Not all Troponin Elevations Are Myocardial Infarctions
Jeremais A, Gibson CM. Narrative review: alternative causes for elevated cardiac troponin levels when acute coronary syndromes are excluded. Ann Intern Med. 2005;142:786-791.
Troponins are regulatory proteins that control the calcium-mediated interaction of actin and myosin during muscle contraction. All muscle tissue contains troponins, but cardiac troponin T and I have amino acid sequences that are different from skeletal and smooth muscle troponins, allowing them to be detectable by monoclonal antibody-based assays.
In the event of reversible or irreversible cell damage—or possibly even from transiently increased cell membrane permeability—cardiac troponins are released from myocytes into circulation. This characteristic provides a sensitive test for detecting myocardial injury and damage; however, this test is not specific for acute coronary syndromes. And any disorder that causes myocyte damage may cause an elevated troponin.
The 2002 American College of Cardiology/American Heart Association practice guidelines for unstable angina and non-ST-segment elevation myocardial infarction acknowledge that the myocardial necrosis signified by troponin elevation may not necessarily be caused by atherosclerotic coronary artery disease. Such nonthrombotic troponin elevation can be caused by four basic mechanisms, as discussed by Dr. Jeremias and Dr. Gibson.
- Demand ischemia refers to a mismatch between myocardial oxygen demand and supply in the absence of flow-limiting epicardial stenosis. Conditions such as sepsis or septic shock and the systemic inflammatory response syndrome, hypotension or hypovolemia, tachyarrhythmias, and left ventricular hypertrophy can all cause release of cardiac troponin.
- Myocardial ischemia in the absence of fixed obstructive coronary disease can be caused by coronary vasospasm, acute stroke or intracranial hemorrhage, and ingestion of sympathomimetics.
- Direct myocardial damage can be seen in cardiac contusion, direct current cardioversion, cardiac infiltrative disorders such as amyloidosis, certain chemotherapy agents, myocarditis, pericarditis, and cardiac transplantation.
- Myocardial strain occurs when volume and pressure overload of the left and/or right ventricle cause excessive wall tension. Congestive heat failure, acute pulmonary embolism, and chronic pulmonary hypertension can lead to myocardial strain and troponin elevation.
Another condition that can lead to persistently elevated cardiac troponins is end-stage renal disease. This elevation may be due to small areas of clinically silent myocardial necrosis, an increased left ventricular mass, or possibly from impaired renal troponin excretion. Although troponins are believed to be cleared by the reticuloendothelial system, recent evidence shows that troponin T is fragmented into molecules that are small enough to be renally excreted.
In summary, elevated troponin can be found in many clinical settings and is associated with impaired short- and long-term survival. TH