Cardiac Troponins

Cardiac troponins. What are they, and do they have any relevance to the family physician?

Troponins are found in both skeletal and cardiac muscle where they are involved with actin and myosin in muscle contraction. Troponin T and I are relatively specific for cardiac muscle. They are released during acute myocardial ischemia and can be measured like the CPK-MB.

Several recent studies have shown that troponins are very sensitive and specific markers of myocardial-cell injury and as such can have prognostic significance.

The Global Use of Strategies to Open Occluded Coronary Arteries in Acute Coronary Syndromes GUSTO-IIa trial showed in a prospective study that elevations of the troponin T isoenzyme (above 0.1 mg per milliliter) were associated with significantly higher mortality within 30 days, both in the total study population (11.8 percent vs. 3.9 percent) and in all electrocardiographic subgroups.

The Thrombolysis in Myocardial Ischemia Phase IIIB (TIMI IIIB) study was a retrospective study looking at troponin I elevations. Its findings were similar to the GUSTO -IIa trial. Cardiac troponin I levels of 0.4 mg per milliliter or above were associated with a significantly higher mortality within 42 days than were lower levels (3.7 percent vs. 1.0 percent).

A strong relationship between long term cardiac outcomes and elevation of troponin T was also found in the Fragmin during Instability in Coronary Artery Disease (FRISC) trial. In this study, the risk of death or myocardial infarction during five months of follow-up increased significantly with increasing levels of troponin T measured within the first 24 hours.

It is apparent then, that knowing the troponin level might affect the sort of care the patient receives immediately post MI. Those with elevated levels may require more aggressive treatment than those with low levels. The FRISC study showed that long-term treatment with low-molecular-weight heparin was associated with a 48 percent reduction in 42-day mortality, as compared with a group of 327 patients without troponin T elevations, in whom this treatment had no benefit.

So therein lies the significance of cardiac troponins - as predictors of future cardiac events. There are still some unanswered questions about such things as cost effectiveness, the accuracy of bedside tests, whether or not serial tests are useful in quantifying the size of an infarct and whether serial tests can be used as a measure of the success of reperfusion therapy. Future studies will no doubt help answer some of these questions.

- John Hickey

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