence that its origin is ischemic, early coronary angiography may be performed. The results of that test may in turn indicate the need for coronary angioplasty or bypass surgery.

Serious complications of myocardial infarction tend to be delayed; most often they develop between the second and seventh days. Medical staff must be alert to complications at the earliest possible moment since the patient's life may depend on immediate intervention.

Fortunately, most patients will not experience such life-threatening complications and will feel well within 48 hours of the attack. At that point their chance of survival increases to 90–95 percent. The focus of medical management then shifts to rehabilitation. Cardiac rehabilitation in cases of myocardial infarction comprises supervising the patient's gradual resumption of activities and providing psychological support. Myocardial infarction often strikes active, healthy persons without warning, and the prospect of death or disability and uncertainty about the future may have a devastating effect on some patients. Most patients, however, are able to resume a life-style comparable to that prior to the attack, and in many the long-term prognosis is not affected by the attack. Two presidents of the United States, Dwight Eisenhower and Lyndon Johnson, were able to bear the immense stresses of the presidency after recovering from myocardial infarction.

A postinfarction survey of the status of the patient's circulatory system can be performed before discharge from the hospital or, as some physicians prefer, a few weeks later. Prognosis is related to several factors, which can be evaluated by tests: the presence or absence of ischemia as determined by an exercise stress test; the

function of the left ventricular pump as determined by echocardiogram or nuclide ventriculogram; and a tendency to precarious ventricular arrhythmias, as determined by a Holter monitor test. The precise battery of tests is tailored to the individual patient. They help the physician decide such questions as whether continuous drug treatment or some intervention (PTCA or bypass surgery) is needed.

Active persons convalescing from myocardial infarction must decide, under guidance from their physician, whether to resume their previous life-style without restrictions, return to their former occupation with some restrictions, retrain for a less strenuous or stressful occupation, or retire. All patients who recover from myocardial infarction should be encouraged to institute or continue preventive measures against atherosclerosis. This secondary prevention involves more aggressive modification of risk factors than does primary prevention. The main emphasis is on reducing cholesterol by diet and, if necessary, drugs. In many patients it is advisable to continue antianginal therapy or other forms of medical therapy.