Creatine kinase (CK) is found in heart muscle, skeletal muscle, and brain. It is elevated at some time in about 90%-93% (literature range, 65%-100%) of patients with acute MI. In acute MI, CK behaves similarly to AST. In addition, elevations have also been reported in myocarditis and also in some patients with tachyarrhythmias (mostly ventricular) for unknown reasons. Acute liver cell damage, which frequently causes an abnormal AST value, has no effect on CK. This is an advantage, since the situation often arises in which an elevated AST (or LDH) level might be due to severe hepatic passive congestion from heart failure rather than from acute MI.
Use of CK measurements in diagnosing primary diseases of skeletal muscle is discussed elsewhere. A considerable number of conditions associated with acute muscle injury or severe muscle exertion affect CK values. Thus, CK values are usually elevated in muscle trauma, myositis, muscular dystrophy, after surgery, postpartum, after moderately severe exercise (e.g., long-distance running), and in delirium tremens or convulsions. Increased serum values have been reported in about 80% of patients with hypothyroidism (literature range, 20%-100%) and in patients with severe hypokalemia, due to changes induced in skeletal muscle. CK elevation can be due to effects of alcohol on muscle. For example, one study found that CK levels became abnormal after 24-48 hours in the majority of persons following heavy drinking episodes as well as in most patients with delirium tremens. Levels of CK are said to be normal in chronic alcoholics without heavy intake.
CK levels are frequently elevated after intramuscular injection. Since therapeutic injections are common, this probably constitutes the most frequent cause of CK elevation. Specimens must be drawn before injection or at least within 1 hour after injection. Trauma to muscle makes the CK level unreliable for a few days postoperatively.
Although CK is present in brain tissue as well as muscle, reports differ to some extent as to the effect of central nervous system (CNS) disease on serum CK levels. According to one report, CK levels may be elevated in a wide variety of conditions that affect the brain, including bacterial meningitis, encephalitis, cerebrovascular accident, hepatic coma, uremic coma, and grand mal epileptic attacks. Elevation is not always present, and when it is present, the degree of elevation varies considerably. Elevations in some patients in acute phases of certain psychiatric diseases, notably schizophrenia have been reported; the cause is not known. According to one report, CK is elevated in 19%-47% of patients with uremia.
Since the major source for body CK is skeletal muscle, individuals with relatively small muscle mass will tend to have lower normal CK levels than the average person; those with increased muscle mass will tend to have relatively higher normal values. Normal CK values for African-American males are double those for European males; values for African-American and European females are nearly equal in most (but not all) reports.
The major drawbacks of total CK are (1) the relatively short time period after onset of infarction during which the CK value is elevated and (2) false positive elevations due to skeletal muscle injury (especially intramuscular injections).
Creatine kinase isoenzyme measurement. Total CK can be separated into 3 major fractions (isoenzymes): CK-BB (CK-1), found predominantly in brain and lung; CK-MM (CK-3), found in skeletal muscle; and the hybrid CK-MB (CK-2), found predominantly in cardiac muscle. CK isoenzyme assays are now available in most hospital laboratories. Isoenzymes offer a way to detect myocardial damage that minimizes skeletal muscle contribution to CK values.
Creatine kinase MM fraction. CK-MM comprises well over 95% of skeletal muscle CK and about 70%-75% of myocardial CK. Since the total amount of body skeletal muscle is so much greater than myocardium, elevation of the MM fraction is usually due to skeletal muscle injury or hypoxia, including moderately severe or severe exercise, convulsions, inflammation, trauma, intramuscular injection, or muscular dystrophy. Some conditions producing less obvious effects on muscle, such as hypothyroidism and hypokalemia, may also produce CK-MM increase.
Creatine kinase MB fraction. CK-MB can be reported in two ways: percentage of total CK (MB/total CK) or in mass units (either by multiplying total CK by the percentage of MB value or by assaying the MB fraction directly using immunoassay). The most recommended method is to screen with mass unit values, because a low normal MB value divided by a relatively low total CK value can give a misleading, rather high percentage of MB. Skeletal muscle contains mostly CK-MM isoenzyme, but there is also about 3%-5% MB present (the amount depends on the particular muscle assayed). Therefore, serum MB can be increased over baseline to some degree by sufficient skeletal muscle injury as well as by myocardial muscle injury. When acute skeletal muscle hypoxia or other injury of sufficient degree elevates CK-MB levels above the upper limit of the reference range in terms of CK-MB units, CK-MM levels are usually increased at the same time and to a much greater degree. Because of the concurrent CK-MM increase, the CK-MB level (although it may be increased) usually remains less than a small MB/total CK cutoff value (which ranges in the literature from 2.5%-5% in different laboratories). Therefore, when the CK-MB value is increased, it is very helpful to know the total CK value in order to calculate the percentage of MB relative to total CK (the “relative index”). If the MB value in terms of units is not increased, the percentage of MB is not useful and may be misleading (e.g., if normal CK-MB levels are 0-10 units and normal total CK units are 0-40 units, a CK-MB level of 2 units is 20% of a total CK value of 10 units, even though both values are well within reference range). Each laboratory should determine the MB “relative index” experimentally because of considerable differences in MB and total CK methodology and analysis conditions.
The CK-MB level begins to rise 3-6 hours after onset of acute MI, reaches a peak in 12-24 hours, and returns to normal in 24-48 hours (sometimes earlier). There is a rough correlation between the size of the infarct and the degree of elevation. Since small infarcts may not elevate the MB fraction dramatically, it is important to time the collection of specimens so as not to miss the peak values. Some recommend five specimens: one immediately, then at 6, 12, 18, and 24 hours afterward. Others use 4 specimens: one immediately, then at 8, 16, and 24 hours or at 6, 12, and 18 hours. Some use 3 specimens: immediately, 12 hours, and 24 hours.
Some laboratories do not perform CK-MB assay unless the total CK value is elevated. However, reports indicate that about 10% of acute MI patients (literature range 0%-16%) demonstrate elevated CK-MB levels with the total CK value remaining within reference range limits. This is especially common in persons with relatively small muscle mass, whose preinfarct normal total CK value is apt to be in the lower part of the population reference range.
Since acute myocardial infarct may occur with CK-MB values elevated to levels diagnostic of MI but with total CK remaining within its reference range, the question arises whether there is a cutoff point within the total CK reference range below which CK-MB could be omitted. Zero percent to 16% of patients with acute MI are said to have elevated MB with concurrent normal total CK. Unfortunately, there is controversy surrounding this question and little data on which to base an answer. Much of the older literature is invalid because the MB methods were not specific for MB; other reports are hindered because MB results were reported only as a percentage of total CK (e.g., an increase of 10 MB units of activity or mass units is a much smaller percentage of total CK when total CK is high in its reference range than when it is low in its reference range). In addition, in some cases there was not adequate confirmation that acute MI had actually occurred (e.g., typical MB rise and fall pattern or LDH-1 isoenzyme fraction becoming elevated and greater than LDH-2). Frequently the location of the total CK values in its reference range when MB was elevated was not disclosed. It would probably be safe to say that MB would be unlikely to suggest acute MI if the total CK value were in the lower half of its reference range (one of 50 consecutive cases of documented acute MI at my hospital).
There is some dispute as to whether ischemia without actual infarct will elevate either total CK or CK-MB levels. The controversy revolves around the fact that currently there is no way to rule out the possibility that a small infarct may have occurred in a patient who clinically is thought to have only ischemia.
In equivocal cases it is helpful to obtain LDH isoenzyme determinations as well as CK-MB, both to enhance diagnostic specificity and because specimens for CK-MB may have been obtained too late to detect elevation. If a CK isoenzyme sample is obtained immediately, at 12 hours, and at 24 hours; and LDH isoenzyme determinations are performed at 24 and 48 hours, 95% or more acute MIs detectible by these tests will be documented. Of course, if one of the CK-MB determinations is strongly positive, it might not be necessary to obtain further samples.
The question sometimes arises whether both CK-MB and LDH isoenzyme fractionation should be done. At our hospital, CK-MB assay is performed immediately and then 12 and 24 hours later; and LDH isoenzyme study is performed at 24 and 48 hours (both using electrophoresis). In 50 consecutive patients with either the CK-MB level elevated or LDH-1 level elevated accompanied by the LDH-1 level becoming greater than the LDH-2 level, CK level was elevated in the first sample in 28% of cases, initially elevated in the 12-hour sample in 50%, initially elevated in the 24-hour specimen in 4%, and not elevated in 18%. The LDH-1/LDH-2 ratio was initially reversed in the 24-hour specimen in 70%, in the 48-hour specimen in 18%, and in neither specimen in 12%. Therefore, with the commonly used protocol, 12% of patients would have been missed relying on LDH isoenzyme studies alone and 18% with CK-MB alone.
CK-MB fraction and skeletal muscle
Although CK-MB is found mainly in cardiac muscle (25%-30%; range, 14%-42% of total myocardial CK), skeletal muscle CK contains a relatively small amount of CK-MB (3%-5% as noted previously). Skeletal muscle contains two types of muscle fibers. Type 1 predominates in muscles such as the gastrocnemius; total CK of type 1 fibers only contains about 1% CK-MB (range, 0%-4%) in addition to CK-MM. Type 2 fibers predominate in other muscles such as the intercostal and soleus and comprise 40% of the fibers in other muscles such as the quadriceps. The total CK of type 2 fibers contains 2%-10% CK-MB in addition to CK-MM. In Duchenne’s muscular dystrophy, especially in the earlier stages, serum levels of CK-MB may be increased, presumably due to skeletal muscle type 2 involvement, with CK-MM levels increased to a much greater degree. An increase in CK-MB value may also occur in some patients with gangrene or severe ischemia of the extremities. In addition, patients with acute myositis of various types, long-distance runners, idiopathic myoglobinemia, Reye’s syndrome, and Rocky Mountain spotted fever frequently display some degree of CK-MB elevation. One study found that intestinal infarct raises CK-MB (both quantitative and %MB) without reversal of the LDH-1/LDH-2 ratio.
CK-MB fraction in defibrillation, coronary angiography, and coronary angioplasty
In patients undergoing defibrillation or cardiac resuscitation, the CK-MB value is usually normal unless actual myocardial injury (e.g., contusion) takes place. At least one report indicates that the CK-MB value is normal in most patients with myocarditis, pericarditis, and subacute bacterial endocarditis, even when LDH isoenzymes display a “cardiac” pattern. However, other investigators have reported elevated MB values in some patients with active myocarditis. Coronary angiography and even coronary angioplasty are likewise reported to be associated with normal MB values unless some degree of myocardial infarction is occurring. In open heart surgery, such as coronary artery revascularization, a CK-MB isoenzyme elevation raises the question of MI, even though operative manipulation of the heart takes place. Nearly 15% of such patients (ranging from 1%-37%) have displayed CK-MB elevation just before operation. Others have CK-MB increase during institution of cardiopulmonary bypass or during anesthesia induction before actual cardiac surgery begins.
Creatine kinase MB fraction; some technical problems and pseudo-MB substances. CK isoenzyme fractionation can be performed in several ways. The most commonly used techniques are column chromatography, electrophoresis, and immunoinhibition. The accuracy of column chromatography depends to some extent on the method used. The major disadvantage is the tendency in some kits for some MM isoenzyme to carry over into the MB fraction if the value of the MM fraction is greatly increased. In addition, some patients have a variant of BB isoenzyme called “macro-CK” (or “macro-CK type 1”) in which BB isoenzyme becomes complexed to gamma globulin, with the resulting complex migrating electrophoretically between MM and MB instead of in the usual BB location on the opposite (anodal) side of MB from MM. Macro-CK is carried into the MB elution fraction in ordinary CK isoenzyme column methods and, if it is present, can falsely increase apparent MB values. Macro-CK is reported present in about 1% of all CK-MB isoenzyme studies (literature range, 0.2%-1.8%), and in one report it accounted for about 8% of elevated CK-MB results. Another CK-MB method is electrophoresis. Electrophoresis is reasonably sensitive, although there is some variation among different methods. Electrophoresis is more specific than column chromatography (and immunoinhibition methods, discussed later) since the various isoenzyme fractions can be inspected visually, and therefore atypically migrating CK variants such as macro-CK can be detected. However, macro-CK may be falsely included with CK-MB even with electrophoresis when automatic electrophoretic fraction quantitation devices are used or the technologist does not inspect the pattern carefully before manually quantitating the major isoenzyme fractions. Immunoinhibition methods are becoming widely used. Most kits currently available are based on elimination of the M subunit and measurement of the B subunit. This measures both MB and BB together. The rationale for this is the infrequency of elevated BB values. However, macro-CK is detected and produces falsely elevated MB results. Several immunoassay methods that are not affected by macro-CK are now available, and it is helpful to ask the laboratory whether their method will be affected.
Other variant CK isoenzymes besides macro-CK have been reported. The best characterized one is a macromolecular CK isoenzyme variant derived from mitochondria (mitochondrial CK, or macro-CK type 2), which electrophoretically migrates cathodal to MM (on the other side of MM from MB). In the few reports in which data are provided, 0.8%-2.6% of patients who had CK-MB studies done had mitochondrial CK present. It was present more often in patients with cancer than in those with other conditions and tended to be associated with a poor prognosis. In addition to CK isoenzyme variants, there is an enzyme called “adenylate kinase,” present in RBCs as well as certain other tissues, which electrophoretically migrates in the same area as mitochondrial CK. Specimens with visible degrees of RBC hemolysis may contain increased adenylate kinase, which, in turn, may falsely increase (apparent) CK-MB results unless there is some way to detect the problem. Hemolysis, whether artifactual or due to hemolytic or megaloblastic anemia; renal cortex infarct; and some germ cell tumors may also elevate LDH-1 values, which adds to the potential for misdiagnosis of acute MI.
Immunoassay methods specific for MB using monoclonal antibodies are now commercially available; these methods do not detect macro-CK or other atypical CK variants. Some laboratories use nonspecific MB methods only; some screen for elevated MB using a nonspecific method (which is usually less expensive than a specific method) and use a specific method to confirm any elevated specimen result; some use only a specific method. It is important to know whether any laboratory is using only a nonspecific method. In fact, even some of the MB-specific kits may produce false positive results in uncommon cases where the anti-MB antibody cross-reacts with a heterophilic or HAMA antibody against the animal tissue from which the monoclonal anti-MB antibody originated. However, if this happens all MB specimens would likely be elevated at approximately the same degree without the “rise and fall” pattern expected in acute MI.
Creatine kinase BB Fraction. Elevation of the CK-BB value is not common. It is occasionally elevated after pulmonary embolization, and one report indicated that it may occasionally be increased in some patients who undergo cardiopulmonary resuscitation or in some patients in shock. It has also been reported in a few patients with some types of cancer (more often widespread and most frequently in prostate and lung small cell carcinoma) but not sufficiently often to be useful as a screening test. In brain disorders, where one theoretically would expect release of CK-BB, the actual isoenzyme detected in serum is usually CK-MM. In some patients (especially those with chronic renal disease) an unusually accentuated albumin may be mistaken for CK-BB on electrophoresis, since albumin migrates between MB and BB.
CK-MM and CK-MB isoforms in acute myocardial infarction. There are three major CK isoenzymes, but each of the three has now been shown to contain subforms (isoforms). The isoform of CK-MM in myocardium is known as MM3; and when MM3 is released into blood during myocardial injury, carboxypeptidase-N enzyme in serum converts MM3 to MM2 isoform; later, MM2 is further converted to MM1. An MM3/MM1 ratio of more than 1.0 suggests myocardial or skeletal muscle injury. The rapid release of the tissue isoform MM3 from damaged myocardium creates a significant excess of MM3 over MM1 considerably sooner than the serum peak values of either MM3 or MM1 are reached. Therefore, most investigators used the MM3/MM1 ratio instead of one of the individual isoforms. The ratio is said to become abnormal 2-4 hours (or even earlier) after onset of acute MI and peaks before either its individual isoforms or CK-MB. In one study the MM3/MM1 ratio was elevated in 90% of acute MI patients by 6-9 hours, whereas 79% of patients had an elevated CK-MB at the same time interval. Thus, the isoform ratio measurement is similar to the myoglobin measurement regarding early abnormality. Unfortunately, like myoglobin, skeletal muscle also contains MM isoforms; so that cardiac MM isoform assay, while a little more specific than myoglobin, is frequently elevated in various types of skeletal muscle injury and in severe or prolonged physical activity.
CK-MB has only two isoforms: the tissue-specific (unaltered tissue form) MB2 and the serum derivative form MB1, created by action of serum carboxypeptidase-N on MB2 released into blood. Similar to CK-MM isoform measurement, the ratio of MB2/MB1 becomes elevated consistently before elevation of either isoform alone or elevation of CK-MB. A MB2/MB1 ratio greater than 1.0 suggests acute myocardial injury. In several studies, the ratio was elevated in nearly 60%-67% of acute MI patients 2-4 hours after onset of symptoms and in 75%-92% of patients at 4-6 hours (versus 15%-50% of patients when using CK-MB). Also, specificity of the MB isoforms for myocardium is much greater than MM isoforms or myoglobin. However, as in CK-MB itself, the MB isoforms or their ratio may become elevated due to moderately severe or severe skeletal muscle injury. One report suggests that the ratio begins to decline after approximately 12 hours.
The MM3/MM1 ratio is reported to peak at 4.0-5.5 hours after successful reperfusion, with reported sensitivity of about 85%. The MB2/MB1 ratio peaks at about 1.5 hours (range, 0.8-2.3 hours) after successful reperfusion.
Troponin-T
Troponin-T is a regulatory protein located in skeletal and cardiac muscle fibers. Skeletal muscle and myocardium have different forms of this protein, so that the antibody that detects cardiac troponin-T in serum is a specific test for myocardial fiber injury. After onset of acute MI, the troponin-T level begins to increase in about 4-6 hours, peaks at about 11 hours (range, 10-24 hours), and returns to reference range in 10 days or more. Therefore, Troponin-T behaves like CK-MB but remains elevated much longer.
Troponin-I
Troponin-I is a regulatory protein in the troponin cardiac muscle complex. This protein, like troponin-T, is specific for myocardium. The troponin-I level becomes elevated after onset of acute MI in about 4-6 hours (range, 3-19 hours), peaks at about 11 hours (range, 10-24 hours), and returns to reference range in about 4 days (range, 3.5-6 days). Therefore, troponin-I behaves much like troponin-T, except that return to reference range is faster. Response to acute MI is also somewhat similar to CK-MB. However, troponin-I may become elevated a little sooner than CK-MB in the first 4 hours after onset of acute MI (in one study, troponin-I was elevated at or before the fourth hour in 44% of MI patients, whereas when using CK-MB only 17% had elevated levels).
Myoglobin in diagnosis of myocardial infarct
Myoglobin is found in cardiac and skeletal muscle and is currently measured by immunoassay. The studies so far available indicate that serum myoglobin levels after onset of acute MI become elevated in about 3 hours (range, 1.5-6.5 hours), reach a peak in about 9 hours (range, 4-16 hours), and return to normal range in about 30 hours (range, 12-54 hours). By 4 hours serum myoglobin values are elevated in about 50%-60% of patients (range, 26%-100%). By 6-8 hours, about 85%-95% of patients have elevated values (range, 54%-100%). By 12-24 hours, after admission, levels remain elevated in fewer than 50% of patients. The serum myoglobin “false positive” rate in patients with chest pain but without strong evidence of acute MI has been reported to range from 0% to 50%. The major attraction of myoglobin in acute MI is elevation in a substantially greater percentage of patients with acute MI than seen with CK-MB before the desirable cutoff point of 6 hours after onset of MI, during which thrombolytic therapy has greatest benefit. However, the substantially greater false positive rate compared to CK-MB, mostly due to skeletal muscle etiology, has counterbalanced this advantage.
Myoglobin is excreted in urine by renal glomerular filtration. There is evidence that myoglobinuria may appear as early as 3 hours after onset of infarct symptoms. In several patients, urine values returned to upper normal limits by 30 hours but remained elevated longer in most cases. Another study recorded 90% sensitivity 24 and 48 hours after hospital admission, diminishing to 76% by 72 hours.
Besides cardiac muscle injury, skeletal muscle injury of various etiologies and other conditions may produce myoglobinemia. Since myoglobin is excreted through the kidneys, poor renal function can elevate serum levels. On the other hand, a sufficient degree of myoglobinuria (or hemoglobinuria) may cause renal failure. To screen for myoglobinuria, the simplest method is a dipstick urine test for blood (hemoglobin); this also reacts with urine myoglobin. Some recommend diluting the urine specimen 1:20 or even 1:40 to avoid detecting minor or clinically unimportant degrees of myoglobinuria. If positive, there are several chemical tests for myoglobin, none of which are completely satisfactory. Serum LDH isoenzyme fractionation by electrophoresis may be helpful; hemolysis elevates LDH-1, whereas myoglobin elevates LDH-4 and 5. An immunologic assay specific for myoglobin (which may have to be performed on serum) is the best diagnostic procedure, if available.
Myosin light chains (MLCs)
Myosin light chains (MLCs) are structural components of myosin, which with actin comprise the contractile proteins of muscle (including cardiac muscle). Each myosin molecule is composed of two heavy chains and two pairs of light chains (MLC-I and MLC-II). MLCs are released about 3-8 hours after onset of acute MI and the MLC value peaks at about 24-36 hours, remaining elevated for 10 days or more. Therefore, MLC assay behaves in the early phase like CK-MB and overall like troponin-T. MLCs are not cardiac-specific, and the MLC value becomes elevated in skeletal muscle injury.
Causes of myoglobinemia and myoglobinuria
Trauma and ischemic disease
Acute myocardial infarct
Muscle arterial ischemia
Surgical procedures
Muscle crush (trauma; pressure while immobile)
Burns; electric shock
Intramuscular injections (conflicting reports)
Metabolic disorders
Alcoholic myopathy
Potassium depletion
Hypothermia
Myxedema
Muscle exertion
Convulsions
Delirium tremens
Severe exercise
Heat cramps
Muscle noninfectious inflammation/degeneration
Systemic lupus
Muscular dystrophies
Dermatomyositis/myositis
Infections/fever
Viral influenza and various viral and bacterial infections
Tetanus
Gas gangrene
Toxicity
Carbon monoxide
Certain drugs
Miscellaneous
Renal failure (myoglobinemia due to poor excretion)
Enzymatic estimation of myocardial infarct size
Several investigators reported that there was a reasonably good correlation between infarct size and the peak value of either total CK or CK-MB. Most investigators favor CK-MB because of its greater specificity for myocardium (especially in view of the effect of intramuscular injections on total CK). However, some reports indicate that infarct location is as important as size, and other reports did not find consistent clear-cut differences in prognosis between a considerable number of infarcts with moderate differences in CK-MB values. In addition to total CK and CK-MB, most other tests used to detect acute MI have also been used to estimate infarct size, but none to date has proved completely satisfactory.
Detection of reperfusion after thrombolytic agents. After interventional therapy for acute MI, not all patients obtain reopening of the affected coronary artery. This information would be useful to assess the need for additional therapy or perhaps for more invasive measures such as balloon angioplasty or coronary artery bypass. Several investigators have found that enzymes released after myocardial fiber damage reach their peak earlier than would be the case if reperfusion did not take place. This is explained on the basis of release of accumulated enzyme after reperfusion occurs (“washout phenomenon”). The reperfusion peak for CK-MB was reached at an average of about 13 hours, whereas the peak for nonreperfused patients was reached at an average of about 18-22 hours. There is disagreement in the reports whether peak enzyme values are greater in the reperfused patients. In addition, although there is clear separation between the mean time duration before the peak in the two groups, in most reports there is overlap between the two groups regarding the time duration of individual patients, with some nonreperfused patients reaching a peak in the same time range as that of some reperfused patients. However, some of the overlap may be due to spontaneous reopening of coronary occlusion or arterial spasm in some nontreated patients. At least partially because of the overlap between patient values, early CK-MB peaking in one study showed a sensitivity of only 69% in detection of reperfusion. Since most of the reports to date have included only relatively small groups of patients, more definitive studies are needed. That includes the CK-MB isoform MB2/MB1 ratio, which is said to peak 1.5 hours after reperfusion (range, 0.8-2.3 hours), substantially earlier than CK-MB.
Radionuclide heart scan
Heart scanning can now be performed in two ways. Scan for acute MI is done with technetium pyrophosphate or certain other radiopharmaceuticals. These agents localize in acutely damaged myocardium, producing a focal area of increased radioactivity. The scan is not reliable less than 24 hours after onset of infarct and returns to normal by 6-7 days after infarct. Best results are obtained with transmural infarcts and with locations in the anterior and lateral walls of the left ventricle. Subendocardial infarcts, especially small ones, are much more likely to be missed. Another consideration is the necessity of transporting the patient to the scanner, unless the institution is one of the few that have a portable unit. Combined use of CPK and LDH isoenzyme determinations has lessened scan necessity in diagnosis. Heart scanning may, however, assist in the diagnosis of acute MI that occurs during or after cardiac surgery, when enzyme or isoenzyme diagnosis is not reliable. Some additional areas of difficulty in myocardial scanning include the fact that size of the scan abnormality cannot at present be reliably correlated with infarct size. Ventricular aneurysms may concentrate the phosphate radiopharmaceuticals in a manner suggesting infarct, and this unexplained capability may persist for years.
Heart scanning can also be done by using radioactive elements, such as thallium, that localize in viable myocardium. Scars and infarcts are seen as areas without uptake. Areas of ischemia (if sufficiently large) also may be detected by scanning before and after exercise. The optimal scan time for acute MI is within 6 hours after onset of chest pain. After 24 hours, reports indicate that as many as 25% of patients may not demonstrate a lesion. Therefore, optimal scan times for thallium and for pyrophosphate are quite different.
Ultrasound imaging of the heart (echocardiography) in two-dimensional B-mode is now widely used to visualize cardiac ventricle wall motion. A focal motion defect is suggestive of myocardial damage, although acute injury and scar cannot always be differentiated. This imaging technique is attractive because it is relatively easy to do with present-day equipment; is relatively inexpensive; and has good sensitivity compared to other scanning modalities. In addition, left ventricular ejection fraction (used as a parameter of left ventricular function) can be estimated. However, like ultrasound in general, much depends on operator scanning technique and interpretation. M-mode ultrasound can also be used to evaluate small ventricular wall areas.