One tenth of all emergency department visits, representing approximately eight million patients, are evaluated each year for chest pain. Differentiating which patients actually have a real cardiac problem from those who have a less serious cause for their chest pain can be very tricky. Making the wrong decision in this setting can be deadly. Fortunately, a number of new tools are emerging to assist us in this important area. Please join me in welcoming Drs. Morrow and Antman to Cyberounds^®. They are experts in the area of Acute Coronary Syndromes and have put together a very helpful guide to the new tools available for evaluating patients with acute onset chest pain.

-- Richard W. Smalling, M.D., Ph.D., Cardiovascular Moderator

Individuals presenting to medical attention with chest discomfort without ST segment elevation represent an extremely heterogeneous population. Integrated with the clinical history, physical exam and electrocardiogram, biochemical cardiac markers provide important diagnostic and prognostic information in the evaluation of these patients. Appropriately utilized, cardiac markers:

1. facilitate the rapid diagnosis of an acute coronary syndrome;
2. contribute to the exclusion of myocardial injury as the etiology of chest discomfort; and
3. enable effective risk stratification and triage of patients with chest symptoms.
4. Finally, these markers may also identify patients who will derive the most benefit from particular therapeutic interventions.

The past decade has witnessed an impressive expansion of data regarding new cardiac markers as well as new assays and uses for well established markers. Optimal use of these markers in the clinical setting requires selection based on the specific advantages and disadvantages of each marker and details of the individual clinical presentation.

The Primary Benchmark - Creatine Kinase MB

Creatine kinase (CK) is present in both skeletal and cardiac muscle and is released into the blood when myocyte necrosis occurs. While total CK is a sensitive marker of myocardial necrosis, it has poor specificity because of its high concentration in skeletal muscle cells. CK exists as three isoenzymes, (CK-MM, CK-MB, CK-BB), which may be separated by electrophoresis. Though CK-MM is present in high concentration in both skeletal and cardiac muscle, the CK-MB isoenzyme has a much higher concentration in cardiac myocytes and, thus, offers improved sensitivity and specificity for detection of myocardial necrosis compared with total CK. Still, this isoenzyme constitutes 1.0-3.0% of the CK in skeletal muscle and is present in minor quantities in intestine, diaphragm, uterus and prostate. Thus, its specificity may be impaired in the setting of major injury to these organs, especially skeletal muscle.

Measured with rapid, reliable and cost-efficient automated assays, CK-MB remains heavily utilized in clinical practice and is the standard against which new cardiac markers have been compared. CK-MB rises within three to four hours after the onset of myocardial injury and falls to normal ranges within 24-36 hours (Table 1).

Serial measurements, documenting the characteristic rise and fall, are important in maintaining specificity for the diagnosis of acute myocardial infarction (AMI). This temporal course does not permit the very early detection of myocardial necrosis (one to three hours) but does allow for the determination of re-infarction during the initial few days after presentation.

New Assays for CK-MB

Radioimmunoassay, agarose gel electrophoresis and ion exchange chromatographic techniques for determination of CK-MB measure enzyme activity, which is reported in international units/liter. These techniques have now largely been supplanted by more specific and sensitive immunoassays using specific monoclonal antibodies to CK-MB. These assays which determine CK-MB mass in nanograms per milliliter are not only more accurate than the activity assays but also more efficient and fully automated producing results within 30 minutes.

Specific subforms of the CK-MB isoenzyme have now been identified. These isoforms are related through the action of a carboxypeptidase enzyme which cleaves a terminal lysine residue from CK-MB2 producing CK-MB1. Measurement of these isoforms may prove useful in the very early detection of AMI and is discussed in detail later in this Cyberounds^®.

CK-MB

Advantages

• Rapid, cost-efficient, accurate assays
• High sensitivity and specificity in most clinical settings
• Ability to detect re-infarction

Disadvantages

• Loss of specificity in setting of skeletal muscle disease or injury including surgery
• Poor sensitivity during very early myocardial infarction (< 6 hours after symptom onset) or late after symptom onset (> 36 hours)

Powerful Tools for Risk Stratification - The Cardiac Troponins

Troponin I and T are two sub-units of the troponin complex which is bound to actin and modulates the interaction between actin and myosin in myocytes. In contrast to CK-MB, these proteins have isoforms that are unique to cardiac myocytes and may be measured by assays using monoclonal antibodies specific to the cardiac form. Thus, cardiac troponin I and T (cTnI, cTnT) are highly specific to myocardium and are not normally found circulating in the blood of healthy individuals. Cardiac troponins have a greater proportional rise above the reference value during myocardial necrosis than CK-MB and perform well as highly sensitive and specific markers of AMI. cTnT and cTnI may, therefore, offer important additional diagnostic information in the case of normal or borderline elevation of CK-MB or concomitant skeletal muscle injury. Troponin rises with a time course similar to CK but remains elevated for up to 14 days after cardiac injury (Table 1). Cardiac troponins are, therefore, also particularly useful for the diagnosis of cardiac events having occurred several days up to one to two weeks previously, when CK will have already returned to normal levels. However, the troponins are poor detectors of early re-infarction.

The cardiac troponins have also proven to be effective for cardiovascular risk assessment for individuals presenting with chest pain syndromes suspicious for AMI. Among patients with non-ST elevation acute coronary syndromes in the Thrombolysis in Myocardial Infarction (TIMI) III trial, researchers observed a gradient of risk for 42-day mortality with increasing levels of cTnI at presentation.(1) Similarly, the GUSTO IIa investigators found that, in a population of patients presenting with acute myocardial ischemia with or without ST elevation, those with elevated cTnT were at higher 30-day mortality risk.(2) In this study, cTnT was a stronger predictor of outcome than either ECG or CK-MB. Finally, when detected using rapid qualitative assays, elevated cTnI or cTnT have been shown to identify those at higher risk for death or non-fatal MI by 30 days among 773 individuals presenting to the emergency department with non-traumatic chest pain.(3) Further, patients in this study with two negative cardiac troponin test results pain, separated by four hours (with one at least six hours after the onset of symptoms), were at low risk of cardiac events by 30 days.

The finding of elevated cardiac specific troponins among some individuals presenting with acute coronary syndromes with normal CK-MB deserves particular attention. The absence of cTnT/cTnI in the circulation of normal individuals together with a greater proportional rise relative to CK-MB in the setting of myocardial necrosis endow the cardiac troponins with a superior signal-noise ratio, enabling the detection of even minor degrees of myocardial necrosis. This feature of the cardiac troponins and the finding of elevated cTnI or cTnT in up to 30% of patients with unstable angina have together fostered the concept of "microinfarction", a term alluding to minor myocyte necrosis not detected by CK-MB.(4) Alternatively, the isolated elevation of cardiac troponins may result from increased permeability of the myocyte plasma membrane and release of the two to six percent of troponin which exists free in the cytosol during ischemia without cellular necrosis. Regardless of the precise mechanism, an elevated cTnI or cTnT at presentation among patients with chest pain suspicious for MI but with normal CK-MB has been associated with significantly higher risk for adverse cardiovascular outcomes, including death, recurrent non-fatal MI and need for revascularization.

Cardiac-specific Troponins

Advantages

• Powerful tool for risk stratification
• High sensitivity and specificity even in setting of skeletal muscle injury or surgery
• Detection of recent MI up to two weeks after onset

Disadvantages

• Poor ability to detect re-infarction
• Poor sensitivity during very early myocardial infarction (< 6 hours after symptom onset)

Early Detection - Myoglobin and CK-MB Isoforms

Myoglobin has long been recognized as a cardiac marker but has been limited in its use by poor specificity related to its presence in skeletal muscle. However, a recent renewal of interest in myoglobin has been sparked by its potential as a very early marker of myocardial necrosis. Myoglobin concentration may begin to rise as early as one hour after onset of myocyte damage and returns to normal within 12-24 hours. A series of clinical trials have examined use of myoglobin for early detection of myocardial infarction among patients presenting prior to six hours after symptom onset, with reported sensitivities of 79 - 100%.(5),(6),(7) Further, the combination of myoglobin and CK-MB in point-of-care rapid assay devices has appeared promising for the early exclusion of MI with a negative predictive value of 100% by four hours after presentation in one study.(8),(9) Finally, given the extremely rapid rise and decline of myoglobin after resolution of coronary arterial occlusion, this marker shows particular promise as an early non-invasive indicator of successful reperfusion.(10),(11),(12)

Also promising for the early detection of infarction is the measurement of CK-MB isoforms. As CK-MB2 may be released into the circulation as early as one hour after the onset of infarction, a relative increase in the proportion of CK-MB in the MB2 isoform (CK-MB2/CK-MB1 > 1.7) may be indicative of the early stages of infarction. In a study of patients presenting to Houston area hospitals with chest pain within six hours of symptom onset, CK-MB isoforms measured at presentation and one hour later demonstrated high sensitivity and specificity (92 and 89%) compared with sensitivities of 79 and 50-60% for myoglobin and troponin.(13),(14) Determination of these isoforms, using high voltage electrophoretic assays in experienced laboratories, offers an opportunity to detect myocardial injury in the very early hours of infarction. It remains to be determined whether these results can be reproduced outside of a dedicated research center.

Myoglobin

Advantages

• Early detection of myocardial infarction

Disadvantages

• Very poor specificity in setting of skeletal muscle injury or disease

CK-MB Isoforms

Advantages

• Early detection of myocardial infarction

Disadvantages

• Specificity profile similar to CK-MB
• Current assays require special expertise

Novel Cardiac Markers -- Looking Beyond Myocardial Necrosis

The biochemical markers discussed to this point are cellular constituents released into the circulation at various intervals after the onset of myocardial necrosis. As such, they may be detected and contribute to the diagnosis of AMI. In contrast, a new generation of novel cardiac markers center on the detection of events preceding this single, albeit important, endpoint in ischemic heart disease.(15) These candidate cardiac markers, such as coagulation proteins and cellular adhesion molecules, reflect crucial elements involved in the evolution of an acute coronary syndrome, including plaque rupture, platelet aggregation and thrombosis, as well as myocardial necrosis.

Experimental and clinical investigations continue to advance our understanding of the pathophysiology of unstable coronary syndromes and offer new directions for risk assessment in ischemic heart disease. For example, multiple lines of investigation have converged to implicate inflammatory processes as central contributors to atherogenesis and plaque compromise. Thus, several mediators of the inflammatory response, including acute phase proteins, cytokines and cellular adhesion molecules, have been examined as potential indicators of underlying atherosclerosis as well as risk of acute cardiovascular events.

As the prototypical acute phase reactant, C-reactive protein (CRP) has been the focus of much of the clinical investigation in this arena. The recent development of high-sensitivity assys for CRP has enabled the detection of elevation of CRP (hs-CRP) within the normal range and provided a reliable method for the assessment of low levels of systemic inflammation. There have now been at least six clinical studies exploring the prognostic value of hs-CRP in acute coronary syndromes and demonstrating a predictive capacity of hs-CRP levels determined at presentation or discharge with respect to cardiovascular outcome both during the initial hospitalization and up to one year thereafter.(16) Moreover, measurement of hs-CRP has been found to add to the prognostic information offered by clinical history, electrocardiography and the cardiac specific troponins. Further, a series of prospective epidemiologic studies have demonstrated a consistent and strong association between elevated baseline hs-CRP and a higher risk of future myocardial infarction among apparently healthy men and women without recognized cardiovascular disease.(16)

Additional research, clarifying the potential clinical role of hs-CRP as well as other participants in the inflammatory response such as serum amyloid A, specific cytokines and cellular adhesion molecules, is ongoing. Though not yet routinely employed in the clinical setting, fully automated, high-efficiency assays for inflammatory markers including hs-CRP will likely soon be available for clinical use. Thus, well-designed prospective studies will be important to evaluate the clinical efficacy of these inflammatory indicators in combination with established cardiac markers for the stratification of cardiovascular risk. Similar investigation is ongoing with markers of platelet function and hemostasis, such as glycoprotein IIb/IIIa occupancy, p-selectin and von Willebrand's factor.

The Future - Targeted Therapy

New cardiac markers, such as the cardiac specific troponins, have proven to add significantly to the clinical evaluation and electrocardiogram in the stratification of cardiovascular risk among patients presenting with chest pain syndromes. It is our hope, as clinicians, that this early risk assessment will direct the appropriate initiation of interventions which might ameliorate the risk of adverse outcomes. Recent evidence from clinical trials suggests that this may be the case. Data from the Fragmin in Unstable Coronary Artery Disease (FRISC) trial have shown that the relative risk reduction achieved with treatment with low-molecular weight heparin was significantly greater among those patients with elevated baseline cTnT.(17) Similarly, data from the CAPTURE trial demonstrate that the benefit of treatment with the glycoprotein IIb/IIIa antagonist abciximab was statistically significant only in those with elevated cTnT and that the benefit of treatment prior to percutaneous coronary intervention was limited to those with a positive baseline cTnT.(18),(19) These data support the notion that these cardiac markers identify patients who will derive the most benefit from aggressive therapy.

Furthermore, clinical investigations using novel markers, such as hs-CRP levels, extend this concept by raising the possibility that these markers may have a role in targeting specific therapeutic interventions relative to a dominant pathophysiologic contribution to the disease process. For example, in the Physicians Health Study, in which participants were randomly assigned to low dose aspirin (325 mg PO QOD) or placebo, the magnitude of risk reduction was found to increase with rising levels of hs-CRP.(20) Though the mechanism underlying these observations is not clear, these data raise the possibility that the anti-inflammatory properties of aspirin, as well as its anti-platelet effects, may contribute to the efficacy of this drug in cardiovascular disease prevention.

Similarly, a potential interaction between hs-CRP and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibition has been demonstrated in a nested case-control analysis of patients enrolled in the CARE trial, where the magnitude of risk reduction with therapy was greatest in those with evidence of inflammation, (risk reduction attributable to pravastatin therapy = 54%), as compared to those without evidence of inflammation (risk reduction = 25%).(21) Again, though the pathophysiologic reasons for this observed relationship are not defined, it is interesting to speculate whether nonlipid effects of the HMG-CoA reductase inhibitors, such as modulation of immune function or antiproliferative effects on vascular smooth muscle, are at play.

Conclusion

Biochemical cardiac markers are an important tool in the evaluation of individuals with chest pain suspicious for ischemia. Among the markers of myocardial necrosis, CK-MB is an effective, widely used initial test in the diagnosis of AMI with more recently developed MB mass assays offering even higher sensitivity and specificity. The cardiac specific troponins perform very well for early risk stratification of patients with acute ischemic syndromes with and without traditional criteria for myocardial infarction and may identify those who will receive particular benefit from aggressive medical therapy or early revascularization. In addition, they extend our ability to detect recent infarction and discriminate myocardial necrosis in the setting of skeletal muscle injury or surgery or equivocal data from CK-MB. Recent data highlight the potential of CK-MB isoforms and myoglobin as the most sensitive markers in the very early hours after symptom onset.

Finally, a number of novel biochemical markers reflecting key pathophysiologic elements in the genesis of the acute coronary syndrome are receiving attention in ongoing clinical trials. With increasing appreciation for the heterogeneity of etiologies contributing to the progression of ischemic heart disease for any individual, these markers may serve to identify the dominant factor(s) for a given patient and direct specific targeted therapy. A few examples to support this concept have already manifested in the interaction between inflammatory markers and certain preventive therapies. The future may be in the development of marker panels utilizing markers of early and later necrosis, as well as platelet function, coagulation and inflammatory processes, which will influence the triage and treatment of patients with acute ischemic syndromes.