Acute Myocardial Infarction

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75 Acute Myocardial Infarction

Angina pectoris was recognized in the 18th century; myocardial infarction (MI), however, was described approximately 200 years later. Simultaneous to the identification of MI was the initial introduction and subsequent application of the electrocardiogram (ECG)—the first objective method of assessing the coronary origin of the presentation. In fact, early clinician investigators described the evolving “electrographic” changes during angina in 1918.¹ Over the next 50 years, angina pectoris and MI were further characterized and diagnosed; unfortunately, however, the management of ischemic heart disease did not progress as significantly. From this point in medical history until the 1960s, management consisted primarily of pain relief coupled with strict bed rest for prolonged periods and management of resultant congestive heart failure (CHF); acute complications such as cardiogenic shock and sudden cardiac death were invariably fatal events. Subsequently, the introduction and widespread use of cardiopulmonary resuscitation, external defibrillation, and antidysrhythmic agents gave the clinician powerful new tools in the management of sudden cardiac death and other malignant dysrhythmias. Overall management, however, was still aimed at the complications of ischemic heart disease rather than the syndrome itself.

With recognition of the thrombotic nature of the acute coronary syndrome within the last several decades, the stage was set for the next most significant advance in the management of more acute forms of ischemic heart disease, namely acute myocardial infarction (AMI). Early coronary angiography coupled with intraarterial administration of streptokinase ushered in the era of acute reperfusion therapies, certainly the most significant advancement in the recent past. Clinicians were now able not only to treat the acute complications of the illness but also to interrupt, if not halt, the primary process, thereby markedly reducing morbidity and mortality. Furthermore, aggressive antiplatelet and anticoagulant therapies as well as intracoronary stenting have increased rates of patency and reduced coronary reocclusion and reinfarction.

The most recent efforts in this important area of acute cardiac care focus on rapid recognition of acute coronary syndrome (ACS), use of various adjunctive therapies, and restoration of coronary perfusion. When applied to the patient with ST-segment elevation myocardial infarction (STEMI), this process can be described as a STEMI “system of care.” In this system of care, STEMI is rapidly recognized; emergent reperfusion therapy, whether it be accomplished via medical fibrinolysis or catheter-based percutaneous coronary intervention (PCI), is quickly initiated while adjunctive antiplatelet and anticoagulant therapies are administered. This system of care spans from the ambulance with prehospital 12-lead ECG through the emergency department (ED) to the cardiac catheterization laboratory and coronary care unit (CCU).

Epidemiology

Globally, cardiovascular disease now ranks as the leading cause of death. It now causes one third of all deaths worldwide. The World Health Organization (WHO) in conjunction with the Centers for Disease Control and Prevention (CDC) published the Atlas of Heart Disease and Stroke; in this report, the WHO/CDC note a combined death toll of 17 million persons per year, with a potential increase to 24 million people per year by 2030.² In the United States, ischemic heart disease, particularly acute forms of the illness, is the leading cause of death for adults. Unfortunately, half of these deaths result from sudden cardiac death unrelated to ACS, usually within the first 2 hours of symptom onset, either out of hospital or soon after arrival in the ED. Fifteen percent of the fatalities occur prior to age 65 years, with the majority in women. The “burden” placed on medical centers and other acute care facilities is tremendous, with an approximate 8 million people having been admitted to hospital in the past 20 years; 20% of these admissions involve AMI. Furthermore, while death from coronary heart disease has decreased in North America and many western European countries, there is an increased mortality in developing countries.^3,⁴

According to the American Heart Association,⁵ coronary heart disease caused approximately 1 of every 6 deaths in the United States in 2006. In 2010, an estimated 785,000 Americans will have a new coronary event, and approximately 470,000 will have a recurrent attack. It is estimated that an additional 195,000 “silent” first MIs occur each year. These events usually occur in patients over the age of 40 years, with an increasing occurrence as one ages. Approximately every 25 seconds, someone in the United States will have a coronary event, and approximately every minute someone will die of one such event.⁵

Pathophysiology

Ischemic heart disease describes an entire spectrum of illness, ranging from acute to chronic entities related to coronary artery disease, including angina pectoris, AMI, cardiomyopathy and malignant dysrhythmia. Acute coronary syndromes have been defined as unstable angina pectoris (USAP) and AMI. In the past, AMI was separated into Q-wave (transmural) and non–Q wave (nontransmural) events. This terminology was replaced by myocardial infarction with associated ST elevation (STEMI) and infarction without elevation of the ST segment (non-STEMI or NSTEMI). In STEMI, the patient’s symptoms and ECG are relied upon to drive treatment. When diagnostically abnormal ST-segment elevation is not present, a rise in serum markers over time can indicate an NSTEMI, assuming the appropriate clinical conditions exist. While this terminology is still used, MI has been further defined and categorized to reflect the many possible clinical situations (please refer to the following discussions for further delineation of AMI).

Historically, the two primary intracoronary pathophysiologic events underlying the development of ACS include thrombus formation and vasospasm. In the setting of either a structurally normal artery or preexisting coronary artery disease, initial endothelial damage produces platelet aggregation and resultant thrombus formation. In most cases, disruption of an atherosclerotic plaque provides the endothelial injury. Occlusion of the coronary artery then results, ranging from minimal, transient, asymptomatic obstruction to complete occlusion usually associated with prominent symptomatology, namely AMI. Coronary artery obstruction can lead to myocardial ischemia, hypoxia, acidosis, and ultimately AMI. Vasospasm results when locally active substances are coupled with systemic mediators to produce a cascade of events resulting in worsened myocardial perfusion. Isolated vasospasm followed by thrombus is involved in approximately 10% of AMIs. Refer to Figure 75-1 for a depiction of the acute pathophysiology of AMI.

Figure 75-1 Pathophysiology of acute myocardial infarction.

A, Normal coronary flow without obstructive lesions. B, Lipid accumulation with plaque formation and ultimate rupture, leading to thrombus formation and vasospasm. C, Significant compromised flow within the coronary artery, resulting in clinical manifestation, as noted in D, with ST-segment elevation, likely accompanied by chest discomfort or other symptoms.

(Figures courtesy Ashok Subramanian, MD.)

In the last decade, the definition of MI has evolved. The European Society of Cardiology and the American College of Cardiology published consensus criteria for “redefinition” of MI in 2000.⁶ These criteria reflected the improvements in biomarker testing. Then in 2007, working groups from these organizations along with the World Heart Federation and American Heart Association published the “Universal Definition of Myocardial Infarction.”⁶ This expanded definition classifies infarction based on clinical situations resulting in myocardial necrosis/cell death.⁶

The term myocardial infarction should be used when there is evidence of myocardial necrosis in a clinical setting consistent with myocardial ischemia. Under these conditions, any one of the following criteria meets the diagnosis for myocardial infarction; the various subcategories of acute myocardial infarction are referred to as types 1 to 5 ⁶:

Type 1. Spontaneous myocardial infarction related to ischemia due to a primary coronary event such as plaque erosion. The type 1 AMI demonstrates a typical rise and/or fall of cardiac biomarkers (preferably troponin), with at least one value above the 99th percentile of the upper reference limit (URL) together with evidence of myocardial ischemia manifested by at least one of the following:

• Symptoms of ischemia

• ECG changes indicative of new ischaemia (new ST-segment and/or T-wave changes or a new left bundle branch block [LBBB])

• Development of pathologic Q waves on the ECG

• Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality

Type 2. Myocardial infarction secondary to ischemia due to either increased oxygen demand or decreased supply (e.g., coronary artery spasm, coronary embolism, anemia, arrhythmias, severe hypertension, or significant hypotension)

Type 3. Sudden, unexpected cardiac death involving cardiac arrest, often with symptoms suggestive of myocardial ischemia, and accompanied by presumably new ST elevation, or new LBBB, and/or evidence of fresh thrombus by coronary angiography and/or at autopsy, but death occurring before blood samples could be obtained or at a time before the appearance of cardiac biomarkers in the blood

Type 4. Myocardial infarction associated with PCI, without or without intracoronary stent. For percutaneous coronary interventions (PCI) in patients with normal baseline troponin values, elevations of cardiac biomarkers above the 99th percentile URL are indicative of periprocedural myocardial necrosis. By convention, increases of biomarkers greater than 3 × 99th percentile URL have been designated as defining PCI-related myocardial infarction. A subtype related to a documented stent thrombosis is recognized.

Type 5. Myocardial infarction associated with CABG. For coronary artery bypass grafting (CABG) in patients with normal baseline troponin values, elevations of cardiac biomarkers above the 99th percentile URL are indicative of periprocedural myocardial necrosis. By convention, increases of biomarkers greater than 5 × 99th percentile URL plus either new pathologic Q waves, or new LLLB, or angiographically documented new graft or native coronary artery occlusion, or imaging evidence of new loss of viable myocardium have been designated as defining CABG-related myocardial infarction. Also, pathologic findings of an acute myocardial infarction define the type 5 AMI.⁶

Criteria for prior MI includes the following ⁶:

• Development of new pathologic Q waves with or without symptoms

• Imaging evidence of a region of loss of viable myocardium that is thinned and fails to contract, in the absence of a nonischemic cause

• Pathologic findings of a healed or healing MI

Additional issues to consider in the pathophysiology of AMI focus on initial primary illness or concurrent medical events. Such considerations obviously have significant potential for impact on additional diagnostic and therapeutic issues; these presentations are reasonably likely in the undifferentiated, ill critical care patient. In the type 2 AMI presentation, the patient with shock of varying causes may experience AMI secondary to the physiologic insult placed on the heart. For instance, the patient with distributive shock resulting from urosepsis or the patient with hypovolemic shock due to gastrointestinal hemorrhage may experience either NSTEMI or STEMI. Furthermore, metabolic poisons such as cyanide, carbon monoxide, and hydrogen sulfide can disrupt myocardial cellular function, resulting in ACS.

Clinical Features

The history—and the clinician’s interpretation of the available history—is vital. In the critical care unit, however, the patient may be unable to offer a thorough history because of either active illness or instrumentation such as endotracheal intubation. If available, an appropriate history will enable the clinician to focus the evaluation, provide adequate therapies, secure a safe disposition, and minimize the need for additional investigations.

Angina pectoris, the chest pain associated with ACS, by definition includes a sense of choking, strangulation, or constriction. Common descriptions of the discomfort include not only pain but also pressure, squeezing, fullness, or heaviness. In some patients, the symptoms are perceived as gastrointestinal. The location for angina is substernal and left chest with radiation to the shoulders, arms, neck, or jaw. Patients with AMI, however, may also present with pain in the right chest. The duration of chest pain is valuable in determining its cause. Angina pectoris generally is short-lived, lasting less than 15 minutes. Patients with AMI usually experience more than 30 minutes of chest pain. Intermittent, sharp, localized chest discomfort lasting less than several seconds usually is not due to ACS. The symptoms of angina pectoris improve dramatically within 2 to 5 minutes after rest or nitroglycerin. If the pain persists for more than 10 minutes, the diagnosis of ACS or a noncardiac origin should be considered. Caution is also advised in the chest pain patient who appears to respond to an antacid; overreliance on this response as a major decision point in “ruling out” ACS is not encouraged. Many AMI patients experience associated symptoms such as dyspnea, diaphoresis, nausea, vomiting, dizziness, and anxiety; these various symptoms may be the primary complaint in patients presenting with AMI.

Risk factors that increase the likelihood for atherosclerosis and AMI—male gender, family history, cigarette smoking, hypertension, hypercholesterolemia, and diabetes mellitus—should be sought. Personal habits such as cigarette smoking and use of illicit drugs, particularly sympathomimetic substances such as cocaine, should be reviewed. Artificial or early menopause and the use of contraceptive pills may increase the likelihood of ischemic heart disease in women. If a patient has a history of coronary artery disease, a risk-factor analysis is unwarranted, because the risk of coronary artery disease is 100%.

There has been disagreement over whether these coronary risk factors should be considered in the clinician’s medical decision making. An early report ⁵ suggested that such factors, which were initially derived because of their ability to predict the development of coronary atherosclerosis and its complications over decades in association with other clinical variables such as ECG interpretation, have minimal predictive value acutely as to whether a patient is currently experiencing an AMI. More contemporary investigation in possible ACS patients suggests that the coronary risk factors do in fact have significant predictive value.^7,^8,⁹ This important issue is still debated by the epidemiologists; for the clinician, a consideration of the risk-factor burden is one feature of the overall diagnostic analysis.

Because angina is a visceral sensation that is often diffuse, some patients may have an anginal equivalent syndrome. Such anginal equivalent presentations describe patients who are experiencing ACS yet do not complain of typical chest pain; rather, these patients note atypical pain, dyspnea, weakness, diaphoresis, or emesis—these complaints, in fact, are the manifestation of the ACS event. Patients with altered cardiac pain perception (e.g., the elderly or patients with long-standing diabetes mellitus) are potentially at risk to present with anginal equivalent syndromes. A recent large survey of 434,877 confirmed AMI patients reported that a significant minority of these individuals—approximately 30%—lacked chest pain on presentation, noting only the anginal equivalent complaints.¹⁰ The most frequently encountered anginal equivalent chief complaint is dyspnea, which is found in 10% to 30% of patients with AMI, often due to pulmonary edema.^10,^11,¹² Isolated emesis and diaphoresis are quite rare.^11,¹²

The geriatric patient may also present atypically with acute weakness (3%–8%) and syncope (3%–5%).¹³ Unexplained sinus tachycardia, bronchospasm resulting from cardiogenic asthma, and new-onset lower extremity edema have all been reported as anginal equivalent presentations for AMI in this age group. Among the very elderly, anginal equivalent syndromes typically involve neurologic presentations with acute mental status abnormalities and stroke. From the perspective of acute delirium, less than 1% of such patients in an ED population with altered mentation will be found to have AMI. AMI associated with acute stroke is noted in approximately 5% to 9% of patients.¹³

Physical Examination

The physical examination in the patient with AMI rarely provides diagnostic confirmation of the illness; the examination can certainly suggest MI yet not confirm its presence. The ECG, serum markers, and other investigations interpreted in the context of the clinical event confirm the diagnosis. Specific examination findings resulting directly from ACS include anxiety, pale appearance, and diaphoresis. In fact, the presence of significant diaphoresis as a physical examination finding is strongly suggestive of AMI.¹⁴ Significant physical examination findings encountered in the AMI patient most often result indirectly from the coronary event and result directly from complications of the AMI. These findings include hypotension, altered mentation, various other signs of poor perfusion, rales and low oxygen saturations related to pulmonary congestion, and heart sounds related to myocardial and/or valvular dysfunction.¹⁵ Both brady- and tachydysrhythmias are seen as well. And, of course, the combination of poor peripheral perfusion—manifested by hypotension unresponsive to hemodynamic support—and pulmonary edema is considered cardiogenic shock.

The physical examination, although crucial to many life-threatening disease processes, is often unhelpful in diagnosing AMI; AMI may be suggested, however, in the patient with obvious cardiac dysfunction manifested by acute pulmonary edema or cardiogenic shock, or both. A change in mental status, poor peripheral perfusion, pronounced tachycardia, hypotension, diaphoresis, rales, jugular venous distension, and S₃ and S₄ heart sounds often provide evidence of significant myocardial dysfunction in patients with AMI. Patients with evidence of myocardial dysfunction, including S₃ heart sound, S₄ heart sound, or rales, on initial presentation are at much greater risk for adverse cardiovascular events, including nonfatal AMI, death, stroke, life-threatening dysrhythmia, and the requirement for cardiac surgery.

Caution should be exercised when attributing a chest wall source for pain based on palpation or movement. To safely relate the chest discomfort to a chest wall origin, the pain must be described as sharp or stabbing (i.e., pleuritic in nature) and be completely reproducible by palpation.¹⁶ Up to 15% of patients with AMI may have some form of tenderness on chest wall palpation.¹⁷

Diagnostic Strategies

Electrocardiogram

In the chest pain patient (or patient with acute cardiorespiratory decompensation suspected of AMI), the ECG can be used to establish the diagnosis of AMI or other noncoronary ailment, select appropriate therapy, determine the response to treatments, determine the correct inpatient disposition location, and predict risk of both cardiovascular complication and death. The ECG is an extremely powerful diagnostic study, which, if used in appropriate fashion, can guide the clinician in the evaluation of the chest pain patient suspected of AMI. In fact, the ECG provides pivotal information in the patient with STEMI, allowing its diagnosis and guiding acute resuscitative therapies. In other coronary-related ailments, the ECG can provide useful information regarding diagnosis and management. An understanding of its shortcomings, however, in this application will only improve its use. From the perspective of the ECG diagnosis of AMI, the ECG has numerous shortcomings, including the “normal” and “nondiagnostic” interpretations, evolving AMI patterns, the NSTEMI ECG presentation, confounding and mimicking patterns, and the isolated acute posterior wall AMI.

The ECG may manifest a range of ECG abnormalities (Figure 75-2) in the patient with potential AMI, including the prominent T wave, T-wave inversion, ST-segment depression, ST-segment elevation, and QA waves, among other findings. The earliest ECG finding resulting from STEMI is the hyperacute T wave, which may appear minutes after the interruption of blood flow; the R wave also increases in amplitude at this stage. The hyperacute T wave, a short-lived structure that evolves rapidly on to ST-segment elevation over a 5- to 30-minute period, is often asymmetric with a broad base; these T waves are also associated not infrequently with reciprocal ST-segment depression in other ECG leads. Such a finding on the ECG is transient in the AMI patient; either apparent or progressive ST-segment elevation is usually encountered at this stage. As the infarction progresses, the hyperacute T wave evolves into the giant R wave, particularly in the anterior wall AMI. The giant R wave is a transition structure from the hyperacute T wave to typical ST-segment elevation; it essentially is a large monophasic R wave with pronounced ST-segment elevation. Prominent T waves may be seen in patients with AMI as well as hyperkalemia, acute myopericarditis, benign early repolarization, left ventricular hypertrophy, and bundle branch block.