Knowledge of cardiac enzymes when used appropriately saves lives. These enzymes, also called cardiac markers, are substances found either exclusively or predominantly in the heart. When the heart is damaged or in distress, such as during or after a heart attack, these enzymes may leak into the bloodstream and allow for prompt diagnosis, treatment and better outcomes of cardiac abnormalities.
Troponins
Troponins are proteins found in muscle. Troponin I is more cardiac specific and is currently the gold standard for diagnosing cardiac muscle damage. Normal levels are usually less than 0.4 ng/mL. It is very useful in diagnosis because blood levels increase within four hours of a heart attack, peak after 12 hours and remain detectable for as long as seven days. Troponins may also be increased in heart failure and inflammation or infection of the heart muscle.
Creatine Kinase
Abbreviated as CK-MB, creatine kinase is an indicator of cardiac muscle damage. Levels begin to increase about four to eight hours after a heart attack, peak within 12 to 24 hours and usually return to normal within two to three days. This makes the enzyme useful in detecting another heart attack within seven days of an initial attack, when blood troponin levels are still present. Normal levels are between 0 and 3 ng/mL.
Lactate Dehydrogenase
LDH-1 is a form of the enzyme lactate dehydrogenase, which is specific for the heart. Levels increase about 10 to 12 hours after a heart attack and may remain in the blood for 10 to 14 days. Another form, LDH-2, usually stays constant during a cardiac condition, therefore the LDH-1:LDH-2 ratio is also an important index of cardiac damage. LDH-1 levels may also increase in conditions that damage red blood cells.
Other Markers
Aspartate transaminase levels, also called AST, increase relatively early in acute heart injury. However, AST is also increased in conditions that damage the liver. Levels of a protein called myoglobin may also rise in cardiac muscle injury. However, this protein is present in other muscle and is not specific for heart problems. Brain natriuretic peptide levels usually increase in patients with heart failure. A new enzyme, glycogen phosphorylase isoenzyme BB, made by the heart tissue, may be useful in early diagnosis of heart muscle death. These cardiac enzymes serve as ancillaries to the diagnosis of acute cardiac conditions.
References
- The University of Utah Eccles Health Sciences Library: Myocardial Infarction
- "Molecular and Cellular Biochemistry"; Glycogen phosphorylase Isoenzyme BB in Diagnosis of Myocardial Ischaemic Injury and Infarction; Ernst-Georg Krause, et al.; 1996
- Quest Diagnostics Patient Health Library: Cardiac Enzyme Studies
