Triage^® Cardiac Panel: Cardiac Markers

When the heart muscle is damaged, cell walls break down, permitting certain proteins and enzymes, normally contained within the cells, to leak into the blood stream. These substances, called cardiac markers, can then be detected in blood samples with specialized immunoassays. The most common cardiac markers used in the evaluation of chest pain and acute myocardial infarction are CK-MB, Myoglobin and Troponin I.

CK-MB

CK-MB is most widely used in hospitals to detect myocardial damage due to its relative cardiospecificity; its primary value lies in its familiarity to clinicians as it has been available for quite some time. However, CK-MB is not an ideal marker to use alone because its levels do not rise early enough to make a rapid diagnosis and it may be elevated in other conditions. CK-MB levels typically exceed the upper limit of normal within three to eight hours of the onset of acute myocardial infarction, peaking within 10 to 24 hours, and returning to normal within two to three days. Additionally, CK-MB elevations occur as a result of chronic muscle disease and exercise.

Myoglobin

Myoglobin is a protein that transports oxygen in muscle tissue, including the myocardium and skeletal muscle. Myoglobin is also present in smooth muscle. Following injury to any of these muscles, it appears in the blood more rapidly than any other marker. As early as one hour following the onset of chest pain, when CK-MB levels are still in the range of normal, myoglobin levels may already be elevated. This rapid appearance is due to myoglobin’s location in the cell and its low molecular weight.

Myoglobin exhibits high clinical sensitivity for acute myocardial infarction but poor specificity. Every patient with acute myocardial infarction will have elevated myoglobin in the early hours following the onset of chest pain; however, myoglobin elevations may also be indicative of skeletal muscle injury.

The American College of Cardiology/American Heart Association (ACC/AHA) guidelines for the management of patients with unstable angina and non-ST-segment elevation myocardial infarction, recommend the use of myoglobin, a highly sensitive marker, within the first four to eight hours after onset in ruling out myocardial necrosis.

Troponin I

The troponin complex, found on the thin filament of the muscle contractile apparatus, consists of three protein sub-units: troponin I, troponin T and troponin C. Cardiac specific troponin I is used as an aid in the diagnosis of myocardial infarction since it becomes elevated in the blood approximately four to eight hours following myocardial injury or necrosis and remains elevated for several days. In addition to its utility in diagnosis, elevated troponin I levels convey prognostic information and have been shown to identify patients having an increased risk of death.

According to the ACC/AHA guidelines, a cardiac-specific troponin (such as troponin I) is the preferred marker for diagnosing cardiac injury.⁴ However, troponin I does have some disadvantages as well, including low sensitivity in the early phase of myocardial infarction (< six hours after symptoms onset) and its limited ability to detect late minor reinfarction.

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