Troponin

Editor-in-Chief: C. Michael Gibson, M.S., M.D. [mailto:mgibson@wikidoc.org]; Associate Editor-In-Chief:

Overview
Troponin is a complex of three proteins that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle. Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic reticulum and release calcium into the sarcoplasm. Some of this calcium attaches to troponin, causing a conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction. Troponin is found in both skeletal muscle and cardiac muscle, but the specific versions of troponin differ between types of muscle. The main difference is that the TnC subunit of troponin in skeletal muscle has four calcium ion binding sites, whereas in cardiac muscle there are only three.

Discussions of troponin often pertain to its functional characteristics and/or to its usefulness as a diagnostic marker for various heart disorders. When cardiac injury occurs (such as in case of an acute MI), these intracellular proteins are then released into the bloodstream. Along with the patient's history and the electrocardiogram, the release of these enzymes forms the basis of the diagnosis of ST elevation myocardial infarction.

Until the 1980s, the enzymes SGOT and LDH were used to assess cardiac injury. In the early 1980s it was found that disproportional elevation of the MB subtype of the enzyme creatine kinase (CK) was very specific for myocardial injury. More recently, troponin sub-units I or T, have been used as an even more specific marker of myonecrosis. The 2007 Joint ESC/ACCF/AHA/WHF Task Force for the definition of myocardial infarction emphasized the importance of both elevated cardiac biomarkers and clinical evidence for MI.

Physiologic Role of Troponins in the Absence of Disease
Both cardiac and skeletal muscles are controlled by changes in the intracellular calcium concentration. When calcium rises, the muscles contract, and when calcium falls the muscles relax.

Troponin is a component of thin filaments (along with actin and tropomyosin), and is the protein to which calcium binds to accomplish this regulation. Troponin has three subunits, TnC, TnI, and TnT. When calcium is bound to specific sites on TnC, tropomyosin rolls out of the way of the actin filament active sites, so that myosin (a molecular motor organized in muscle thick filaments) can attach to the thin filament and produce force and/or movement. In the absence of calcium, tropomyosin interferes with this action of myosin, and therefore muscles remain relaxed.

Troponin I has also been shown to inhibit angiogenesis in vivo and in vitro.

Individual subunits serve different functions:
 * Troponin C binds to calcium ions to produce a conformational change in TnI
 * Troponin T binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex
 * Troponin I binds to actin in thin myofilaments to hold the troponin-tropomyosin complex in place

Diagnostic Use
Two subtypes of troponin (cardiac troponin I and T) are very sensitive and specific indicators of damage to the heart muscle (myocardium). One of the more common uses of troponin is to determine if a patient with chest pain has sustained death of the myocytes (heart muscle cells) as a result of thrombotic (blood clot related) occlusion of a coronary artery which would warrant urgent medical or interventional therapy. The level of troponin is measured in the bloodstream and it is used to differentiate between unstable angina (no elevation of troponin, the myocardium is not irreversibly damaged) versus either non ST elevation MI or ST elevation MI (heart attack) in patients with chest pain. Troponin is a simple yet potent tool for risk stratification. If a patient is troponin positive, and has signs and symptoms of ischemic heart disease (substernal chest pain or pressure, electrocardiographic EKG changes), then an early invasive strategy is warranted. This should be preceded by aggressive antiplatelet and antithrombin therapy (see acute coronary syndromes).

Use of Troponin to "Rule Out" or "Rule In" a Heart Attack
While it is commonly said that a negative troponin "rules out" a non ST elevation MI or ST elevation MI, it should be noted that among those patients who are ultimately found to be troponin positive, approximately 20% will be troponin negative at the time of the initial testing. These patients are sometimes referred to as an "MI in evolution" and form the basis for checking serial or multiple troponins in a monitored setting. A negative troponin on a single test does not exclude the possibility that the patient has ongoing myonecrosis.

On the other hand, a positive troponin does not "rule in" (i.e. does not allow one to conclude) that there is a thrombotic occlusion of the epicardial artery. As shown below, there are a wide variety of other causes for an elevated troponin.

Role of Pre-Test Probability in the Positive Predictive Value of Troponin
If troponin values are checked in a population with a low pre-test probability of disease, then the positive predictive value of the test will drop dramatically. Patients who have a low pre-test probability of an acute coronary syndrome (negative family history, few or absent cardiac risk factors, absent EKG changes, absent ischemic chest pain) do not derive benefit from an early invasive strategy. Identification of an alternate source of the troponin other than an acute coronary syndrome should be sought and the underlying disease treated. Not all troponin positive patients should undergo cardiac catheterization. Indeed, patients with a low pre-test probability of heart disease should not be treated with aggressive antiplatelet therapy, antithrombin therapy, diagnostic catheterization and revascularization.

Absence of Troponin Elevation in the General Population
Among 3,557 individuals in the general population, only 0.7% of patients had a troponin ≥0.01 microg/L, which is > than the 99th percentile of the reference range. The underlying disease state in those individuals with elevated troponins included chronic kidney disease, heart failure, left ventricular hypertrophy and diabetes.

Prognostic Value of an Elevated Troponin in the Absence of Thrombotic Acute Coronary Syndromes
Even in the absence of an acute coronary syndrome, and in the presence of "normal coronary arteries" on diagnostic cardiac catheterization, an elevated troponin is associated with adverse outcomes. There is no data to suggest that aggressive antiplatelet and / or antithrombin strategies improve clinical outcomes in these patients who are often critically ill from non-cardiac conditions. Aspirin is safe to administer to these patients.

1. Injury or Death of Heart Muscle Cells due to Reduced Oxygen Supply
In these scenarios there is reduced blood flow to the heart muscle:
 * a. Epicardial Vessel Occlusion:
 * This is the cause of an acute coronary syndrome and should be the trigger for an early invasive strategy. This form of myonecrosis is classified as a Type I MI in the new universal definition of MI proposed by the 2007 Joint European Society  of Cardiology/American College of Cardiology/American Heart  Association/World Health Federation (ESC/ACCF/AHA/WHF).
 * b. Epicardial vessel spasm
 * c. Microvascular obstruction:
 * Microvascular disease, cocaine abuse, stress cardiomyopathy, subarachnoid hemmorhage, transplant vasculopathy, and hypertrophic  cardiomyopathy may all lead to troponin elevations due to small vessel disease or spasm.
 * d. Hypoperfusion:
 * Heart block and hypotension may lead to hypoperfusion of the myocardium.

2. Injury or Death of the Heart Muscle Cells due to Increased Demands for Oxygen
Classic example of this would the patient with tachycardia and strenuous exercise such as a marathon or the patient with new onset atrial fibrillation and a rapid ventricular response. This form of myonecrosis is classified as a Type II MI in the new universal definition of MI proposed by the 2007 Joint European Society of Cardiology/American College of Cardiology/American Heart Association/World Health Federation (ESC/ACCF/AHA/WHF). Demand can be increased due to tachycardia, increased oxygen consumption and changes in loading conditions. Sepsis is associated with troponin elevations in part due to this mechanism, and in part due to the increased permeability of myocytes during sepsis.

3. Injury or Death of the Myocytes due to Increased Stretch of the Cells
Patients with congestive heart failure, pulmonary hypertension and acute valvular disorders such as aortic insufficiency often have dilated hearts. Both subendocardial ischemia and excess stretch due to the dilation may damage the myocytes. Increase demand due to tachycardia and subendocardial ischemia may play a role as well.

4. Direct Injury of the Heart Muscle Cells

 * a. Inflammation and Infection
 * Classic examples of this would be inflammatory diseases such as pericarditis, myocarditis, Kawasaki's disease and inflitrative diseases such as amyloidosis, hemochromatosis, sarcoidosis,  and scleroderma.
 * b. Trauma to the Heart Muscle Cells
 * Classic examples of this would be ablation, blunt trauma, cardiac surgery, cardioversion, defibrillation, endomyocardial biopsy.

5. Drug toxicity or toxins
Drug toxicity such as high-dose chemotherapy  and compounds such as adriamycin,  5-flurouracil,  herceptin,  snake venom

6. Leakage of Troponin from the Heart Muscle Cells
A classic example is sepsis.

7. Reduced Clearance of Troponin from the Bloodstream
A classic example of this would include renal failure.

"False Positive" Troponin Elevations That Are Not Due to Thrombotic Coronary Occlusion
Again, it should be re-emphasized that while a troponin elevation reflects myocardial injury, thrombotic occlusion of an epicardial coronary artery is just one of many causes of myocardial injury. Non-thrombotic causes of an elevated troponin are often referred to as "false positive" causes of a troponin elevation. Troponin release in the context of coronary thrombosis and vessel occlusion is due to irreversible damage (myocyte necrosis or cell death) with the release of the intracardiac enzymes into the bloodstream as the myocyte's cell membranes break down. However, in the absence of thrombotic occlusion of a coronary artery, troponin can also be released from myocytes in the absence of necrosis or cell death. This release can occur as a result of changes in the permeability of the cell membrane. Sepsis for instance can cause the breakdown of troponin to lower-molecular-weight fragments that can then leak into the bloodstream through a myocyte membrane that is also rendered more porous by sepsis. The fact that patients who survive sepsis do not have an irreversible decline in LV function supports this mechanism as well. Among patients who have an elevated troponin and a normal angiogram, a very small study of 21 patients identified the following as the underlying causes :
 * 47% No clear precipitant
 * 28% Tachycardia
 * 10% Strenuous exercise
 * 10% Pericarditis
 * 5% Congestive heart failure

Non-Thrombotic Cardiac Causes of Troponin Elevation

 * Ablation procedures to treat arrhythmias
 * Amyloidosis and other cardiac infiltrative disorders
 * Aortic dissection
 * Atrial septal defect closure
 * Cardiac contusion
 * Cardiac surgery and heart transplant
 * Cardioversion
 * Defibrillation and defibrillator implantation
 * Congestive heart failure
 * Coronary artery vasospasm
 * Dilated cardiomyopathy
 * Endomyocardial biopsy
 * Heart block
 * Heart failure
 * Hypertrophic cardiomyopathy
 * Hypotension
 * Kawasaki disease. Troponin elevations have been variably associated with Kawasaki's disease and it has been speculated that the troponin elevation may reflect the underlying myositis.
 * Myocarditis
 * Percutaneous coronary intervention
 * Pericarditis
 * Pulmonary hypertension
 * Radiofrequency ablation
 * Stress cardiomyopathy (Apical ballooning syndrome, Takotsubo Cardiomyopathy)
 * Supraventricular tachycardia including atrial fibrillation
 * Transplant vasculopathy

Non-cardiac Causes of Troponin Elevation

 * Burns, especially if it affects >25 percent of body surface area
 * Critical illness, e.g. sepsis
 * Drug toxicity such as high-dose chemotherapy and compounds such as adriamycin, 5-flurouracil, herceptin, snake venom
 * Exercise (e.g. marathon)
 * Hypovolemia
 * Infiltrative disorders like amyloidosis, hemochromatosis, sarcoidosis, and scleroderma
 * Pulmonary embolism
 * Intracranial hemorrhage
 * Pulmonary hypertension
 * Renal failure
 * Respiratory failure
 * Rhabdomylysis with cardiac injury
 * Subarachnoid hemorrhage
 * Scorpion venom
 * Stroke
 * Sympathomimetic ingestion

Technical Aspects
Cardiac troponin T (cTnT) and I (cTnI) are measured by immunoassay methods. A single manufacturer distributes cTnT but a host of diagnostic companies make cTnI methods available on many different immunoassay platforms.

Drug-induced cardiotoxicity is common to all classes of therapeutic drugs. It is essential that cardiotoxicity is detected with a high degree of sensitivity and specificity. The newly developed troponins are especially useful in this context