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What Is a Cardiac Enzyme?

By Nicole Etolen
Updated: Jan 29, 2024

Cardiac enzymes are released into the blood stream when the heart sustains damage. They include the enzyme creatine phosphokinase (CPK) and the protein troponin. Typically, these enzymes are found in very low levels throughout the body, but when the heart muscle is damaged, they leak out in larger amounts. A cardiac enzyme study measures the levels of the two substances, allowing doctors to determine if a cardiac event, such as a heart attack, has occurred.

Physicians usually order a cardiac enzyme study when a patient presents with the symptoms of a heart attack. The procedure is performed by drawing blood from the patient, typically from a vein in the arm or hand, and sending it to a lab for analysis. While the study only requires a single blood draw, it actually consists of two separate tests. In some cases, only one of the two is ordered.

The troponin test is usually the preferred test for determining whether a patient has suffered a heart attack. The presence of this cardiac enzyme is indicative of heart injury, and levels tend to remain elevated for a longer time than CPK. In normal conditions, levels of troponin are so low that they are barely detectable. Slightly elevated levels indicate some type of heart damage, while significantly elevated levels indicate that a heart attack has occurred.

The CPK test is also used to diagnose heart attack, but it is not as reliable because elevated levels of this cardiac enzyme can occur with brain or muscle injury. Thyroid disorders can also produce an abnormal result. A CPK isoenzyme test can be used to determine the exact type of CPK present in the blood, which in turns helps determine where the damage occurred. The test is usually repeated every two to three days while the patient is in the hospital, as a rise and fall of the enzyme can further help diagnose certain conditions.

Several factors can alter the results of a cardiac enzyme study, including other pre-existing heart conditions, certain cholesterol-lowering medications, and heavy alcohol use. Patients with muscular dystrophy and certain autoimmune diseases may also have elevated levels of these enzymes. Recent surgery or cardiopulmonary resuscitation can interfere with the results as well.

In addition to a cardiac enzyme study, physicians may also order a test to measure the levels of myglobin, another protein found in the heart muscle. Additional diagnostic tools, such as an electrocardiography and a physical exam, are typically used along with the cardiac enzyme study. The patient’s symptoms and past medical history are also taken into account.

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