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Cardiac Enzymes and Heart Attacks

By Rosalyn Carson-DeWitt, MD

Updated October 07, 2008

(LifeWire) - Cardiac markers (also called cardiac enzymes) are substances that are released in to the bloodstream when the heart muscle is damaged. These markers can be measured in a laboratory to help diagnose a heart attack (myocardial infarction). The cardiac markers that are most useful in making a determination of heart muscle damage include:

  • Troponin
  • Creatine kinase (CK)
  • Myoglobin (Mb)

About Troponin

Troponin is a component of the heart's muscle fibers, and the level of troponin in the blood is considered the most important cardiac marker used for assessing heart attacks. Troponin is the most sensitive (that is, it can be detected at low levels) and specific (its presence has a high probability of indicating cardiac muscle damage) of the cardiac markers.

When the heart is deprived of oxygen, the muscle fibers are damaged, and their components (including troponin) leak in to the bloodstream. Within 3 to 4 hours after a heart attack, blood levels of two types of troponin (cTnI and cTnT) begin to increase. Troponin levels peak at about 12 to 16 hours and stay elevated for up to 2 weeks.  Blood is usually drawn to check troponin levels as an individual arrives at the hospital with chest pain, and then they are checked every 4 to 6 hours after that. Higher troponin levels indicate greater heart muscle damage, but even smaller heart attacks can be detected by measuring troponin.

About Creatine Kinase (CK)

These enzymes are important participants in the chemical reactions that are necessary for the heart muscle's quick generation of energy. CK, however, is not just present in heart muscle cells; it's actually present inside all muscle cells throughout the body, as well as in brain and lung cells. Since different forms of CK may be increased because of skeletal muscle injury, liver disease or kidney disease, a specialized test is used to determine what percentage of the total CK is due to the type produced by damaged heart muscles (called "CK-MB").

After a heart attack, levels of CK-MB follow a particular, predictable pattern. CK-MB levels begin to rise within about 3 to 6 hours after a heart attack, with the highest CK-MB levels occurring about 12 to 24 hours after the heart attack. Within about 12 to 48 hours of a heart attack, the CK-MB in the bloodstream will return to normal levels. To see this pattern, a series of blood tests will be drawn at various times after the start of the patient's initial symptoms -- usually immediately upon arrival at the hospital, and then every 6 to 8 hours for the next 24 hours.

About Myoglobin (Mb)

Myoglobin is a protein found in both skeletal and heart muscle. Although myoglobin levels are not specific for heart damage, they rise very early in a heart attack, making them useful for the early, provisional detection of a heart attack. Myoglobin rises within about 2 hours of a heart attack, peaks at 8 to 12 hours after the heart attack and returns back to normal levels at about 20 to 36 hours after the heart attack. Myoglobin levels are usually drawn upon arrival at a hospital, and then every 2 to 3 hours thereafter for several cycles. A persistently normal myoglobin level can rule out heart muscle damage. Although an elevated myoglobin can suggest a heart attack, because myoglobin can be elevated in many other conditions, an elevated troponin level is required to make a definitive diagnosis.

Sources:

Antman, Elliott. "ST-Elevation Myocardial Infarction: Pathology, Pathophysiology, and Clinical Features." Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. Ed. Peter Libby, Robert O. Bonow, Douglas L. Mann, Douglas P. Zipes. Philadelphia: Saunders, 2007. Chapter 51.



"Blood Tests for Rapid Detection of Heart Attack." AmericanHeart.org. 2008. American Heart Association. 4 Sep. 2008. <http://www.americanheart.org/presenter.jhtml?identifier=4477>.



Butler, Kenneth H., and Sharon A. Swencki.  "Chest Pain: A Clinical Assessment. " Radiology Clinics of North America 44:21(2006): 165-79. 4 Sep. 2008. <http://www.radiologic.theclinics.com/article/S0033-8389(05)00180-6/abstract>.



Cayle, William E. "Diagnosing the Cause of Chest Pain." American Family Physician. 72:10(2005): 2012-21. 4 Sep. 2008 <http://www.aafp.org/afp/AFPprinter/20051115/2012.pdf>.



Jaffe, Allen S. "Use of Biomarkers in the Emergency Department and Chest Pain Unit." Cardiology Clinics. 23:41 (2005): 453-65. 5 Sep. 2008 <http://www.cardiology.theclinics.com/issues/contents?issue_key=S0733-8651%2805%29X0019-4>.



Senger, Amy K., and Allen S. Jaffe. "The Use of Biomarkers for the Evaluation and Treatment of Patients with Acute Coronary Syndromes." Medical Clinics of North America 91:41(2007): 657-81. 5 Sep. 2008  <http://linkinghub.elsevier.com/retrieve/pii/S0025712507000430>.


LifeWire, a part of The New York Times Company, provides original and syndicated online lifestyle content. Rosalyn Carson-DeWitt, MD, works as a medical writer, editor and consultant in Durham, NC. She served as editor-in-chief for two multivolume MacMillan encyclopedias: The Encyclopedia of Drugs, Alcohol, and Addictive Behavior and Drugs, Alcohol and Tobacco: Learning About Addictive Behavior. She worked on the 18th edition of the Merck Manual of Diagnosis and Therapy, and has written thousands of print and online articles for health care providers and consumers.

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