43- Troponin and the Sarcomeres

KYJ-43 Troponin.
Let's zoom down deep
Into a muscle cell.

The muscle cell of Striated muscle (striped muscle) has molecular structures called Sarcomeres.
These are strands of proteins wrapped around each other that physically shorten in length when electrically stimulated.   The two primary filaments are called Actin  (thin filaments) and Myocin (thick filaments).

These Sarcomeres may number in the thousands inside each muscle cell.

Binding these Sarcomeres together is a protein glue called Troponin.  Think of it as cheese melted on a pizza gooing all the other stuff together.

There are three types of troponin. They are called T, C and I.  All striped muscle including cardiac muscle and skeletal muscle has Troponin C.  But types of Troponin I and T are specific to cardiac muscle cells.

If a heart muscle cell is injured or damaged, this troponin leaks out of the damaged cell, and stains the interstitial fluid surrounding the cell.  It leaches into the lymphatic system which drains eventually into the blood stream.

If detected in the blood (on blood tests) it is a direct indicator that cardiac muscle injury/ damage has occurred.

Normal blood troponin is functionally zero- ie, troponin is an intramuscular substance, not an intravascular substance.  That said, a tolerance of less than 0.04 mcg/L is considered ok.

If Troponin I&T are elevated >0.04 then this is highly suggestive of cardiac damage, and one of three diagnostic criteria for myocardial infarction. The other two criteria being ST elevation on ECG, and or a pain history that sounds cardiac typical.

Troponin can take 4-6 hours to reach the blood, and up to 12 hours to peak during a cardiac episode.  Once elevated, can be present in blood for 5-14 days.

The I-Stat point of care blood test is a common method to test for troponin at the bedside, and in around 10 mins can support a diagnosis of MI.

For those with an unclear ECG based diagnosis, the patient can be admitted onto a ward for serial enzymes at 6 and 12 hours or 4 and 9 hours post onset of pain.  

Because of troponin's ability to linger in the blood, it is a handy test for those patients that state they had chest pain yesterday or last week.

It is, to date, our most sensitive blood test for MI diagnosis.