These assays were recently evaluated by two large multicenter studies revealing corroborating results. Several new high-sensitivity troponin assays can detect lower concentrations of circulating troponins compared with previous assays. Their levels start to rise 3 to 12 hours after the onset of ischemia, peak at 12 to 24 hours, and may remain elevated for 8 to 21 days (troponin T) or 7 to 14 days (troponin I). There are two isoforms of cardiac troponin: T and I. The cardiac troponins regulate the interaction of actin and myosin and are more cardiac-specific than CK-MB. Blood sampling for biomarker determination is recommended at hospital admission, at 6 to 9 hours, and at 12 to 24 hours if the earlier samples were negative and the clinical index of suspicion is high (see Chap. Determinations of total CK, aspartate aminotransferase, and lactate dehydrogenase are no longer recommended. CK-MB, because of its more rapid appearance and disappearance from the blood, can be used (1) in patients presenting early after symptom onset (2) to time the onset of injury if the troponin is increased and (3) to detect reinfarction later in the hospital course. 13 CK-MB is the best alternative, if cardiac troponin assays are not available. 12 Cardiac troponins are currently the preferred biomarkers for myocardial damage because of their high sensitivity and specificity. 7, 8 ST-segment elevation in these additional leads is suggestive of posterior involvement.ĭamaged cardiomyocytes release several proteins in the circulation, including myoglobin, creatine kinase (CK) and its myocardial band isoenzyme (CK-MB), troponins (I and T), myoglobin, aspartate aminotransferase, and lactate dehydrogenase (see Chap. Sensitivity of detecting posterior myocardial infarction may be enhanced by the use of additional ECG leads (V7-V9) designed to represent the posterolateral myocardium. 6 Depending on the affected artery (right coronary vs circumflex), posterior myocardial infarction is frequently associated with either inferior or lateral injury with ECG manifestations of STEMI corresponding to those areas of myocardium. Because no leads on the standard 12-lead ECG directly represent the posterior myocardium, isolated infarction of this area may be difficult to diagnose but is typically manifested by ST-segment depression in V1-V3, a mirror image of anterior myocardial infarct. 5 ST-segment elevation in lead V1, in the setting of inferior myocardial infarction suggests RV involvement.
5 In a study of STEMI patients, ST-segment elevation in lead aVR that was greater than or equal to the extent of ST-segment elevation in lead V1 had 81% accuracy for diagnosing left main occlusion. ST-segment elevation in lead aVR is more frequent in patients with left main artery occlusion than in patients with left anterior descending coronary artery or right coronary artery occlusion.
15): ST-segment elevation in the inferior, anterior, or high lateral leads is seen with infarction of the corresponding areas of myocardium. The specific leads with ST-segment elevation can help localize the infarct (see Chap. 3 Despite these improvements, AMI continues to be a major public health problem, and it has been estimated that the number of years of life lost because of an AMI is 15 years, and the cost to American society (both direct and indirect) is $165.4 billion per year. 2 The in-hospital mortality rate also has declined from 18% in 1975 to 10% in 2006. 2 The incidence of AMI has declined over the past 2 decades from 244 per 100,000 population in 1975 to 162 per 100,000 population in 2006. 1 Approximately 935,000 Americans suffer from an acute myocardial infarction (AMI) per year, one-third of which are caused by an acute ST-segment elevation myocardial infarction (STEMI). In 2004, it was estimated that worldwide, ischemic heart disease was responsible for 9.4% of all deaths (2.5 million) in low-income countries and 16.3% (1.3 million) of all deaths in high-income countries. Coronary artery disease is the leading cause of morbidity and mortality in Western society and is a worldwide epidemic.
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