Acute Coronary Syndrome

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CHAPTER 52 Acute Coronary Syndrome

Acute coronary syndrome (ACS) is usually diagnosed in the emergency department based on history, physical examination, abnormalities on ECG, and elevations of cardiac serum biomarkers. According to the American Heart Association (AHA) and American College of Cardiology (ACC), high-risk patients recognized as having ACS should undergo early invasive coronary angiography and angiographically directed revascularization.¹ The ultimate goal is to reperfuse and salvage any viable myocardium immediately. Other imaging modalities play minimal role in the evaluation of these patients. ACS is just one of many possible diagnoses when patients present to the emergency department with acute chest pain, however. Other considerations range from acute life-threatening conditions, such as pulmonary embolism, tension pneumothorax, or aortic dissection, to conditions with less morbidity and mortality, such as gastroesophageal reflux or costochondritis.

Acute chest pain accounts for approximately 6 million emergency department visits annually, and is the second most common reason for patients to present to the emergency department in the United States.² Although most emergency department physicians and cardiologists have a low threshold for diagnosing ACS, 2% to 5% of patients are misdiagnosed and discharged inappropriately.³ In these patients, the mortality rate is 1.7 to 1.9 times greater than in patients who are hospitalized. Most patients who are inappropriately discharged with ACS have a nondiagnostic initial assessment. Various noninvasive imaging modalities are available to evaluate patients with acute, but indeterminate, chest pain in the emergency department.

DEFINITION

ACS refers to any group of clinical symptoms compatible with acute myocardial ischemia. Acute myocardial ischemia is precipitated by coronary artery disease (CAD), resulting in chest pain owing to the lack of blood supply to the myocardium. When patients present with chest pain, an ECG is usually performed. The ECG may show myocardial infarction (MI) with or without ST segment elevation. Patients with ischemia-related discomfort but without ST segment elevation are classified as having either unstable angina or non–ST segment elevation myocardial infarction (NSTEMI). Unstable angina is unexpected chest pain or discomfort that usually occurs while at rest. The discomfort may be more severe and prolonged than typical angina, or may be an initial episode of angina. NSTEMI usually progresses to subendocardial acute MI. Patients who have ST segment elevation myocardial infarction (STEMI) are likely to go on to develop a transmural acute MI. ACS is a spectrum of clinical diagnoses, ranging from unstable angina to NSTEMI to STEMI. Other conditions may mimic ACS, and these need to be excluded.

PREVALENCE AND EPIDEMIOLOGY

In the most recent National Hospital Discharge Survey by the National Center for Health Statistics, there were approximately 1.4 million unique hospitalizations because of ACS in the United States in 2005.⁴ ACS is a major cause of morbidity and mortality in Western countries. The prevalence of ACS increases with age and predominates in men among patients younger than 70 years. In patients older than 70 years, the incidence occurs with equal frequency in men and women. Other risk factors that predispose patients to ACS include diabetes mellitus, smoking, hypertension, hypercholesterolemia, hyperlipidemia, a prior cerebrovascular event, inherited metabolic or connective tissue disorder, cocaine or amphetamine use, and occupational stress.

ETIOLOGY AND PATHOPHYSIOLOGY

Atherosclerosis is the most common culprit leading to acute myocardial ischemia because it decreases the blood supply to a portion of myocardium. At first, even in the setting of atherosclerotic coronary plaques, the coronary arteries permit adequate blood flow to counterbalance myocardial demand. As myocardial demand increases, such as during exercise or stress, the decrease in blood supply may become clinically significant, resulting in angina. Initially, the angina may develop during exertion and resolve at rest, and this is termed stable angina.

Many atherosclerotic plaques are composed of rich foam cells (lipid-laden macrophages), inflammatory cells, and a thin fibrous cap. Such plaques are susceptible to sudden rupture or erosion, exposing the thrombogenic surfaces of the plaques, and allowing platelet aggregation and thrombus formation to occur. Distal microembolization of thrombi may also result. These events usually are not limited to one particular coronary artery, but affect other coronary arteries as well.

Less common causes of acute myocardial ischemia include intense focal epicardial coronary spasm (Prinzmetal angina), cardiac emboli, and arterial inflammation caused by or related to infection. Rarely, myocardial ischemia may occur secondary to increased myocardial demand, particularly sepsis, fever, tachycardia, thyrotoxicosis, or anemia. These precipitating conditions usually occur in patients who have underlying stable CAD.

MANIFESTATIONS OF DISEASE

Clinical Presentation

The typical presentation is characterized by substernal tightness, pressure, squeezing, or heaviness with or without pain radiating to the neck or arm. The chest pain, or angina, can be new in onset and can cause marked limitation of physical activity. The pain may be initiated either at rest or during exertion. Unstable angina may last more than 20 minutes during rest or be signified by a change compared with previous angina, with increasing length, frequency, or onset with less exertion. Atypical chest pain, such as epigastric pain or stabbing or pleuritic chest pain, is a common manifestation of myocardial ischemia. Symptoms other than chest pain, such as dyspnea, nausea or vomiting or both, palpitations, syncope, or cardiac arrest, also can be observed. Atypical presentations are more common in patients who are older, diabetic, and female.

Physical examination findings are frequently unremarkable. Signs of hypotension or congestive heart failure, such as new murmurs, gallops, or rales, may be present.

ECG findings of ST segment or T wave changes and elevated serum cardiac biomarkers, especially cardiac troponins, indicate ACS. The results of ECG are often nonspecific, however, and serum cardiac biomarkers, such as creatine kinase MB (CK-MB) isoenzyme and cardiac troponins, may not be useful in patients who present to the emergency department less than 4 to 5 hours after the onset of their symptoms.

Imaging Indications and Algorithm

Although complicated algorithms exist for classifying risk with regard to indications for noninvasive imaging, chest pain can be categorized into three general groups. The first group comprises patients with life-threatening conditions, in particular high-risk patients diagnosed with ACS based on the history and physical examination, ECG findings, and elevated troponins. These patients immediately undergo coronary angiography and angiographically directed revascularization. Emergent medical and surgical intervention is also required for other life-threatening conditions, such as pulmonary embolism, aortic dissection, and tension pneumothorax.

The second group comprises patients who present with acute chest pain that does not require immediate invasive intervention or imaging, such as gastroesophageal reflux disease or costochondritis. These patients are usually triaged and discharged quickly with a recommendation for outpatient follow-up.

The third group comprises patients who usually present to the emergency department with acute chest pain that is indeterminate for ACS because of a nondiagnostic history, physical examination, or ECG. These patients are considered a diagnostic challenge and often require further noninvasive imaging, including chest radiography, echocardiography, multidetector CT, MRI, and radionuclide perfusion imaging as part of further work-up.

Imaging Technique and Findings

Radiography

Conventional chest radiography is the imaging modality used initially for screening noncardiac etiologies of acute chest pain, including pneumonia, pneumothorax, or rib fractures. Chest pain of cardiac origin also can be indirectly evaluated using chest radiography. Coronary arterial stenosis may be hinted by coronary artery calcification, which is seen in the “coronary triangle”—the mid-upper portion of the left cardiac silhouette on the frontal chest radiograph. In addition, circumferential calcifications may be seen within the expected location of myocardial wall, suggesting prior MI (Fig. 52-1). On the lateral chest radiograph, “tram-track” hyperdensities along the aortic arch correspond to atherosclerotic calcifications in the aorta, which can be independently correlated with coronary heart disease.⁵

FIGURE 52-1 A 58-year-old man presented to the emergency department with chest pain. Posteroanterior and lateral chest radiographs show a circumferential thin band of calcification (arrows) in the left mid-anterior myocardium, likely representing prior MI.

Atherosclerotic calcifications overlying the borders of the heart may also indicate previous episodes of MI.⁶^,⁷ Specifically, hyperdensity or calcifications of the left heart border, often curvilinear, may signify prior infarction of the left ventricular anterolateral wall. Focal outpouching of the left heart border may also represent postinfarct myocardial pseudoaneurysm or aneurysm.⁸

Chest radiography may not only indicate coronary heart disease and prior MI, but it may also be indicative of acute myocardial ischemia. Acute myocardial ischemia may manifest as mild to severe congestive heart failure, ranging from cephalization of blood flow to the upper lobes on erect chest radiography to diffuse interstitial or alveolar pulmonary edema.

Ultrasonography

In risk stratifying and evaluating patients who present to the emergency department with acute chest pain, stress echocardiography is a versatile imaging modality. The prognostic information obtained is equivalent to radionuclide perfusion imaging and may be used to exclude ACS.⁹^,¹⁰ Stress echocardiography provides functional and anatomic information by evaluating baseline ventricular function (e.g., ejection fraction, wall motion), valvular function, aortic root morphology, and pericardial anatomy. If a study shows reduction in ejection fraction or wall motion abnormality, especially as an interval change from previous assessment, the findings suggest myocardial ischemia.

Stress echocardiography has limitations in the acute emergency department setting. Patients who have had interval resolution of their acute chest pain symptoms, nontransmural or small MI, or other structural abnormality of the heart may not have wall motion abnormalities or reduced ejection fraction, and may have false-negative results. In addition, causes of chest pain outside the heart are often not well evaluated by echocardiography. Conversely, the portability of stress echocardiography is beneficial, although its use may be limited by dependence on operator experience and restricted availability during off-hours.

Computed Tomography

In recent years, CT has become a noninvasive alternative to invasive coronary angiography in addition to its long-standing role in more general evaluation of the thorax. As the number of detectors has increased from 4 to 320, and gantry rotation speeds have increased, the spatial and temporal resolution for cardiac imaging acquisition has dramatically improved. The current generation of multidetector CT scanners provides accurate anatomic assessment of the coronary vasculature, allowing visualization to at least the third generation of coronary vessels. Compared with invasive coronary angiography, multiple studies have found multidetector CT to have sensitivity of 83% to 99%, specificity of 86% to 98%, positive predictive value of 66% to 93%, and negative predictive value of 97% to 99% in the evaluation of significant coronary arterial stenosis.¹¹

Cardiac multidetector CT approaches the capability of invasive coronary angiography, the gold standard, in its ability to evaluate coronary arterial stenosis. Multidetector CT has several additional capabilities that can be valuable to emergency department physicians and cardiologists (Figs. 52-2 to 52-5). Cardiac multidetector CT can be used to assess the quantity and composition of atherosclerotic plaques and, potentially, to differentiate stable from unstable plaques.¹²^,¹³ Cardiac multidetector CT also can evaluate myocardial function by assessing left ventricular wall motion and ejection fraction on the basis of images obtained throughout the different phases of the cardiac cycle.¹⁴ Dynamic imaging can be performed to assess perfusion of the myocardium.

FIGURE 52-2 A 56-year-old man presented to the emergency department with chest pain. A-C, Curved planar reconstructions of the right coronary (A), left anterior descending (B), and left circumflex (C) arteries show punctate calcified plaque in the left main artery before bifurcation, resulting in 30% stenosis (arrow in C). The remaining arteries show no evidence of CAD. D, Two-dimensional map of the coronary vasculature is assessed, including one diagonal (D1) and two left marginal (M1 and M2) arteries. LAD, left anterior descending; LCX, left circumflex; RCA, right coronary artery.

FIGURE 52-3 A 43-year-old man presented to the emergency department with chest pain. A-C, Curved planar reconstructions of the left anterior descending (A), left circumflex (B), and right coronary (C) arteries show no evidence of CAD.

FIGURE 52-4 A 78-year-old man presented to the emergency department with atypical chest pain and was diagnosed with three-vessel CAD with predominantly calcified plaques. A-C, Using curved planar reconstructions, a 50% to 70% stenosis involving the proximal and mid left anterior descending artery (arrow in A), and a 30% to 50% stenosis in the proximal left circumflex artery (arrow in B) are seen. The right coronary artery also shows a proximal lesion with a 30% to 50% stenosis (arrow in C). D, Volumetric reconstructions show scattered atherosclerotic changes of the vessels. A right dominant posterior descending artery is also seen.

FIGURE 52-5 A 56-year-old man presented to the emergency department with chest pain and shortness of breath. A, Curved planar reconstruction of the right coronary artery shows small and nondominant mild calcified plaque at the ostium (arrow). B, Scattered foci of calcified plaque formation involve the proximal, mid, and distal left anterior descending artery, resulting in 30% stenoses (arrows). C, A calcified plaque (arrow) in the proximal first obtuse marginal branch results in significant narrowing with several smaller calcified plaques distally. D, The left circumflex artery shows no evidence of disease.

Before actual multidetector CT imaging, oral or intravenous β blockers, particularly metoprolol, are administered to reduce heart rate and prevent beat-to-beat variability, with the purpose of reducing motion artifacts and improving image quality. Cardiac images are typically acquired when the patient is breath-holding during mid-inspiration to allow smooth flow of nonionic intravenous contrast agent into the right atrium, resulting in homogeneous enhancement of the heart. With breath-holding, an anatomic triggering mechanism is used to allow rapid injection of intravenous contrast agent. ECG gating is crucial for optimal image acquisition and reconstruction of evenly spaced phases of the cardiac cycle. The reconstructed images are performed in a limited field of view, with the axial images centered on the heart. In the “triple rule-out” chest pain protocol, which is directed at excluding pulmonary embolism and aortic dissection in addition to CAD, an additional large field of view reconstruction, usually at 70% or 80% of the R–R interval, is obtained.

Currently, two protocols—comprehensive or triple rule-out multidetector CT of the chest and dedicated coronary CT angiography—are available when a patient presents to the emergency department with acute chest pain, depending on the specific clinical suspicion related to the chest pain. The comprehensive or triple rule-out protocol images the entire chest. In a patient who has acute, but nonspecific chest pain, a triple rule-out protocol may be used to evaluate life-threatening diagnoses other than ACS, particularly pulmonary embolism and aortic dissection, in addition to the coronary arteries. The dedicated coronary CT angiography protocol is more appropriate when the patient’s presentation strongly suggests a coronary etiology for the chest pain. By using a limited field of view and excluding structures superior to the aortic arch and inferior to the cardiac apex, the dedicated protocol reduces image acquisition time and radiation dose, and provides the best possible resolution of the coronary arteries.

Images of a standard multidetector CT scan of the chest are acquired from the thoracic inlet to the upper portion of the abdomen without the use of ECG gating and with a wide field of view. The dedicated coronary CT angiography protocol requires the use of ECG gating and a limited field of view. A major challenge of the triple rule-out protocol is the fusion of these two methods. Currently in our institution, the triple rule-out protocol is obtained using ECG gating and an unrestricted field of view. Images are acquired in a caudocephalic direction, the reverse of dedicated coronary CT angiography, with the goal of minimizing artifacts caused by respiration. For optimal image quality, a slightly larger volume of intravenous contrast agent is administered to maintain simultaneous peak contrast levels in the coronary and pulmonary arteries (Table 52-1).

TABLE 52-1 Sample Protocol Parameters for 64-Detector Multidetector CT for Acute Chest Pain Evaluation

Protocol	Dedicated CT Angiography	Triple Rule-Out
kVp	120	120
mAs/slice	800	600
Field of view	220	400
Thickness (mm)	0.67	0.9
Increment (mm)	0.33	0.45
Rotation time (sec)	0.4	0.4
Direction	Cephalocaudal	Caudocephalad
Time (sec)	9-10	15-18
Injection protocol	6 mL/sec (80 mL contrast), then 5 mL/sec (50 mL contrast/saline), then 5 mL/sec (50 mL saline)	5 mL/sec (80 mL contrast), then 2 mL/sec (50 mL contrast), then 2 mL/sec (50 mL saline)
Bolus tracker	Ascending aorta	Descending aorta

Several preliminary studies have shown dedicated coronary CT angiography to be a useful noninvasive imaging modality in the evaluation of patients with acute chest pain in the emergency department. In one study, all patients received standard of care assessment to rule out ACS, including invasive coronary angiography.¹⁵ Patients also underwent CT angiography to assess the presence of coronary atherosclerotic plaque and significant coronary artery stenosis (≥50%). Compared with standard of care, coronary CT angiography had 100% sensitivity and 82% specificity. The absence of coronary plaque and significant arterial stenosis excluded ACS in 100% of patients.

The use of the comprehensive or triple rule-out multidetector CT protocol has also been studied in patients with acute chest pain in the emergency department. In a separate study of this algorithm, all patients underwent a triple rule-out multidetector CT scan immediately after a serum creatinine value and an initial history, physical examination, and ECG were obtained.¹⁶ After CT, patients proceeded to receive the standard of care evaluation and treatment. One month after discharge, medical records were reviewed by consensus. Overall, the sensitivity, specificity, positive predictive value, and negative predictive value for CAD were 83%, 96%, 83%, and 96%, respectively. If noncardiac etiologies were also assessed, the sensitivity and positive predictive value increased to 87%. Comprehensive multidetector CT has the potential to evaluate cardiac and noncardiac causes of acute chest pain.

Another study has shown cardiac multidetector CT to be just as accurate in detecting and excluding ACS as the current standard of care using radionuclide perfusion imaging.¹⁷ In addition, the median diagnostic time and cost of care were significantly reduced. Cardiac multidetector CT can be as safe and efficacious as the standard of care, and the efficiency (diagnostic time and cost of care) is improved in evaluating emergency department patients with acute chest pain.

To date, there are several limitations to the widespread adoption of cardiac multidetector CT in the emergency department. First, the postprocessing of data after image acquisition is time-consuming and labor-intensive, restricting its use during off hours. Second, the radiation burden is still high (often >10 mSv), even with the use of current technologies, such as ECG-controlled tube current modulation, which can potentially reduce the exposure. Radionuclide perfusion imaging (8 to 10 mSv) and invasive coronary angiography also have substantial radiation doses, however. Third, although the triage of a negative result in cardiac multidetector CT is straightforward, the course of a positive result is less certain and remains to be defined (Table 52-2). Finally, a larger, multi-institutional clinical study using cardiac multidetector CT to evaluate ACS is still required to establish the role of multidetector CT in acute chest pain assessment.

TABLE 52-2 University of Maryland Triage for Cardiac Multidetector CT Studies

Risk Category for ACS	CT Interpretation	Clinical Guideline
High	Coronary calcification (Agatston score) >400	Admission
	Hard or soft plaque
	Stenosis >70% in any vessel
	Stenosis >50% in left main artery
Medium	Coronary calcification 100-400	Cardiology consultation
	Stenosis 30%-70% in any vessel
Low	Coronary calcification <100	Follow-up with preventive cardiology
Negative	Normal study	Follow-up with primary care provider

Magnetic Resonance Imaging

MRI can be used to assess myocardial function, perfusion, and viability.¹⁸ Myocardial function assessment is based on the evaluation of myocardial wall motion. Assessments of myocardial perfusion and viability require intravenous administration of gadolinium contrast material and acquisition of dynamic images during the immediate and delayed (10 to 15 minutes) phases. These findings can facilitate the detection of myocardial ischemia, postischemic stunning or hibernation, and MI. In one study, cardiac MRI detected and diagnosed ACS with a sensitivity of 84% and specificity of 85%.¹⁹

Cardiac assessment using MRI is primarily based on myocardial wall motion and perfusion and viability images obtained after contrast administration. If wall motion abnormality is observed, but the perfusion and viability (delayed) images are normal, the myocardium may have endured an ischemic insult, a situation termed myocardial stunning. If wall motion and the perfusion (immediate postgadolinium) images are abnormal, but no abnormal uptake of the contrast material is seen in the viability (delayed) images, postischemic hibernation is diagnosed. If viability images show uptake of gadolinium within the myocardium, as shown by high signal intensity, the finding is consistent with MI (Fig. 52-6).

FIGURE 52-6 A 60-year-old man presented to the emergency department with chest pain. A and B, MRI viability images in short-axis (A) and vertical long-axis (B) views of the left ventricle show delayed contrast enhancement involving the mid to basal portion of the inferior, subendocardial wall of the left ventricle (arrows), consistent with subendocardial infarct of the myocardium.

Compared with radionuclide myocardial imaging and stress echocardiography, MRI is able to overcome some disadvantages. The anatomic and functional assessments of the myocardium are better because of improved spatial and temporal resolution. In addition, cardiac MRI may be available 24 hours a day, particularly in larger centers. In addition, there is no radiation dose exposure because no ionizing radiation is used in MRI, in contrast to radionuclide myocardial perfusion. The disadvantages of MRI include the greater length of time for image acquisition, distance of the scanner from the emergency department in most departments, and lack of expertise at many facilities. From a practical standpoint, cardiac MRI is more often used to assess chronic myocardial ischemia or infarction than acute chest pain.

Nuclear Medicine

In patients with low to intermediate risk of ACS, radionuclide or myocardial perfusion imaging is typically a part of a cardiac evaluation protocol for risk stratification.²⁰ Either during or immediately after an acute episode of chest pain, Tc 99m tetrofosmin or sestamibi is injected. Image acquisition is obtained using single photon emission computed tomography (SPECT) 45 to 60 minutes after injection of the radionuclide. Two sets of images are acquired to reflect myocardial blood flow at the time of the injection: one set of images with adenosine infusion or exercise stress and a second set of images at rest.

Radionuclide perfusion imaging is a powerful imaging modality in the diagnosis of ACS. The sensitivity, specificity, and negative predictive value are 92%, 71%, and 99%, respectively.²¹ As a result, a negative radionuclide perfusion study in a patient with acute chest pain in the emergency department suggests minimal or no risk of an adverse cardiac event.²² Myocardial perfusion imaging can also be used for prognosis and risk stratification for future cardiac events.²³ If a perfusion defect is shown, the patient is at higher risk of having a cardiac event during hospitalization or at follow-up.

The main purpose of myocardial perfusion study is to assess the myocardial function by evaluating inducible perfusion abnormality, myocardial viability, and regional and global myocardial wall motion. Inducible perfusion abnormality is diagnosed if the uptake of radiotracer in a certain region of the myocardium is reduced during stress imaging. During the rest images, the myocardium takes up the radiotracer normally, suggesting the ischemic territory is reversible, and the perfusion abnormality is inducible by stress (Fig. 52-7). Conversely, if the radiotracer is not taken up by the myocardium during the rest images, the myocardium is considered nonviable. Abnormality in wall motion can also be evaluated by visualizing the decrease or lack of contractility in the region of myocardial ischemia and infarct.

FIGURE 52-7 A 55-year-old woman presented to the emergency department with atypical chest pain. Stress images show radiotracer uptake defect in the apical anterior/anterolateral myocardium (arrows). On rest images, the uptake defect is reversed, and the myocardial uptake of the radiotracer is normal, suggesting reversible myocardial ischemia.

Although radionuclide perfusion is a powerful component of the acute chest pain evaluation armamentarium, there are several drawbacks that hinder its widespread use. Radionuclide perfusion imaging is restricted in its ability to assess the myocardial structure and anatomy because of technical limitations in spatial resolution and photon attenuation. In addition, isotope preparation, decay, and licensing, and restrictive availability during off-hours limit its use. In most hospitals, any radionuclide perfusion study requires transporting a potentially unstable patient because nuclear medicine departments are rarely located adjacent to the emergency department.

Angiography

Cardiac catheterization and coronary angiography are currently the standard in the evaluation of high-risk patients with ACS. Cardiac catheterization provides a measure of intracardiac pressure, oxygen saturation, and cardiac output. Left ventricular function can also be evaluated. Coronary angiography remains the gold standard for delineating coronary anatomy and diagnosing CAD. Coronary angiography is used to assess the site, severity, and morphology of lesions and provides a qualitative measurement of coronary blood flow. Collateral vessels can also be identified.

Percutaneous coronary intervention (PCI) using angioplasty and stent placement to revascularize the myocardium may be performed in the same setting. Compared with thrombolytic therapy, PCI is associated with a lower mortality, lower reinfarction rate, and lower frequency of stroke.²⁴^,²⁵ PCI also restores flow to the occluded coronary artery with greater speed and frequency and with a high success rate.²⁶

Classic Signs

Obstruction to coronary flow and myocardial wall motion and perfusion abnormalities may be evident using one or more of the previously described techniques.

DIFFERENTIAL DIAGNOSIS

The differential diagnoses that should be considered other than ACS, especially if the patient presents to the emergency department with atypical chest pain, can be categorized as either life-threatening conditions or conditions with less morbidity and mortality. Life-threatening diagnoses include aortic dissection or leaking aneurysm, cardiac tamponade, cardiomyopathies, esophageal rupture, hypertensive emergencies, perforated gastric ulcer, pneumothorax, and pulmonary embolism. Less emergent conditions include anxiety, asthma, costochondritis, esophagitis, gastroenteritis, myocarditis, and pericarditis. Many life-threatening conditions cannot be diagnosed solely based on clinical presentation. Various imaging modalities are essential to differentiate and treat these conditions emergently.

TREATMENT OPTIONS

Medical

ACC/AHA guidelines recommend all initial therapy be implemented in the emergency department based on an institution-specific, written protocol.²⁷ Patients presenting to the emergency department with symptoms suggestive of an acute myocardial event should be evaluated within 10 minutes after arrival. Early assessment includes confirmation of hemodynamic stability and securing the airway, breathing, and circulation; obtaining a 12-lead ECG; and administering oxygen to maintain saturation greater than 90%, intravenous analgesia (most commonly morphine sulfate), oral aspirin (162 to 325 mg), and sublingual nitroglycerin (0.4 mg per 5 minutes for a total of three doses) if not contraindicated. Initial history, physical examination, and laboratory work are also necessary.

All patients with suspected or definite ACS should be admitted to the hospital. High-risk patients should be admitted to a coronary care unit. Patients with a low risk for major complications can be admitted to a telemetry unit. Resuscitative medical therapy and equipment should be readily available, including lidocaine, atropine, an external or internal pacemaker, and a defibrillator.

In patients with acute STEMI whose onset of symptoms is of less than 12 hours’ duration, reperfusion therapy with primary PCI or thrombolysis is essential. If PCI is unavailable, thrombolytic therapy (combination of heparin plus tissue plasminogen activator or streptokinase or both) must be administered promptly (in <30 minutes) in patients without contraindications for thrombolysis. If there is a relative delay in performing primary PCI (>60 minutes), thrombolytic therapy is also recommended. In patients with unstable angina or NSTEMI, thrombolytic therapy should not be administered.

Several medications should also be given to all patients with ACS and without contraindications, regardless of reperfusion therapy, including clopidogrel, glycoprotein IIb/IIIa inhibitors, β blockers, and calcium channel blockers. Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers may also be given. If a patient is to undergo reperfusion therapy, data have shown that clopidogrel (300 or 600 mg) and glycoprotein IIb/IIIa inhibitors reduce thrombotic complications.

Surgical/Interventional

The ACC/AHA Task Force recommends the use of PCI with possible stent placement for any patient with acute STEMI who presents within 12 hours of onset of symptoms, and who can undergo the procedure within 90 minutes of presentation.²⁷ For unstable angina and NSTEMI, only high-risk patients should undergo early PCI management. High-risk indicators include (1) recurrent angina at rest or with low-level activity despite intensive medical therapy; (2) elevated cardiac troponin levels; (3) new, or presumably new, ST segment depression; (4) recurrent angina or ischemia with heart failure; (5) high-risk findings on noninvasive stress testing; (6) depressed left ventricular function (ejection fraction <40%) on noninvasive study; (7) hemodynamic instability; (8) sustained ventricular tachycardia; (9) PCI within the previous 6 months; and (10) previous coronary artery bypass graft (CABG) surgery.²⁸ Otherwise, medical management is recommended.

As described in the most recent ACC/AHA practice guideline, CABG surgery may offer a survival advantage compared with medical management in patients with unstable angina and left ventricular dysfunction, especially in patients with three-vessel CAD.²⁹ There is as yet no medical evidence, however, to suggest CABG surgery is better than PCI in patients with unstable angina. No study has shown that CABG surgery should be performed in patients with NSTEMI or STEMI in the acute setting.