Enzymes are proteins present in cells universally which act as catalysts in various biochemical reactions. Damage to cells of whatever cause may result in enzyme release into the blood stream.

When

Cardiac or heart muscle enzymes are frequently requested in patients with chest pain who are suspected of having had a heart attack or myocardial infarct.

How

Gloves are worn during this procedure for the protection of the blood taker. A tourniquet is placed around the arm to enlarge the veins and a sterile needle inserted into the vein to draw off the appropriate amount of blood. The tourniquet is released and the needle withdrawn. Pressure is applied for a few minutes to the site. 3 lots of abnormal cardiac enzymes are required to make a diagnosis of myocardial ischaemia

Side effects

Although it is the most commonly performed invasive procedure in the UK it can promote much anxiety. To help avoid bruising the tourniquet is released prior to withdrawing the needle. Firm pressure is applied to the site after withdrawing the needle for at least 2 minutes. Keep the arm straight and advise the client not to bend their elbow.

Accuracy

Technical accuracy is high

Interpretation

Appropriate interpretation in relation to clinical history chest pain and other symptoms is required. The main cardiac enzymes are creatinine kinase (CK) MB isoenzyme, troponin (T), aspartate transaminase (AST) and alpha-hydroxybutyrate dehydrogenase (HBD).

Each enzyme shows an increase in activity which appears at slightly different times following the event and their activities remain elevated for different lengths of time. Therefore estimation of the activity of these enzymes over a 2-3 day period following myocardial infarction is mandated. AST is not specific for heart muscle damage.

CK levels rise early and disappear most rapidly whilst HBD shows a more delayed rise and fall. The latter is therefore more useful for late diagnosis.