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1/16/24, 10:25 PM Realizeit for Student Introduction Acute coronary syndrome (ACS) is an emergent situation characterized by an acute onset of myocardial ischemia that results in myocardial death (i.e., MI) if definitive interventions do not occur promptly. (Although the terms coronary occlusion,...

1/16/24, 10:25 PM Realizeit for Student Introduction Acute coronary syndrome (ACS) is an emergent situation characterized by an acute onset of myocardial ischemia that results in myocardial death (i.e., MI) if definitive interventions do not occur promptly. (Although the terms coronary occlusion, heart attack, and myocardial infarction are used synonymously, the preferred term is myocardial infarction.) The spectrum of ACS includes unstable angina, NSTEMI, and STsegment elevation myocardial infarction (STEMI). Acute Coronary Syndrome and Myocardial Infarction Pathophysiology In unstable angina, there is reduced blood flow in a coronary artery, often due to rupture of an atherosclerotic plaque. A clot begins to form on top of the coronary lesion, but the artery is not completely occluded. This is an acute situation that can result in chest pain and other symptoms that may be referred to as preinfarction angina because the patient will likely have an MI if prompt interventions do not occur. In an MI, plaque rupture and subsequent thrombus formation result in complete occlusion of the artery, leading to ischemia and necrosis of the myocardium supplied by that artery. Vasospasm (sudden constriction or narrowing) of a coronary artery, decreased oxygen supply (e.g., from acute blood loss, anemia, or low blood pressure), and increased demand for oxygen (e.g., from a rapid heart rate, thyrotoxicosis, or ingestion of cocaine) are other causes of MI. In each case, a profound imbalance exists between myocardial oxygen supply and demand. The area of infarction develops over minutes to hours. As the cells are deprived of oxygen, ischemia develops, cellular injury occurs, and the lack of oxygen results in infarction, or the death of cells. The expression “time is muscle” reflects the urgency of appropriate treatment to improve patient outcomes. Approximately every 40 seconds, an American will have an MI (Benjamin et al., 2019), and many of these people will die as a result. Early recognition and treatment of patients presenting with an MI will improve their chances of survival. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zbfuwFjUs0mdxkPeVey4KH2F7i%2fH0LC0NH7inQLoUzK%2fM… 1/7 1/16/24, 10:25 PM Realizeit for Student Various descriptions are used to further identify an MI: the type (NSTEMI, STEMI), the location of the injury to the ventricular wall (anterior, inferior, posterior, or lateral wall), and the point in time within the process of infarction (acute, evolving, or old). The differentiation between NSTEMI and STEMI is determined by diagnostic tests and is explained later in this chapter. The 12-lead ECG identifies the type and location of the MI, and other ECG indicators, such as a Q wave, and patient history, identify the timing. Regardless of the location, the goals of medical therapy are to relieve symptoms, prevent or minimize myocardial tissue death, and prevent complications. Clinical Manifestations Chest pain that occurs suddenly and continues despite rest and medication is the presenting symptom in most patients with ACS. Some of these patients have prodromal symptoms or a previous diagnosis of CAD, but others report no previous symptoms. Patients may present with a combination of symptoms, including chest pain, shortness of breath, indigestion, nausea, and anxiety. They may have cool, pale, and moist skin. Their heart rate and respiratory rate may be faster than normal. These signs and symptoms, which are caused by stimulation of the sympathetic nervous system, may be present for only a short time or may persist. In many cases, the signs and symptoms of MI cannot be distinguished from those of unstable angina; hence, the evolution of the term acute coronary syndrome. Electrocardiogram The 12-lead ECG provides information that assists in ruling out or diagnosing an acute MI. It should be obtained within 10 minutes from the time a patient reports pain or arrives in the ED. By monitoring serial ECG changes over time, the location, evolution, and resolution of an MI can be identified and monitored. The ECG changes that occur with an MI are seen in the leads that view the involved surface of the heart. The expected ECG changes are T-wave inversion, ST-segment elevation, and development of an abnormal Q wave. Because infarction evolves over time, the ECG also changes over time. The first ECG signs of an acute MI are usually seen in the T wave and ST segment (Urden et al., 2019). As the area of injury becomes ischemic, myocardial repolarization is altered and delayed, causing the T wave to invert. Myocardial injury also causes ST-segment changes. The ST segment is normally flat on the ECG tracing. The injured myocardial cells depolarize normally but repolarize more https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zbfuwFjUs0mdxkPeVey4KH2F7i%2fH0LC0NH7inQLoUzK%2fM… 2/7 1/16/24, 10:25 PM Realizeit for Student rapidly than normal cells, causing the ST segment to rise at least 1 mm above the isoelectric line (the area between the T wave and the next P wave is used as the reference for the isoelectric line). This change is measured 0.06 to 0.08 seconds after the end of the QRS—a point called the J point (Urden et al., 2019). An elevation in the ST segment in two contiguous leads is a key diagnostic indicator for MI (i.e., STEMI). https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zbfuwFjUs0mdxkPeVey4KH2F7i%2fH0LC0NH7inQLoUzK%2fM… 3/7 1/16/24, 10:25 PM Realizeit for Student The appearance of abnormal Q waves is another indication of MI. Q waves develop within 1 to 3 days because there is no depolarization current conducted from necrotic tissue (Urden et al., 2019). A new and significant Q wave is 0.04 seconds or longer and 25% of the R wave depth. An acute MI may also cause a significant decrease in the height of the R wave. During an acute MI, injury and ischemic changes are usually present. An abnormal Q wave may be present without STsegment and T-wave changes, which indicates an old, not acute, MI. For some patients, there is no persistent ST elevation or other ECG changes; therefore, an NSTEMI is diagnosed by blood levels of cardiac biomarkers. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zbfuwFjUs0mdxkPeVey4KH2F7i%2fH0LC0NH7inQLoUzK%2fM… 4/7 1/16/24, 10:25 PM Realizeit for Student Using the information presented, patients are diagnosed with one of the following forms of ACS: •Unstable angina: The patient has clinical manifestations of coronary ischemia, but ECG and cardiac biomarkers show no evidence of acute MI. •STEMI: The patient has ECG evidence of acute MI with characteristic changes in two contiguous leads on a 12-lead ECG. In this type of MI, there is a significant damage to the myocardium. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zbfuwFjUs0mdxkPeVey4KH2F7i%2fH0LC0NH7inQLoUzK%2fM… 5/7 1/16/24, 10:25 PM Realizeit for Student •NSTEMI: The patient has elevated cardiac biomarkers (e.g., troponin) but no definite ECG evidence of acute MI. In this type of MI, there may be less damage to the myocardium. During recovery from an MI, the ST segment often is the first ECG indicator to return to normal. Q-wave alterations are usually permanent. An old STEMI is usually indicated by an abnormal Q wave or decreased height of the R wave without ST-segment and Twave changes. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zbfuwFjUs0mdxkPeVey4KH2F7i%2fH0LC0NH7inQLoUzK%2fM… 6/7 1/16/24, 10:25 PM Realizeit for Student Echocardiogram The echocardiogram is used to evaluate ventricular function. It may be used to assist in diagnosing an MI, especially when the ECG is nondiagnostic. The echocardiogram can detect hypokinetic and akinetic wall motion and can determine the ejection fraction. Laboratory Tests Cardiac enzymes and biomarkers, which include troponin, creatine kinase (CK), and myoglobin, are used to diagnose an acute MI. Cardiac biomarkers can be analyzed rapidly, expediting an accurate diagnosis. These tests are based on the release of cellular contents into the circulation when myocardial cells die. Troponin Troponin, a protein found in myocardial cells, regulates the myocardial contractile process. There are three isomers of troponin: C, I, and T. Troponins I and T are specific for cardiac muscle, and these biomarkers are currently recognized as reliable and critical markers of myocardial injury (Norris, 2019). An increase in the level of troponin in the serum can be detected within a few hours during acute MI. It remains elevated for a long period, often as long as 2 weeks, and it therefore can be used to detect recent myocardial damage. It should be noted that cardiac troponin levels may rise during inflammation and other forms of mechanical stress on the myocardium. These include sepsis, heart failure, and respiratory failure (Felker & Fudim, 2018). Creatine Kinase and Its Isoenzymes There are three CK isoenzymes: CK-MM (skeletal muscle), CK-MB (heart muscle), and CK-BB (brain tissue). CK-MB is the cardiac-specific isoenzyme; it is found mainly in cardiac cells and therefore increases when there has been damage to these cells. Elevated CK-MB is an indicator of acute MI; the level begins to increase within a few hours and peaks within 24 hours of an infarct. Myoglobin Myoglobin is a heme protein that helps transport oxygen. Like the CK-MB enzyme, myoglobin is found in cardiac and skeletal muscle. The myoglobin level starts to increase within 1 to 3 hours and peaks within 12 hours after the onset of symptoms. An increase in myoglobin is not very specific in indicating an acute cardiac event; however, negative results can be used to rule out an acute MI. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zbfuwFjUs0mdxkPeVey4KH2F7i%2fH0LC0NH7inQLoUzK%2fM… 7/7