Heart enzymes

and Eva Rudolf-Müller, doctor

Jens Richter is editor-in-chief at Since July 2020, the doctor and journalist has also been responsible as COO for business operations and the strategic development of

What are heart enzymes?

Enzymes are proteins that perform special 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 heart damage are often - scientifically incorrect - summarized under the term "heart enzymes". This also includes, for example, hormones and protein building blocks from heart muscle cells.

This fuzziness in the definition has no meaning for clinical use. It is much more important that the heart enzymes can identify heart damage as quickly and precisely as possible, in the case of a heart attack, for example, often shortly after the event. The heart enzymes mainly include the following substances:

Heart enzyme creatine kinase (CK)

Creatine kinase is an important enzyme in the energy metabolism in muscle cells. Depending on the cell type, the enzymes differ slightly, one speaks here of different isoenzymes, which are identified by a letter addition. The isoenzyme CK-MB can primarily be assigned to the heart and can therefore indicate cell damage in the heart muscle.

However, about three percent of the cardiac enzyme CK-MB is also found in the skeletal muscles, so that the CK-MB can also be increased due to intense muscle exertion or after muscle injuries (for example after a fall). It is therefore important to determine the CK-MM enzyme, which is primarily associated with the skeletal muscles, in addition to the CK-MB.

The ratio of the two isoenzymes is therefore of particular significance. If CK-MB and CK-MM are both elevated, this suggests that they come 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 heart damage is likely.

A problem with the CK-MB for its suitability as a heart attack indicator is that the heart enzyme only increases characteristically four hours after the event. This means that a very recent infarction can be overlooked if the CK-MB is determined exclusively. Another advantage is that the doctor can use the CK-MB to estimate the extent of the affected damage to the heart, since the blood concentration of the CK-MB correlates with the size of the heart muscle damage.

Cardial Troponin (cTnI / cTnT)

Troponin is a regulator protein within a working unit of the muscle cell, the actin filament. It is very organ specific. Cardiac troponin (cTnl and cTnT) only occurs in the heart and is therefore a very sensitive marker. Its concentration in the blood increases measurably if only one gram of the heart muscle is damaged. The laboratory value is not affected by injury or cell damage elsewhere in the body.

However, an increase in troponin is only detectable just under four hours after a heart attack. This means that troponin is not suitable for early diagnosis of myocardial infarction. Since it still leaks from the damaged cells after several days, the blood count can be elevated even one to two weeks after a heart attack. In the case of severe kidney dysfunction, the troponin level may be increased, even without a heart attack, as it accumulates in the blood and cannot be excreted.

Myoglobin

Myoglobin is a muscle protein that is of crucial importance for the oxygen supply to the muscle cells. Viewed in isolation, the value is very imprecise for heart diagnostics, because myoglobin occurs in all muscle cells in the body. When muscle cells are damaged, myoglobin escapes from the cell into the blood. The value rises quickly after a heart attack or any other muscle damage and is therefore suitable for detecting a heart attack very early.

Aspartate aminotransferase (AST / GOT)

The aspartate aminotransferase is actually not a specific cardiac enzyme. It is important for the sugar metabolism in the skeletal muscle cells as well as in the heart muscle. If cells die, AST (GOT) is released and more of it gets into the blood.

However, the concentration only rises eight to twelve hours after the heart attack. Since the other cardiac enzymes are superior in their expressiveness, the AST determination is no longer relevant for the diagnosis of heart attack today.

Lactate dehydrogenase (LDH)

Strictly speaking, lactate dehydrogenase is also not a heart enzyme, but an important protein in the energy metabolism of muscles and organs. It is an enzyme that is found in all cells of the body. So it is not specific for the heart and therefore does not play a role in acute heart attack diagnosis. But it can be used to estimate the size of an infarct.

Heart hormone BNP

BNP is a hormone whose precursor (proBNP) is produced by the heart muscle cells. When the workload on the heart increases, more proBNP is released into the bloodstream in order to relieve the heart: via increased excretion of sodium and widening of the blood vessels.

For this, the proBNP is split in equal parts into BNP that is active in the circulation and biologically inactive NT-proBNP. The latter can be proven particularly well and thus serves the doctor as an indicator of the stress on the heart - for example in the case of cardiac insufficiency. BNP is therefore suitable both for diagnosis and for monitoring the progress of cardiac insufficiency.

When do you determine the heart enzymes?

The doctor will have the heart enzymes determined from a blood sample if, based on certain symptoms or examination results, he suspects that the patient has suffered a heart attack or has another serious heart disease (e.g. an inflammation of the myocardium or a weak heart). Typical signs of heart disease are for example:

Weakness and shortness of breath during exercise or even at rest
sudden, severe pain in the chest and / or shoulder area or in the upper abdomen
Cold perspiration
anxiety
The skin and lips are pale or bluish
Changes in the EKG (electrocardiogram)

Since certain cardiac enzymes are elevated for a certain time after a heart attack or in the case of a long-standing heart disease, some values are suitable for so-called progress monitoring. The doctor can tell from the characteristic pattern of the laboratory parameters whether a diseased or damaged heart muscle is healing or whether a drug treatment (e.g. for cardiac insufficiency) is working well.

Heart enzymes: reference values

To interpret the cardiac enzyme values, the doctor must compare the measured values with a table of normal values, the so-called reference values. Here you will find an overview of the most important values and their informative value for heart diagnostics!

Cardiac enzyme	Reference value	meaning
CK-MB	0 - 25 U / l or <6% of the total CK	Increased in: myocardial infarction or myocarditis
Troponin (cTnT / cTnI)	<0.4 µg / l	Acute diagnostics: myocardial infarction (> 2.3 µg / l) and myocarditis (0.4-2.3 µg / l)
BRANCH	Men: 10 - 50 U / l Women: 10 - 35 U / l	Follow-up control of heart muscle damage, but also of liver / biliary tract diseases
LDH	Men: 135 - 225 U / l Women: 135 - 215 U / l	unspecific, suitable for monitoring the progress of a heart attack
NT-pro BNP	Age, gender and laboratory dependent: Men <50 years: <84 pg / ml Men 50 - 65 years: <194 pg / ml Women <50 years: <155 pg / ml Women 50 - 65 years: <222 pg / ml	Increased in the case of cardiac insufficiency and prolonged pressure load on the left ventricle

When are the heart enzymes lowered?

Cardiac enzymes are of medical importance only when they are elevated. In patients with a healthy heart, they are only found in low concentrations in the blood.

When are the heart enzymes elevated?

Increased concentrations of cardiac enzymes (or other hormones and proteins of the heart) indicate damage or overloading of the heart muscle tissue. These mainly occur with the following diseases or injuries:

Heart attack
Inflammation of the heart muscle (myocarditis)
Heart failure (heart failure)
Bruising of the heart muscle (heart contusion)
Coronary heart disease

What to do if the cardiac enzyme values change?

If the cardiac enzymes are noticeably elevated during a routine check, the cause must always be found. For this purpose, the doctor will carry out further examinations (e.g. ECG, echocardiogram, cardiac catheter or MRI).

Heart enzymes are also specifically determined to check and confirm a suspected disease that has arisen from typical symptoms or test results.

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