Cardiac markers are the substances released from the heart muscle when it is damaged as a result of myocardial infarction more commonly known as a heart attack.
Human heart like any other organ of the human body is made up of cells. These cells are also prone to damage like any other cell. When the heart is damaged, the cell membrane is ruptured and the contents of the heart muscle cells leak out into the surrounding areas, the most important and clinical one being the blood. Since blood can be analyzed via drainage from the superficial veins, the levels of the leaked substances can provide information regarding damage to the heart itself. The most important substances that are taken into account while performing blood tests regarding the cardiac markers include the following:
- Cardiac Troponin
- Creatine Kinase (CK)
- Lactate Dehydrogenase (LDH)
- Myoglobin (Mb)
Cardiac markers , although, are now central to the definition of Acute Myoglobin Infarction (AMI), however they are not useful in the diagnosis of myocardial infarction in the acute phase because depending upon the marker, it can take about 2-24 hours for the level to increase in blood and the fact that the blood tests themselves require substantial time for performing properly.
Troponin is released during MI from the cytosolic pool of the myocytes. Its subsequent release is prolonged with degradation of actin and myosin filaments. Because it has increased specificity compared with CK-MB, troponin is a superior marker for myocardial injury. Troponins can also calculate infarct size but the peak must be measured in the 3rd day.
CK resides in the cytosol and facilitates the movement of phosphate into and out of the mitochondria. It is present in large number of tissues however because of this fact it is less specific than troponin. Its levels also rise in muscle diseases and exercise. However it is relatively specific when skeletal muscle injury is not present.
Lactate dehydrogenase catalyses the conversion of pyruvate to lactate. LDH-1 isozyme is normally found in the heart muscle and LDH-2 is found predominately in blood serum. A high LDH-1 level to LDH-2 suggest MI. LH is not as specific as troponin. LDH levels are also high in tissue breakdown or hemolysis. It can mean cancer, meningitis, encephalitis, or HIV.
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. 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.