Cardiac enzymes ― also known as cardiac biomarkers ― include myoglobin, troponin and creatine kinase. Historically, lactate dehydrogenase, or LDH, was also used but is non-specific. Cardiac enzymes are released into the circulation when myocardial necrosis occurs, as seen in myocardial infarction.
Myoglobin is released into circulation with any damage to muscle tissue, including myocardial necrosis. Because skeletal muscle contains myoglobin, this measurement is quite nonspecific for MIs. The benefit in myoglobin is that a detectable increase is seen only 30 minutes after injury occurs, unlike in troponin and creatine kinase, which can take between 3 and 4 hours.
The enzymes troponin I and troponin T are normal proteins that are important in the contractile apparatus of the cardiac myocyte. The proteins are released into the circulation between 3 and 4 hours after myocardial infarction and remain detectable for 10 days following. This long half-life allows for the late diagnosis of MI but makes it difficult to detect re-infarction, as can occur in acute stent thrombosis after percutaneous coronary intervention, or PCI. There are a number causes for troponin elevation not related to myocardial infarction; however, troponin elevation is much more sensitive than myoglobin and even creatine kinase.
Creatine kinase (CK)
Creatine kinase ― also known as creatine phosphokinase, or CPK ― is a muscle enzyme that exists as isoenzymes. The MB type is specific to myocardial cells, whereas MM and BB are specific to skeletal muscle and brain tissue, respectively. The CK level increases approximately 3 to 4 hours after MI and remains elevated for 3 to 4 days. This makes it useful for detecting re-infarction in the window of 4 to 10 days after the initial insult; troponin remains elevated for 10 days, making it less useful for this purpose.
1. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine
2. Hurst’s the Heart, 13th Edition