The healthy functioning of the heart is dependent upon the availability of oxygen. This oxygen availability may be compromised by the slow deposition of cholesterol-rich atheromatous plaques in the coronary arteries. As these deposits increase, a point is reached at which the oxygen supply cannot be met at times of peak demand, for example at times of strenuous exercise. As a consequence, the heart becomes temporarily ischaemic (‘lacking in oxygen’) and the person experiences severe chest pain, a condition known as angina pectoris (‘angina of effort’). Although the pain may be severe during such events, the cardiac cells temporarily deprived of oxygen are not damaged and do not release their cellular contents. However, if the arteries become completely blocked either by the plaque or by a small thrombus (clot) that is prevented from flowing through the artery by the plaque, the patient experiences a myocardial infarction (MI, ‘heart attack’) characterised by the same severe chest pain, but in this case the pain is accompanied by irreversible damage to the cardiac cells and the release of their cellular contents. This release is not immediate, but occurs over a period of many hours. From the point of view of the clinical management of the patient, it is important for the clinician to establish whether or not the chest pain was accompanied by a myocardial infarction. In about one-fifth of the cases of a myocardial event, the patient does not experience the characteristic chest pain (‘silent myocardial infarction’), but again it is important for the clinician to be aware that the event has occurred. Electrocardiogram (ECG) patterns are a primary indicator of these events, but in atypical presentations ECG changes may be ambiguous and additional evidence is sought in the form of changes in specific serum proteins and enzymes. The time course of release of these MI marker proteins is illustrated in Figure 1. In current practice, usually only creatinine kinase (CK) and one of the cardiac muscle proteins, troponin T or troponin I, are used:
• Creatine kinase (CK): This enzyme converts phosphocreatine (important in muscle metabolism) to creatine. CK is a dimeric protein composed of two monomers, one denoted as M (muscle), the other as B (brain), so that three isoforms exist: CK-MM, CK-MB and CK-BB. The tissue distribution of these isoenzymes is significantly different such that heart muscle consists of 80–85% MM and 15–20% MB, skeletal muscle 99% MM and 1% MB and brain, stomach, intestine and bladder predominantly BB. CK enzyme activity is raised in a number of clinical conditions, but since the CK-MB form is almost unique to the heart, its raised activity in serum gives unambiguous support for a myocardial infarction, even in cases in which the total CK activity remains within the reference range. A rise in total serum CK activity is detectable within 6 hours of a myocardial infarction and the serum activity reaches a peak after 24–36 hours (Figure 1). However, a rise in CK-MB is detectable within 3–4 hours, has 100% sensitivity within 8–12 hours and reaches a peak within 10–24 hours. It remains raised for 2–4 days.
• Troponin: The troponins I and T are protein components of a complex that regulates the contractility of the myocardial cells. The concentration in serum increases at the same rate as CK-MB after a myocardial infarction, has a similar time for 100% sensitivity and for peak time, but it remains raised for several days after the onset of symptoms. Troponin T is assayed by a ‘sandwich’ immunological assay in which troponin T in the sample reacts with a biotinylated monoclonal cardiac troponin T-specific antibody and a monoclonal troponin T antibody labelled with a ruthenium complex. The measurement of serum troponin I or T is widely used to rule out cardiac damage in patients with chest pain – if there is no significant change when measured at presentation and again at 3–6 hours, then MI can be ruled out and the patient discharged or monitored under less intensive conditions. Newer troponin assays with high sensitivity allow this time interval to be reduced. Since troponin levels remain raised for several days following a myocardial infarction they can be helpful in identifying delayed presentation of MI (Figure 1).
• Total CK activity: This is assessed by coupled reactions with hexokinase and glucose-6-phosphate dehydrogenase in the presence of N-acetylcysteine as activator, and the measurement of increase in absorbance at 340 nm or by fluorescence polarisation (primary wavelength 340 nm, reference wavelength 378 nm):
creatine phosphate + ADP ⇌ creatine + ATP
D-glucose + ATP ⇌ D-glucose-6-phosphate + ADP
D-glucose-6-phosphate + NAD(P)+ ⇌ D-6-phosphogluconate + NAD(P)H + H+
• CK-MB activity: This is assessed by the inhibition of the activity of the M monomer by the addition to the serum sample of an antibody to the M monomer. This inhibits CK-MM and the M unit of CK-MB. The activity of CK-BB is unaffected, but is normally undetectable in serum, hence the remaining enzyme activity in serum is due to the B unit of CK-MB. It is assayed by the above coupled assay procedure and the activity doubled to give an estimate of the CK-MB activity. An alternative assay uses a double antibody technique: CK-MB is bound to anti-CK-MB coated on microparticles, the resulting complex washed to remove non-bound forms of CK and anti-CK-MM conjugated to alkaline phosphatase added. It binds to the antibody–antigen complex, is washed to remove unbound materials and assayed using 4-methylumbelliferone phosphate as substrate, the released 4-methylumbelliferone being measured by its fluorescence and expressed as a concentration (μg l −1 ) rather than as enzyme activity.
Fig1. Serum enzyme activity as well as myoglobin and troponin I concentration changes following a myocardial infarction. Changes are expressed as a multiple of the upper limit of the reference range. Values vary according to the severity of the event, but the time course of each profile is characteristic of all events. In current practice in the UK, only troponin and CK or CK-MB are used.
The measurement of CK and troponin concentrations, together with plasma potassium, glucose and arterial blood gases, is routinely used to monitor the recovery of patients following a myocardial infarction. A patient may experience a second myocardial infarction within a few days of the first. In such cases, the pattern of serum enzymes shown in Figure 1 is repeated, with the new pattern being superimposed on the remnants of the first profile. CK-MB is the best initial indicator of a second infarction since the levels of troponin may not reflect a secondary event.
The sensitivity and specificity of an electrocardiogram (ECG) and diagnostic enzymology in the management of heart disease are complementary. The specificity of an ECG is 100%, whilst that of enzyme measurements is 90%; however, the sensitivity of ECG is 70%, whilst that of troponin measurement can be up to 94% depending on the cut-off value used.
