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October 1, 2013 Volume 77, Number 10
SEE Question


A 62-year-old woman presents to the emergency department with central chest pain that began one hour earlier. Her medications include aspirin, lisinopril, and atorvastatin. Her heart rate is 110 beats/min, blood pressure is 140/90 mm Hg, and pulse oximetry is 98 percent. Electrocardiography shows sinus rhythm and nonspecific ST-T changes. Which of the following measurements using the new, highly sensitive cardiac troponin (cTn) assays provides the BEST diagnostic accuracy for acute myocardial infarction (AMI)?


(A) An absolute change in serial cTn levels


(B) A percentage change in serial cTn levels


(C) The cTn level measured at presentation


(D) The cTn level measured six hours following presentation



The early diagnosis of AMI is crucial for improving patient outcomes. AMI can be diagnosed on the basis of symptoms, electrocardiographic (ECG) changes, biomarkers of cardiac necrosis, and cardiac imaging. ST elevation myocardial infarction can be readily identified by the characteristic ECG changes. However, diagnosing AMI when confronted with nonspecific ECG changes can be challenging.


Cardiac troponins (cTn) are promptly released into the bloodstream following myocardial necrosis, but they do not elucidate the mechanism of myocardial injury. Troponin elevations may be encountered in many other situations besides AMI, such as myopericarditis, heart failure, kidney disease, critical illness, following cardiac surgery and percutaneous coronary intervention, and even in otherwise healthy individuals.


Per the accepted definition, AMI may be diagnosed in a patient with symptoms consistent with myocardial ischemia in the presence of an acute rise and fall in serum troponin, with at least one value above the 99th percentile upper reference limit. It is recommended that standard troponin assays be serially measured at presentation and six to eight hours later. With the introduction of the newer-generation, highly sensitive assays, the measurement window may be further decreased to one or two hours.


The ongoing multicenter prospective Advantageous Predictors of Acute Coronary Syndromes Evaluation study recently examined the role of absolute versus relative changes in highly sensitive cTn values in the evaluation of patients (n = ~1,200) presenting to the emergency department with symptoms of myocardial ischemia. Irrespective of baseline cTn levels, both absolute and relative changes were higher in AMI patients. However, an absolute change had greater accuracy than a percentage change in diagnosing AMI as early as one to two hours after presentation. A single troponin elevation measured either at presentation or six hours later does not indicate ongoing necrosis and is not specific for AMI.


An absolute change in cTn from baseline to one or two hours later can expeditiously distinguish AMI from other diagnoses.


Limitations of this study include the use of a homogeneous patient population (European); this measure has not been externally validated at other times, in other locations, and in different patient populations.



Answer: A



 

Bibliography:

  • Reichlin T, Irfan A, Twerenbold R, et al. Utility of absolute and relative changes in cardiac troponin concentrations in the early diagnosis of acute myocardial infarction. Circulation. 2011;124(2):136-145.
  • Thygesen K, Alpert JS, Jaffe AS, et al. Third universal definition of myocardial infarction. J Am Coll Cardiol. 2012;60(16):1581-1598.

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