What are cardiac enzymes?
Enzymes are proteins that perform specific tasks in body cells. If the cells are damaged, the enzymes enter the bloodstream and can be measured with a blood test. Blood values determined in the laboratory that indicate cardiac damage are often grouped – scientifically not entirely correctly – under the term “cardiac enzymes”. These then include, for example, hormones and protein building blocks of the heart muscle cells.
Heart enzyme Creatine kinase (CK)
Creatine kinase is an important enzyme of energy metabolism in muscle cells. Depending on the cell type, the enzymes differ slightly; they are referred to as different isoenzymes, which are identified by a letter suffix. The isoenzyme CK-MB can be assigned primarily to the heart and can thus indicate cell damage to the heart muscle.
Therefore, the ratio of the two isoenzymes is particularly significant. If CK-MB and CK-MM are both elevated, this tends to indicate an origin from the skeletal muscles. If, however, the CK-MM is normal, while the cardiac enzyme CK-MB has risen significantly above the upper limit, then cardiac damage is likely.
Cardiac Troponin (cTnI/cTnT).
Troponin is a regulatory protein within a working unit of the muscle cell, the actin filament. It is highly organ specific. Cardiac troponin (cTnl and cTnT) is found only in the heart and is therefore a very sensitive marker. Its concentration in the blood rises measurably even if only one gram of the heart muscle is damaged. The laboratory value is not influenced by injuries or cell damage elsewhere in the body.
Myoglobin
Myoglobin is a muscle protein that plays a crucial role in supplying oxygen to the muscle cell. For cardiac diagnostics, the value, considered in isolation, is very inaccurate, because myoglobin is found in all muscle cells of the body. If muscle cells are damaged, myoglobin leaks out of the cell into the blood. The value rises rapidly after a heart attack or any other muscle damage and is therefore suitable for detecting a heart attack very early.
Aspartate aminotransferase is not actually a specific cardiac enzyme. It is important for sugar metabolism in skeletal muscle cells as well as in the heart muscle. When cells die, AST (GOT) is released and enters the bloodstream in increased quantities.
However, the concentration does not increase until eight to twelve hours after the heart attack. Since the other cardiac enzymes are superior in their informative value, AST determination is no longer significant for diagnosing myocardial infarction.
Lactate dehydrogenase (LDH)
Cardiac hormone BNP
BNP is a hormone whose precursor (proBNP) is produced by the heart muscle cells. If the load on the heart increases, more proBNP is released into the bloodstream to relieve the heart: via increased excretion of sodium and widening of the blood vessels.
When do you determine the cardiac enzymes?
The doctor has the cardiac enzymes determined from a blood sample if he suspects, on the basis of certain symptoms or examination findings, that the patient has suffered a heart attack or is suffering from another serious heart disease (e.g., myocarditis or cardiac insufficiency). Typical signs of heart disease include:
- Weakness and shortness of breath during physical exertion or even at rest.
- Cold sweat
- Anxiety
- Paleness or bluish discoloration of the skin and lips
- changes in the ECG (electrocardiogram)
Cardiac enzymes: reference values
To interpret the cardiac enzyme values, the physician must compare the measured values with a table of standard values, the so-called reference values. Here you will find an overview of the most important values and their significance for cardiac diagnostics!
|
Cardiac enzyme |
Reference value |
Meaning |
|
CK-MB |
0 – 25 U/l or < 6 % of total CK |
Elevated in: Myocardial infarction or myocarditis. |
|
Troponin (cTnT/cTnI) |
< 0.4 µg/l |
|
|
AST |
Men: 10 – 50 U/l Women: 10 – 35 U/l |
Follow-up for myocardial damage, but also for liver/gallbladder diseases |
|
Men: 135 – 225 U/l Women: 135 – 215 U/l |
unspecific, suitable for follow-up of myocardial infarction |
|
|
NT-pro BNP |
Age-, sex-, and laboratory-dependent: Men < 50 yrs: < 84 pg/ml Men 50 – 65 y.: <194 pg/ml Women < 50 y.: < 155 pg/ml Women 50 – 65 y.: < 222 pg/ml |
Elevated in heart failure and prolonged left ventricular pressure load |
Cardiac enzymes only have medical significance if they are elevated. In patients with a healthy heart, they are present in the blood only in low concentrations.
When are cardiac enzymes elevated?
Increased concentrations of cardiac enzymes (or other hormones and proteins of the heart) indicate damage or overload of the heart muscle tissue. These occur primarily in the following diseases or injuries:
- Heart attack
- @ inflammation of the heart muscle (myocarditis)
- Crushing of the heart muscle (cardiac contusion)
- Coronary artery disease
What to do in case of altered cardiac enzyme values?
If the cardiac enzymes are conspicuously elevated during a routine checkup, the cause must always be found. To do this, the doctor will perform further tests on you (e.g. ECG, echocardiogram, cardiac catheterization or MRI).