D3 Cardiac markers

Topic

Cardiac markers are substances measured in the blood to assess two main processes: direct damage to the heart muscle—evaluated via troponins and the cardiac fraction of creatine kinase (CK-MB)—and ventricular wall stress caused by increases in pressure or volume—evaluated via natriuretic peptides.

Cardiac troponins I and T are structural components of heart muscle fibers and normally remain within the cardiomyocytes. When these cells undergo irreversible necrosis, troponin is released into the bloodstream. High-sensitivity assays allow for the detection of extremely low concentrations, using a reference limit corresponding roughly to the 99th percentile of the healthy population; exceeding this limit indicates myocardial damage.

Following a heart attack, troponin levels follow a characteristic pattern. Initially, levels rise within one to three hours due to the release of the cytoplasmic fraction; they subsequently continue to rise as troponin bound to the cellular structure is released, peaking between approximately 12 and 24 hours. Finally, levels decline over several days, taking anywhere from five to ten days to return to normal. This rapid initial rise enables the use of protocols that compare the concentration at admission with a second measurement taken one hour later. Both an elevated initial value and a significant change during that interval assist in the rapid diagnosis or exclusion of a heart attack.

Natriuretic peptides operate via a different mechanism. They are actively synthesized and released by cardiomyocytes when ventricular wall tension increases due to elevated pressure or filling volume. Their concentration tends to rise with the severity of ventricular dysfunction, making them important biochemical markers for heart failure.

The values ​​used to interpret them depend on age. The reference text indicates approximate values ​​of 450 pg/L for individuals under 50 years of age, 900 pg/L for those between 50 and 75, and 1,800 pg/L for those over 75. Part of this difference is linked to changes in renal clearance; consequently, the same concentration may carry different clinical significance depending on the patient's characteristics.

Prior to the introduction of troponins, CK-MB was the primary marker for cardiac injury. While it is more heart-specific than total creatine kinase—which also rises with skeletal muscle injury—it is less sensitive than high-sensitivity troponins for detecting minor myocardial injury. Nevertheless, it remains useful in specific situations.

One such instance is the detection of a second infarction while troponin levels remain elevated from the first event. Since CK-MB clears from the blood much more rapidly, a new rise in levels can indicate fresh necrosis. It can also provide valuable information during cardiac surgery, where direct manipulation of the heart causes troponin elevations that make it difficult to distinguish expected damage from a complication.

The diagnostic performance of these markers can be assessed using curves that compare—across various cutoff points—the proportion of affected patients correctly identified against the proportion of healthy individuals erroneously classified as ill. The area under this curve summarizes discriminative capacity: a maximum value represents perfect discrimination, while a value near 0.5 corresponds to classification no better than chance. According to the text, high-sensitivity troponin achieves an area exceeding 0.97 for infarction.

Marker results must be interpreted in conjunction with pre-test clinical probability. Likelihood ratios derived from positive or negative results allow for the adjustment of this probability using Bayesian reasoning, yielding a patient-specific post-test probability of infarction. Taken together, troponins identify myocardial damage and its temporal evolution, natriuretic peptides reflect ventricular stress, and CK-MB remains useful in specific situations such as reinfarction. The combination of these markers, their changes over time, and the pre-test clinical probability allows for a more accurate assessment of whether acute cardiac damage or ventricular overload is present, or if neither process is occurring.

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