Requiem for a Heavyweight: The Demise of Creatine Kinase-MB

Cardiac troponin: a critical review of the case for point-of-care testing in the ED

The measurement of cardiac troponin concentrations in the blood is a key element in the evaluation of patients with suspected acute coronary syndromes, according to current guidelines, and contributes importantly to the ruling in or ruling out of acute myocardial infarction. The introduction of point-of-care testing for cardiac troponin has the potential to reduce turnaround time for assay results, compared with central laboratory testing, optimizing resource use. Although, in general, many point-of-care cardiac troponin tests are less sensitive than cardiac troponin tests developed for central laboratory-automated analyzers, point-of-care systems have been used successfully within accelerated protocols for the reliable ruling out of acute coronary syndromes, without increasing subsequent readmission rates for this condition. The impact of shortened assay turnaround times with point-of-care technology on length of stay in the emergency department has been limited to date, with most randomized evaluations of this technology having demonstrated little or no reduction in this outcome parameter. Accordingly, the point-of-care approach has not been shown to be cost-effective relative to central laboratory testing. Modeling studies suggest, however, that reengineering overall procedures within the emergency department setting, to take full advantage of reduced therapeutic turnaround time, has the potential to improve the flow of patients through the emergency department, to shorten discharge times, and to reduce cost. To properly evaluate the potential contribution of point-of-care technology in the emergency department, including its cost-effectiveness, future evaluations of point-of-care platforms will need to be embedded completely within a local decision-making structure designed for its use.

Release of tissue-specific proteins into coronary perfusate as a model for biomarker discovery in myocardial ischemia/reperfusion injury

Diagnosis of acute coronary syndromes is based on protein biomarkers, such as the cardiac troponins (cTnI/cTnT) and creatine kinase (CK-MB) that are released into the circulation. Biomarker discovery is focused on identifying very low abundance tissue-derived analytes from within albumin-rich plasma, in which the wide dynamic range of the native protein complement hinders classical proteomic investigations. We employed an ex vivo rabbit model of myocardial ischemia/reperfusion (I/R) injury using Langendorff buffer perfusion. Nonrecirculating perfusate was collected over a temporal profile of 60 min reperfusion following brief, reversible ischemia (15 min; 15I/60R) for comparison with irreversible I/R (60I/60R). Perfusate proteins were separated using two-dimensional gel electrophoresis (2-DE) and identified by mass spectrometry (MS), revealing 26 tissue-specific proteins released during reperfusion post-15I. Proteins released during irreversible I/R (60I/60R) were profiled using gel-based (2-DE and one-dimensional gel electrophoresis coupled to liquid chromatography and tandem mass spectrometry; geLC-MS) and gel-free (LC-MS/MS) methods. A total of 192 tissue-specific proteins were identified during reperfusion post-60I. Identified proteins included those previously associated with I/R (myoglobin, CK-MB, cTnI, and cTnT), in addition to examples currently under investigation in large cohort studies (heart-type fatty acid binding protein; FABPH). The postischemic release profile of a novel cardiac-specific protein, cysteine and glycine-rich protein 3 (Csrp3; cardiac LIM domain protein) was validated by Western blot analysis. We also identified Csrp3 in serum from 6 of 8 patients postreperfusion following acute myocardial infarction. These studies indicate that animal modeling of biomarker release using ex vivo buffer perfused tissue to limit the presence of obfuscating plasma proteins may identify candidates for further study in humans.

A novel strategy for evaluating the effects of an electronic test ordering alert message: Optimizing cardiac marker use

Background:

Laboratory ordering functions within computerized provider order entry (CPOE) systems typically support the display of electronic alert messages to improve test utilization or implement new ordering policies. However, alert strategies have been shown to vary considerably in their success and the characteristics contributing to an alert's success are poorly understood. Improved methodologies are needed to evaluate alerts and their mechanisms of action.

Materials and Methods:

Clinicians order inpatient and emergency department laboratory tests using our institutional CPOE system. We analyzed user interaction data captured by our CPOE system to evaluate how clinicians responded to an alert. We evaluated an alert designed to implement an institutional policy restricting the indications for ordering creatine kinase-MB (CKMB).

Results:

Within 2 months of alert implementation, CKMB-associated searches declined by 79% with a corresponding decline in CKMB orders. Furthermore, while prior to alert implementation, clinicians searching for CKMB ultimately ordered this test 99% of the time, following implementation, only 60% of CKMB searches ultimately led to CKMB test orders. This difference presumably represents clinicians who reconsidered the need for CKMB in response to the alert, demonstrating the alert's just-in-time advisory capability. In addition, as clinicians repeatedly viewed the alert, there was a “dose-dependant” decrease in the fraction of searches without orders. This presumably reflects the alerting strategy's long-term educational component, as clinicians aware of the new policy will not search for CKMB when not indicated.

Conclusions:

Our analytic approach provides insight into the mechanism of a CPOE alert and demonstrates that alerts may act through a combination of just-in-time advice and longer term education. Use of this approach when implementing alerts may prove useful to improve the success of a given alerting strategy.

Isolated creatine kinase-MB rise with normal cardiac troponins: a strange occurrence with difficult interpretation

A 37-year-old man is admitted to the hospital for retrosternal chest pain lasting more than 30  min and nonspecific ECG findings. Serial assays of cardiac biomarkers reveal an isolated elevation of creatine kinase-MB and negative troponin levels. A coronary angiography shows normal vessels in the presence of a Thrombolysis in Myocardial Infarction (TIMI) 2 flow. How should this patient be managed and treated? Is it a myocardial infarction? We here provide a review of the relevant literature and suggest that such a strange condition, for which several explanations are possible, involves a worse prognosis than for normal creatine kinase-MB and troponins.

Biochemical markers in acute coronary syndrome

Owing to their higher risk for cardiac death or ischemic complications, patients with acute coronary syndrome (ACS) must be identified from other causes of chest pain. Patients with acute coronary syndrome are divided into categories based on their electrocardiogram; those with new ST-segment elevation and those who present with ST-segment depression. The subgroups of patients with ST-segment elevation are candidates for immediate reperfusion, while fibrinolysis appears harmful for those with non-ST elevation myocardial infarction. There is increasing evidence to encourage appropriate risk stratification before deciding on a management strategy (invasive or conservative) for each patient. The TIMI, GRACE or PURSUIT risk models are recommended as useful for decisions regarding therapeutic options. Cardiac biomarkers are useful additions to these clinical tools to correctly risk stratify ACS patients. Cardiac troponin is the biomarker of choice to detect myocardial necrosis and is central to the universal definition of myocardial infarction. The introduction of troponin assays with a lower limit of detection will allow for earlier diagnosis of patients who present with chest pain. Analytical and clinical validations of these new assays are currently in progress. The question is whether the lower detection limit of the troponin assays will be able to indicate myocardial ischemia in the absence of myocardial necrosis. Previous to the development of ultrasensitive cardiac troponin assays free fatty acids unbound to albumin and ischemia modified albumin were proposed as biochemical markers of ischemia. Advances in our knowledge of the pathogenesis of acute coronary thrombosis have stimulated the development of new biomarkers. Markers of left ventricular performance (N-terminal pro-brain natriuretic peptide) and inflammation (e.g. C-reactive protein) are generally recognized as risk indicators. Studies suggest that using a number of biomarkers clinicians can risk stratify patients over a broad range of short and long term cardiac events. Nevertheless, it is still under debate as to which biomarker combination is best preferred for risk prediction. This review will focus on recent practice guidelines for the management of patients with ACS as well as current advances in cardiac biomarkers, their integration into clinical care and their diagnostic, prognostic and therapeutic utility.

High-sensitivity troponin assays in the evaluation of patients with acute chest pain in the emergency department

Evaluating patients with acute chest pain presenting to the emergency department remains an ongoing challenge. The spectrum of etiologies in acute chest pain ranges from minor disease entities to life-threatening diseases, such as pulmonary embolism, acute aortic dissection or acute myocardial infarction (MI). The diagnosis of acute MI is usually made integrating the triad of patient history and clinical presentation, readings of 12-lead ECG and measurement of cardiac troponins (cTn). Introduction of high-sensitivity cTn assays substantially increases sensitivity to identify patients with acute MI even at the time of presentation to the emergency department at the cost of specificity. However, the proportion of patients presenting with cTn positive, non-vascular cardiac chest pain triples with the implementation of new sensitive cTn assays increasing the difficulty for the emergency physician to identify those patients who are at need for invasive diagnostics. The main objectives of this mini-review are 1) to discuss elements of disposition decision made by the emergency physician for the evaluation of chest pain patients, 2) to summarize recent advances in assay technology and relate these findings into the clinical context, and 3) to discuss possible consequences for the clinical work and suggest an algorithm for the clinical evaluation of chest pain patients in the emergency department.

Implementation of high sensitivity cardiac troponin T measurement in the emergency department

BACKGROUND

we examined the diagnostic performance of high sensitivity cardiac troponin T (cTnThs) measurement and its ability to predict risk in unselected patients presenting to the emergency department with acute chest pain.

METHODS

we conducted a retrospective analysis of 137 consecutive patients with chest pain (age range, 66 ± 16 years; 64% male). A final diagnosis of acute myocardial infarction was made using the "old" (cTnT fourth-generation assay, ≥ 0.04 microg/L) or the "new" cutpoint (cTnThs ≥ 0.014 microg/L).

RESULTS

the adjudicated final diagnosis of acute myocardial infarction significantly increased from 20 to 35 patients (a 75% increase) and troponin-positive nonvascular cardiac chest pain from 10 to 30 (a 200% increase) using cTnThs. The number of patients with unstable angina or troponin-negative nonvascular cardiac chest pain significantly decreased (P <.05). Diagnostic performance of cTnThs levels at admission was significantly higher compared to cTnT levels (area under the curve [AUC] 0.85 vs AUC 0.70; P <.05). cTnThs levels below the detection limit (<0.003 microg/L) had a negative predictive value of 100% to exclude acute myocardial infarction. The event rate during 6 months of follow-up was low in patients with cTnThs levels <0.014 microg/L, while patients with cTnT levels ≥ 0.04 μg/L were at increased, and patients with cTnThs ≥ 0.014 μg/L and cTnT <0.04 microg/L at intermediate risk of death or recurrent myocardial infarction (P = .002). Risk was highest in chest pain patients with dynamic changes of cTnThs levels >30%.

CONCLUSION

the introduction of cTnThs assay displays an excellent diagnostic performance for the workup of patients with chest pain at the time of their initial presentation. Even small increases of cTnThs indicate increased risk for death or myocardial infarction during follow-up.

Peak cardiac troponin-T level, scintigraphic myocardial infarct size and one-year prognosis in patients undergoing primary percutaneous coronary intervention for acute myocardial infarction

Current guidelines recommend troponin T (TnT) as the biomarker of choice in the diagnosis of myocardial infarction. In patients with ST-elevation myocardial infarction (STEMI) however, its role in providing a measurement of infarct size and its association with survival is less well established. We sought to assess the correlation of TnT and creatine kinase-MB (CK-MB) with scintigraphically determined infarct size and to assess the predictive value of all 3 parameters on 12-month mortality. Patients presenting with STEMI managed with primary percutaneous intervention underwent serial TnT and CK-MB measurements at admission and for ≥72 hours after presentation. Before hospital discharge patients underwent assessment of infarct size by technetium-99m sestamibi single-photon emission computed tomographic (SPECT) scan. Clinical follow-up was performed up to 1 year. Data were available for 1,237 patients. Mean age was 62.9 ± 12.9 years. Infarct location was anterior in 509 patients (41%); 75 (6.1%) had cardiogenic shock. Median admission and peak TnT were 0.74 μg/L (0.10 to 2.70) and 3.70 μg/L (1.69 to 6.99), respectively. Corresponding values for CK-MB were 44.1 U/L (21.0 to 108.8) and 160.0 U/L (69.0 to 301.0), respectively. Median infarct size on SPECT scan was 12.0% (3.0 to 25.0) of the left ventricle. Peak TnT and CK-MB demonstrated similar moderate correlation with final infarct size (r = 0.45, p <0.001, and r = 0.41, p <0.001 respectively). This correlation was not affected by Thrombolysis In Myocardial Infarction flow grade after intervention. At 1 year, 47 patients (3.8%) had died. Final infarct size at SPECT scanning better predicted mortality than peak TnT or CK-MB. In conclusion, this study is the largest investigation on the value of cardiac troponin for assessment of infarct size in acute STEMI. Compared to peak CK-MB, peak TnT shows similar correlation with scintigraphic infarct size, although scintigraphic infarct size remains a better correlate of 1-year mortality than either biomarker.

An update on cardiac troponins as circulating biomarkers in heart failure

Circulating troponins and natriuretic peptides are the only biomarkers specifically released from cardiac myocytes that can be determined with robust and sensitive analytical methods, even in healthy subjects. These intracellular proteins are released from reversibly or irreversibly damaged cardiac myocytes into the bloodstream by mechanisms that are not entirely clear. The recent introduction of a new generation of highly sensitive assays of cardiac troponin I or T has not only improved the early diagnosis of acute myocardial infarction but also suggested that there are several causes for troponin release other than acute coronary syndromes. Circulating troponins are elevated in patients with acute or chronic heart failure and are strongly associated with outcome, independently of natriuretic peptides, the benchmark biomarkers in heart failure. In the absence of further experimental evidences, the pathophysiologic basis for the elevation of circulating cardiac troponins in patients with stable chronic heart failure remains speculative.

Macroenzyme creatine kinase in the era of modern laboratory medicine

Among the conditions in which creatine kinase (CK)-MB activity is elevated in the absence of myocardial injury or infarction, macroenzyme (macro) CK merits special attention from clinicians. We present 2 cases, 1 with macro CK type 1 and the other with macro CK type 2, to stress the common clinical situations and diagnostic dilemma that clinicians encounter when evaluating patients with macro CK. Moreover, the rare conditions associated with macro CK, and the phenomenon of spuriously high CK-MB activity out of proportion to total CK, are discussed. The biochemical characteristics, clinical significance and potential implications of macro CK are reviewed within the scope of modern laboratory medicine.

Cardiac troponin may be released by ischemia alone, without necrosis

Whilst it is formally stated that cardiac troponin is only released when cardiac myocytes undergo necrosis, there are a number of clinical situations where troponin is present in the circulation, without any apparent cardiac injury. In these cases, troponin half-life in the circulation is usually substantially shorter than that seen when troponin is released following myocardial infarction with frank necrosis. A mechanism has been described in liver, where large cytoplasmic molecules can pass from the intra- to extra-cellular space without cellular necrosis occurring. This occurs by the formation of membranous blebs which bud off from the plasma membrane of the cell. Blebs develop during cellular ischemia. If the ischemia is limited and re-oxygenation occurs, the blebs may be released into the circulation without rupture of the plasma membrane, resulting in a one-off release of cytoplasmic contents including macromolecules. Evidence from cardiac studies is presented supporting the presence of membranous blebs in cardiac myocytes, enabling troponin to be released from cardiac cells due to ischemia alone, without necrosis.

Identification of cardiac myosin-binding protein C as a candidate biomarker of myocardial infarction by proteomics analysis

Acute myocardial infarction (AMI) is a common cause of death for which effective treatments are available provided that diagnosis is rapid. The current diagnostic gold standards are circulating cardiac troponins I and T. However, their slow release delays diagnosis, and their persistence limits their utility in the identification of reinfarction. The aim was to identify candidate biomarkers of AMI. Isolated mouse hearts were perfused with oxygenated protein-free buffer, and coronary effluent was collected after ischemia or during matched normoxic perfusion. Effluents were analyzed using proteomics approaches based on one- or two-dimensional initial separation. Of the 459 proteins identified after ischemia with one-dimensional separation, 320 were not detected in the control coronary effluent. Among these were all classic existing biomarkers of AMI. We also identified the cardiac isoform of myosin-binding protein C in its full-length form and as a 40-kDa degradation product. This protein was not detected in the other murine organs examined, increased markedly with even trivial myocardial infarction, and could be detected in the plasma after myocardial infarction in vivo, a profile compatible with a biomarker of AMI. Two-dimensional fluorescence DIGE of ischemic and control coronary effluents identified more than 200 asymmetric spots verified by swapping dyes. Once again existing biomarkers of injury were confirmed as well as posttranslational modifications of antioxidant proteins such as peroxiredoxins. Perfusing hearts with protein-free buffers provides a platform of graded ischemic injury that allows detailed analysis of protein release and identification of candidate cardiac biomarkers like myosin-binding protein C.