In 2000, the European Cardiology Community and the American College of Cardiology agreed on the diagnosis of myocardial infarction. To verify the diagnosis, they decided to use an analysis of the level of cardiac troponins, since it is considered more specific in the diagnosis of damage to the heart muscle than creatine phosphokinase, even in cases of minor myocardial necrosis.
Troponins
Troponin is a protein molecule consisting of three subunits (T, C, 1). It is located on actin threads in the muscles of the myocardium and is involved in its contractions. Each subunit performs its specific role in regulating the activity of the heart muscle. In the body, for the most part, bound troponin predominates. The norm in the blood (ng / ml) is 2.0, because only six percent of these proteins are freely in the cytosol.
It was found that with the destruction of cardiomyocytes in the peripheral blood, the level of different fractions of troponins increases. Sometimes this indicator is hundreds or even thousands of times higher than normal.
Function
As mentioned above, troponin binds to the tropomyosin protein. In an inactive state, when the muscle is relaxed, this protein prevents the attachment of myosin to actin. When the action potential reaches the muscle cell, positively charged ions enter the cell cytoplasm through the calcium channels. Part of the calcium binds to troponin and binds it. This allows myosin to reach the actin filaments and cause contraction.
Not only in the myocardium is troponin. The rate of binding is ninety-five percent. The rest is in a free state or distributed in other types of muscle tissue. The key difference is that troponin has four subunits in skeletal muscle, and three in the cardiac muscle.
Physiology
Contraction of all types of muscles is coordinated by changing the intracellular concentration of calcium ions. When their number rises, the muscle contracts, and vice versa. The protein that calcium binds to in the cell is troponin. The norm of its location on tropomyosin corresponds to the length of the latter, namely 40 nanometers.
Troponin has three subunits, each of which has its own functions. When calcium attaches to the C moiety, tropomyosin moves along the actin filament so that myosin can attach to it. If calcium is insufficient, tropomyosin blocks the active centers and prevents myosin from attaching to myofilaments.
Microbiologists and histologists have also found that troponin 1 inhibits the formation of new vessels. Troponin C binds calcium ions, and this causes changes in troponin 1. Troponin T is necessary in order to bind to tropomyosin, and troponin 1 binds to actin, and also helps to actin interact with myosin when calcium ions in the cytoplasm are insufficient.
Preparation and indications for the appointment of analysis
The level of troponin and its norm in peripheral blood is best judged by a quantitative analysis. Some specific preparation is not required for its delivery, but, as in all cases of blood donation, it is better to do this four hours after a meal.
If a patient enters a hospital in an acute period of myocardial infarction, he immediately takes blood for analysis. According to the protocol, it is supposed to be repeated after 4 hours, and then after 8. After a day, troponin is determined once a day for two weeks. In modern medicine, methods have been developed to determine the amount of this protein directly at the patient’s bed using test systems. A negative result makes it possible to exclude such a patient from persons who may develop acute coronary syndrome in the near future.
In addition to determining acute myocardial infarction (MI), an analysis of the amount of troponin is done to identify the subacute form of myocardial infarction, microinfarction, mute heart attack, as well as monitor the quality of thrombolytic therapy and the effect of direct-acting anticoagulants, such as heparin.
Increased troponin concentration 1
Troponin 1 (normal in serum to 0.1 ng / ml) may increase after surgery on the heart. So, when transplanting this organ, the troponin concentration increases to 5 nanograms per milliliter. This value can last up to three months.
Inflammatory diseases and heart injuries cause a slight increase in the level of troponin T, but the dynamics of its changes in the blood differs from that in MI. The rate of troponin in the blood is the same for men and women. This indicator is not related to the gender or age of the patient. Sometimes there may be false positive test results if the patient is in a state of acute alcohol intoxication.
A slight increase in troponin is possible with massive skeletal muscle damage. For comparison: creatine phosphokinase (CPK) in these patients increases significantly, which leads to erroneous registration of myocardial infarction.
Acute myocardial infarction
Troponin 1 (the norm in the blood is up to 0.1 ng / ml) is a pathognomonic sign of acute myocardial infarction only if its value exceeds 100 ng / ml, or more than 99 percentile of normal values. But only in combination with clinical and instrumental symptoms of myocardial ischemia.
As a result of introducing this analysis into medical practice, the number of diagnosed heart attacks has increased, since this test is more sensitive than CPK. In addition, in the prognostic plan, elevated troponin levels are a negative sign. Some researchers claim that the size of a blood clot is directly proportional to the level of troponin in the blood serum.
An increase in the level of this protein in patients with a heart attack is recorded within six hours after the onset of a pain attack. After fourteen days, the level of troponin progressively decreases and returns to normal.
Acute coronary syndrome
The norm of troponin in the blood in acute coronary syndrome (ACS) corresponds to that for myocardial infarction and ranges from 50 to 100 ng / ml. An increase in this protein allows us to differentiate ACS from unstable angina. Sometimes in patients with characteristic clinical symptoms and elevated troponin levels, a serious hemodynamic disorder is not recorded on coronarography. The reason for this phenomenon is the spontaneous formation of a plaque that dissolves during treatment with thrombolytics. The second theory claims that a blood clot just sinks lower in the vessels and is not visible on the screen.
Percutaneous coronary interventions
The level of troponin (normal to 2.0 ng / ml) increases slightly after angioplasty or stent placement. Scientists no longer had doubts that the proteins that are being determined came from cardiomyocytes. The reason for their appearance is temporary, passing ischemia, which is caused by ballooning and compression of the coronary arteries.
Even in the case of myocardial microtraumas, troponins were extremely sensitive, which, of course, was reflected in the peripheral blood test. But, among other things, this indicates that patients with similar changes in protein levels are at high risk of developing coronary syndrome and myocardial infarction. Unfortunately, there is no specific figure that doctors could orient themselves in their predictions. Some pathophysiologists suggest talking about myocardial infarction in cases where the level of troponin is exceeded more than three times.
Surgery
Troponin (normal to 2.0 ng / ml) rises after radical surgery on the heart muscle. This is due to the disadvantages of cardioprotective measures. In theory, they should protect the patient's body from hypoxia, but in practice this is not always possible one hundred percent. But not in all cases, an increase in troponins should be regarded as perioperative myocardial infarction.
The volume and duration of the operation, as well as the initial level of this protein, should be taken into account. The diagnosis of myocardial infarction is possible only by the presence of clinical, laboratory and instrumental symptoms at the same time.
Other conditions
Numerous studies have confirmed the fact that troponin (norm 2.0 ng / ml) is a highly specific marker of myocardial infarction. But there are a number of pathological conditions that are also accompanied by an increase in the amount of this protein in peripheral blood. These include sepsis, pulmonary embolism (pulmonary embolism), heart failure, inflammation of the heart bag, myocarditis, chronic renal failure, excessive exercise, heart transplantation, chemotherapy.
This phenomenon is caused by fever, tachycardia, hypoxia, impaired microvasculature, decreased pressure and anemia. All this leads to myocardial ischemia and the destruction of cardiomyocytes, which leads to an increase in troponin.
Troponin 1 or T
The norm of troponin in the blood is up to 2.0 ng / ml, but today there are methods for determining different fractions of troponin. This allows you to more accurately determine myocardial damage. Over the course of a number of studies, it turned out that troponin-T is more sensitive to renal failure, especially troponin 1, but in other parameters they are similar, therefore they have the same diagnostic value.
The peculiarity of troponin T is also that only one company produces a diagnosticum for it, while for troponin 1 there are several test systems. Because of this, some difficulties may arise in comparing the results. But clinicians are trying to deduce a single formula with which one could neutralize the difference in parameters among the results of different laboratories.