Cardiac enzymes (cardiac markers) are substances that are present in higher concentration within the myocardial cells. Their concentration in blood is very low, but it can increase during myocardial damage, especially when it is accompanied with necrosis of myocardial cells. Such situation leads to leakage of the cardiac enzymes out of the myocardial cells and elevation of their concentration in blood.
The elevation of cardiac markers helps us to confirm damage of the heart muscle and to evaluate its severity. In addition, further changes of the concentration may inform us, whether the situation improves or deteriorates. In a patient with chest pain, the symptoms, ECG and cardiac enzymes are the fundamental pillars that determine the diagnosis of myocardial infarction and play an important role in decision of acute coronary angiography.
However, it is necessary to take into account that there are other causes of elevation of particular cardiac enzymes that are not related to myocardial damage. In addition, in case of myocardial damage, the elevation of cardiac markers occurs with a certain time delay.
Causes of elevation
The typical reason of the heart damage, which causes elevation of cardiac enzymes, is the ischemia (lack of oxygen). This ischemia may be silent, or manifest as angina pectoris (negative or only slightly elevated cardiac enzymes), or as a heart attack (significantly elevated cardiac enzymes). Other causes of elevation of cardiac enzymes include myocarditis (inflammation of the heart muscle), pericarditis (inflammation of the pericardium), certain arrhythmias and heart failure. Certain cardiac enzymes may, however, react to non-cardiac pathologies such as creatine kinase to damage of muscles. Certain increase in serum level of cardiac enzymes is usually found in patients with chronic renal failure, because the enzymes can not be properly excreted and increasingly accumulate.
Basic cardiac enzymes
Troponin is probably the most rewarding and the most widely used cardiac enzyme. It is quite reliable and relatively accurately points to damage of the heart muscle. It increases within few hours after the damage, usually no later than in 6 hours. For this reason, we should be cautious in a patient with chest pain lasting less than six hours and normal troponin level and repeat the check once more after 4-6 hours. On the other hand, the troponin stays elevated for a relatively long period (for about two weeks), which can help to confirm a myocardial damage that has occurred recently.
Creatine kinase (CK)
Creatine kinase is an interesting enzyme, which is located in the heart muscle, skeletal muscles and in brain cells. Its concentration after myocardial damage grows relatively quickly, but it is non-specific. Creatine kinase level also increases in damage of non-cardiac muscles (rhabdomyolysis). Therefore, we try to detect a subtype of CK, which is more specific for myocardial cells. This subtype is abbreviated as CK-MB.
Lactate dehydrogenase (LD)
Lactate dehydrogenase is an enzyme, which is located in virtually all cells of the body. Its increase corresponds to a massive damage of the body cells. The concentration of the LD after the heart attack increases relatively slowly (from several dozens of hours to days) and it is very nonspecific. Therefore, it is usually not used in the diagnosis of cardiac damage.
Aspartate transaminase (AST)
This enzyme is located in myocardial and liver cells. It is a valuable marker of liver damage and its importance in diagnosis of cardiac events is minimal. However, an acute chest pain with elevated AST should be considered as potentially serious.
Myoglobin is a substance similar to hemoglobin. Myoglobin is present in all muscle cells including the myocardial cells and this is certain similarity with the CK. Unlike the creatine kinase, myoglobin has one big advantage. Its elevation is the fastest, occurring within 2-4 hours after the myocardial injury.
What is the time period between the heart damage and reactive elevation of the particular cardiac enzyme?
- Troponin - It elevates in 4-6 hours and the peak occurs in 24 hours after the injury.
- Creatine kinase - It elevates in 4-6 hours and the peak level occurs in 24 hours after the injury.
- Lactate - It elevates in 8-12 hours and the peak level is in 3 days after the injury.
- Aspartate transaminase – It elevates in about 4-6 hours after the injury.
- Myoglobin - It elevates in 2-4 hours and the peak is in about 6-12 hours after the injury.