Biochemical markers

Estimation of enzymes released from necrotic myocardial cells has previously been used as confirmatory evidence of infarction, rather than to make the diagnosis on presentation, since the evolution of enzyme release is as important as the actual values recorded. Typically, in a patient seen within a few hours of infarction onset, the values rise and then fall over hours or days.

Most laboratories record serum glutamine oxaloacetic transaminase (SGOT) or serum aspartate aminotransferase (AST) and creatine kinase (CK). The CK-MB form is more useful as it is specific to heart muscle. These enzymes are usually present by 6 h from onset, peak at 24 h, and fall to normal by 3 to 4 days. The height of the peak (which may be missed by infrequent sampling) is a good guide to the amount of myocardial necrosis. Serum lactate dehydrogenase is released more slowly and persists for longer (2-4 days). The CK enzymes can be fractionated further, and so it is possible to date the time of infarction onset by comparing the ratio of MM to MB subforms.

Other more sophisticated enzyme tests measure serum myoglobin or troponin. These smaller molecules are released much earlier and can be used to diagnose infarction on admission when the ECG is equivocal. Bedside kits are now available. These may help to make a diagnosis of myocardial necrosis, particularly if the ECG is unhelpful because of previous infarctions or bundle branch block. However, these methods still involve some delay which may hinder proper administration of thrombolytic treatment. In practice, they have not yet proved clearly superior to clinical evaluation by the more traditional methods outlined above, except in the case of small infarctions when the value of reperfusion is less clear. Nevertheless, they may be useful as markers of early reperfusion or, more importantly, as markers of failure to reperfuse in the first 60 to 90 min, and so act as pointers for further intervention. Markers arising solely from the myocardium, such as the CK-MB and troponins, are quite specific for myocardial damage, whereas myoglobin and lactate dehydrogenase are non-specific and can be released, for example, by intramuscular injections and liver damage respectively. The sensitivity of these markers varies markedly, mainly depending on the appropriate timing of blood sampling. Troponins and myoglobin are released during the first few hours, AST and CK are released more slowly, and lactate dehydrogenase is somewhat delayed over 24 to 72 h. The sensitivity of these assays also depends on how frequently blood is sampled; once-daily sampling may miss the peak level.

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