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Clinical Practice/Controversy: Clinical Approach to Suspected Acute Myocardial Infarction
Introduction
Chest symptoms suspicious for acute myocardial infarction (MI) are among the most common reasons for emergency evaluation, accounting for six to seven million emergency department (ED) visits each year in the United States. The initial assessment of nontraumatic chest discomfort is challenging because of the broad range of possible causes ( Figure 6-1 ). The primary aim of the ED assessment is to rapidly identify the minority of patients whose symptoms are the manifestation of a life-threatening condition that should not be missed and to initiate appropriate therapy. More than 60% of patients who present with chest symptoms suspicious for MI are hospitalized for further testing, and the remainder undergo additional investigation in the ED. However, in most series of unselected populations, only 5% to 15% are ultimately determined to have an acute coronary syndrome (ACS), and less than 10% are found to have other life-threatening cardiopulmonary conditions. Therefore, an efficient but effective evaluation of this population of patients that avoids the excessive use of testing and minimizes empiric treatment is important.
Pathways for the triage and management of patients with ST-segment elevation on the presenting electrocardiogram (ECG) are described in Chapter 5 . In contrast, strategies for evaluating low-risk patients with a low probability of MI are discussed in Chapter 12 . The present chapter provides a general framework for the clinical approach to the assessment of patients with an intermediate or high probability of MI. Risk factors for MI are discussed in Chapter 2 . The optimal use of cardiac troponin (cTn) is detailed in Chapter 7 , and other biomarkers are considered in Chapter 8 . Diagnostic imaging in the ED is described in Chapter 9 . Chapter 11 provides an in-depth discussion of tools for risk stratification of the patient with established MI.
Goals of the Initial Assessment of Suspected Myocardial Infarction
The fundamental goals of the initial assessment of the patient with chest symptoms suspicious for myocardial ischemia are (1) to assess the probability that the symptoms are caused by underlying myocardial ischemia (diagnosis), and (2) to determine the probability of major cardiovascular complications if the cause of the patient’s presentation is myocardial ischemia (risk stratification). These two concurrent probabilistic assessments rely on the clinical history, the physical examination, ECG, and initial cardiac biomarkers, and are intertwined because each of these elements provides information that influences both the diagnostic and prognostic probabilities (see the section on Clinical Approach to the Patient ). Together, these two probabilistic assessments guide subsequent diagnostic testing, including the use of invasive coronary angiography, triage, and the initiation of empiric medical therapies while the diagnosis is established ( Figure 6-2 ).
Causes of Chest Discomfort
The characteristics of symptoms caused by myocardial ischemia are discussed in this section. The major alternative causes of chest discomfort are summarized in Table 6-1 and described briefly in this section. In general, the initial diagnostic assessment of patients with acute chest discomfort centers around three categories: (1) myocardial ischemia; (2) other cardiopulmonary causes (pericardial disease, aortic emergencies, and pulmonary conditions); and (3) noncardiopulmonary chest pain. High-risk conditions, other than acute MI, to be considered in the differential diagnosis include acute aortic syndrome, pulmonary embolism, tension pneumothorax, and pericarditis with tamponade.
TABLE 6-1
Typical Clinical Features of Major Causes of Acute Chest Discomfort
From Morrow DA: Chest discomfort. In Kasper DL, et al, eds: Harrison’s principles of internal medicine, ed 19, New York, McGraw Hill, 2015.
| System | Condition | Onset/Duration | Quality | Location | Associated Features |
|---|---|---|---|---|---|
| Cardiopulmonary | |||||
| Cardiac | Myocardial ischemia | Stable angina: Precipitated by exertion, cold, or stress; 2–10 min; Unstable angina: Increasing pattern or at rest; MI: Usually >30 min |
Pressure, tightness, squeezing, heaviness, burning | Retrosternal, often radiation to neck, jaw, shoulders, or arms; sometimes epigastric | S 4 gallop or mitral regurgitation murmur (rarely) during pain; S 3 or rales if severe ischemia or complication of MI |
| Pericarditis | Variable; Hours to days; may be episodic |
Pleuritic, sharp | Retrosternal or toward cardiac apex; may radiate to left shoulder | May be relieved by sitting up and leaning forward; Pericardial friction rub |
|
| Vascular | Acute aortic syndrome | Sudden onset of unrelenting pain | Tearing or ripping; knifelike | Anterior chest, often radiating to back, between shoulder blades | Associated with hypertension and/or underlying connective tissue disorder; murmur of aortic insufficiency, loss of peripheral pulses |
| Pulmonary embolism (PE) | Sudden onset | Pleuritic; may be heaviness with massive PE | Often lateral, on the side of the embolism | Dyspnea, tachypnea, tachycardia, and hypotension | |
| Pulmonary hypertension | Variable; often exertional | Pressure | Substernal | Dyspnea, signs of increased venous pressure | |
| Pulmonary | Pneumonia or pleuritis | Variable | Pleuritic | Unilateral, often localized | Dyspnea, cough, fever, rales, occasional rub |
| Spontaneous pneumothorax | Sudden onset | Pleuritic | Lateral to side of pneumothorax | Dyspnea, decreased breath sounds on side of pneumothorax | |
| Noncardiopulmonary | |||||
| Gastrointestinal | Esophageal reflux | 10–60 min | Burning | Substernal, epigastric | Worsened by postprandial recumbency; Relieved by antacids |
| Esophageal spasm | 2–30 min | Pressure, tightness, burning | Retrosternal | Can closely mimic angina | |
| Peptic ulcer | Prolonged; 60–90 min after meals | Burning | Epigastric, substernal | Relieved with food or antacids | |
| Gallbladder disease | Prolonged (h); generally steady and subsides spontaneously | Aching or colicky | Epigastric, right upper quadrant; sometimes to the back and lower chest or scapula | May follow meal | |
| Neuromuscular | Costochondritis | Variable | Aching | Sternal | Sometimes swollen, tender, warm over joint May be reproduced by localized pressure on examination |
| Cervical disk disease | Variable; may be sudden | Aching; may include numbness | Arms and shoulder | May be exacerbated by movement of neck | |
| Trauma or strain | Usually constant | Aching | Localized to area of strain | Reproduced by movement or palpation | |
| Herpes zoster | Usually prolonged | Sharp or burning | Dermatomal distribution | Vesicular rash in area of discomfort | |
| Psychological | Emotional and psychiatric conditions | Variable; may be fleeting or prolonged | Variable; often tightness and dyspnea with feeling of panic or doom | Variable; may be retrosternal | Situational factors may precipitate symptoms; history of panic attacks depression |
Myocardial Ischemia
Onset of myocardial ischemia is precipitated by an imbalance between myocardial oxygen requirements and myocardial oxygen supply, which results in insufficient delivery of oxygen to meet the heart’s metabolic demands. Chest discomfort caused by myocardial ischemia is termed angina pectoris, often referred to simply as angina. The causes and classification of myocardial ischemia into stable angina, unstable angina, non–ST-elevation MI (NSTEMI), and ST-elevation MI (STEMI) are addressed in Chapter 1 . The pathobiology of unstable ischemic heart disease is discussed Chapter 3 and Chapter 4 .
Characteristics of Myocardial Ischemia
Myocardial ischemia can usually be identified from the patient's history and from the ECG. Possible ischemic symptoms include various combinations of chest, upper extremity, mandibular, or epigastric discomfort, or an ischemic equivalent, such as dyspnea or fatigue (see the section on Clinical Approach: History). When myocardial ischemia is sufficiently severe and prolonged in duration (e.g., as short as 20 to 30 minutes), irreversible cellular injury occurs, resulting in MI. Often, the discomfort is diffuse—not localized, nor positional, nor affected by movement of the region—and it may be accompanied by diaphoresis, nausea, or syncope. Because of their prevalence among other common conditions, these symptoms may be incorrectly attributed to gastrointestinal, neurological, pulmonary, or musculoskeletal disorders (see Table 6-1 ). In addition, MI may occur with atypical symptoms or may be asymptomatic. Such atypical presentations are more common in women, older adults, patients with diabetes, or postoperative and critically ill patients.
The clinical characteristics of angina pectoris are highly similar whether the ischemic discomfort is a manifestation of stable ischemic heart disease, unstable angina, or MI, with exceptions being differences in the pattern and duration of symptoms associated with these syndromes. Heberden initially described angina as a sense of “strangling and anxiety.” Chest discomfort characteristic of myocardial ischemia is usually described as aching, heavy, squeezing, crushing, or constricting. However, in a substantial minority of patients, the quality of discomfort is very vague and may be described as a mild tightness, or merely an uncomfortable feeling that sometimes is experienced as numbness or a burning sensation. The site of the discomfort is usually retrosternal, but radiation is common, and usually occurs down the ulnar surface of the left arm; the right arm, both arms, neck, jaw, or shoulders may also be involved ( Figure 6-3 ). These and other characteristics of ischemic chest discomfort pertinent to discrimination from other causes of chest pain are discussed later in this chapter (see the section on Approach to the Patient ).
Stable angina usually begins gradually and reaches its maximum intensity over a period of minutes before dissipating within several minutes with rest or with nitroglycerin. The discomfort typically occurs predictably at a characteristic level of exertion or psychological stress. By definition, unstable angina manifests by self-limited symptoms that may be exertional, but that occur at increased frequency with progressively lower intensity of physical activity or at rest ( Table 6-2 ). Chest discomfort associated with MI is typically more severe, is prolonged (usually ≥30 minutes), and is not relieved by rest.
TABLE 6-2
Three Principal Presentations of Unstable Ischemic Heart Disease
From Anderson JL, Adams CD, Antman EM, et al: ACC/AHA 2007 guidelines for the management of patients with unstable angina/non-ST-elevation myocardial infarction. J Am Coll Cardiol 50:e1–e157, 2007.
| Class | Presentation |
|---|---|
| Rest angina | Angina occurring at rest and prolonged, usually >20 min |
| New-onset angina | New-onset angina of at least CCS class III Severity |
| Increasing (crescendo) angina | Angina that has become distinctly more frequent, longer in duration, or lower in threshold (increased by 1 or more CCS class to at least CCS class III severity) |
CCS , Canadian Cardiovascular Society.
Triggers of Myocardial Ischemia
Myocardial ischemia may be triggered by acute coronary atherothrombosis (see Chapter 3 ), increased myocardial oxygen demand, such as during intense psychological stress, or fever, or by decreased oxygen delivery due to anemia, hypoxia, or hypotension (see Chapter 1 ). Other contributors to stable and unstable ischemic heart disease, such as endothelial dysfunction, microvascular disease, and vasospasm, may also exist alone or in combination with coronary atherosclerosis and may be the dominant cause of myocardial ischemia in some patients. Nonatherosclerotic processes, including congenital abnormalities of the coronary vessels, myocardial bridging, coronary arteritis, and radiation-induced coronary disease, can also lead to coronary obstruction. In addition, conditions associated with extreme myocardial oxygen demand and impaired endocardial blood flow, such as in patients with aortic valve disease, hypertrophic cardiomyopathy, or idiopathic dilated cardiomyopathy, can precipitate myocardial ischemia in patients with or without underlying obstructive atherosclerosis. In the course of their history and physical examination, clinicians should consider each of these potential contributors to the onset of myocardial ischemia.
Nonischemic Causes of Chest Discomfort
Cardiopulmonary Causes
Pericardial and Myocardial Diseases
Inflammation of the pericardium can cause acute chest discomfort. The pain of pericarditis is believed to arise primarily from associated pleural inflammation, and is consequently more common in infectious compared with noninfectious causes of pericarditis, because the former more often involve the pleura. The pain of pericarditis is usually a pleuritic discomfort that is exacerbated by breathing, coughing, or changes in position, and is often referred to the shoulder and neck.
Acute inflammatory and other nonischemic myocardial diseases can also produce chest symptoms (see Table 6-1 ). Takotsubo stress–related cardiomyopathy can cause the abrupt onset of chest pain and shortness of breath, and may mimic acute MI because of associated ECG abnormalities, including ST-segment elevation and elevated biomarkers of myocardial injury.
Acute Aortic Syndromes
Acute aortic syndromes, including aortic penetrating ulcer, intramural hematoma, and frank dissection, are less common but important causes of chest pain (see Table 6-1 ). Acute aortic syndromes typically present with thoracic pain that is often severe, sudden in onset, sometimes described as tearing in quality, and can occur in the midline of the anterior chest. Dissections that begin in the ascending aorta and extend to the descending aorta tend to cause pain in the front of the chest that extends toward the back, between the shoulder blades. Aortic aneurysms without dissection are most often asymptomatic, but these can cause chest pain by compressing adjacent structures. This pain tends to be steady, deep, and occasionally severe. Aortitis, in the absence of dissection, is a rare cause of chest discomfort.
Pulmonary Embolism
Pulmonary and pulmonary vascular conditions that cause chest discomfort usually do so in conjunction with dyspnea. The symptoms are usually pleuritic in nature and may be lateral, in the case of smaller pulmonary emboli, or may be severe and substernal, in the case of massive pulmonary embolism. Massive or submassive pulmonary embolism may also cause syncope, hypotension, and signs of right heart failure.
Other Pulmonary Causes
Primary spontaneous pneumothorax is a rare cause of chest discomfort. The symptoms are usually sudden in onset and dyspnea may be mild. Most pulmonary diseases, including pneumonia and malignancy, that can involve the pleura may cause pleurisy, a knifelike pain that is worsened by inspiration or coughing. Chronic pulmonary hypertension can cause chest pain that may be very similar to angina in its characteristics. Reactive airways diseases can also cause chest tightness, with associated breathlessness.
Noncardiopulmonary Causes
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