Cardiovascular system: Detection of myocardial infarction

The 2007 American College of Cardiology (ACC) definition of myocardial infarction includes the detection of a rise and/or fall of cardiac biomarkers – with at least one value exceeding the 99th percentile of the upper reference limit – with evidence of myocardial damage. Cardiac enzymes that are elevated following myocardial injury include troponin and creatinine kinase.

In the cell, there are three troponin subunits: Troponin Inhibitory unit (TnI), Troponin Tropomyosin binding unit (TnT) and Troponin Calcium binding unit (TnC). Troponin is found in both cardiac and skeletal muscles. Cardiac TnI (cTnI) contains 31 additional amino acid residues on its N-terminus compared with TnI in skeletal muscles. TnT also exists in different forms in cardiac and skeletal muscles, whereas the TnC forms found in skeletal and cardiac muscles are identical. Diagnostic assays use antibodies that are specific for cardiac TnI (cTnI) and cardiac TnT (cTnT); however, cTnT has been shown to also be upregulated in conditions such as muscular dystrophy and renal failure. The detection limit of both cTnI and cTnT levels for most assays is 0.01 ng/mL, and the 99th percentile of these biomarkers in the normal population is reported as 0.04 ng/mL.

Creatine kinase consists of two chains (M and B) that form three isozymes: CK-MB, CK-MM and CK-BB. CK-MM and CK-MB are found in skeletal muscles, so skeletal muscle disease will be associated with elevated CK-MM and CK-MB levels.

Other biomarkers of myocardial Infarction that are no longer used or have uncertain clinical relevance include lactate dehydrogenase, myoglobin, carbonic anhydrase, glycogen phosphorylase, heart fatty acid binding protein, and ischemia-modified albumin.

Figure 1 shows how the levels of cardiac enzymes change in the 160 hours following myocardial infarction.

Figure 1 Factor increase of cardiac enzyme levels above normal during the 160 hours following myocardial infarction.