Chapter 9: The Heart

There are many forms of cardiac disease. This chapter briefly covers the role of biomarkers in acute myocardial infarction (AMI) and congestive heart failure (CHF). The large numbers of other cardiac diseases are not discussed in this chapter because of the relatively minor role of diagnostic clinical laboratory tests in these disorders.

Description

The term AMI is defined as an imbalance between myocardial oxygen supply (ischemia) and demand, resulting in injury to and the eventual death of myocytes. AMI should be used when there is evidence of myocardial necrosis in a clinical setting consistent with acute myocardial ischemia. Such necrosis is most often associated with a thrombotic occlusion superimposed on coronary atherosclerosis. It is now apparent that the process of plaque rupture and thrombosis is 1 of the ways in which coronary atherosclerosis progresses. Total loss of coronary blood flow results in a clinical syndrome associated with an ST-segment elevation MI (STEMI). Partial loss of coronary perfusion, if severe, can lead to necrosis as well, which is generally less severe and is known as non-ST-segment elevation MI (NSTEMI). Both STEMI and NSTEMI are considered type 1 MIs. In instances of myocardial injury with necrosis with a condition other than coronary artery disease (CAD), which contributes to an imbalance between oxygen supply and/or demand (eg, coronary endothelial dysfunction, respiratory failure, hypotension, etc), this MI is a type 2 MI that is secondary to ischemic imbalance. Other ischemic events of lesser severity without myocardial necrosis are designated as angina, which can range from stable to unstable. About 1.7 million patients are hospitalized each year in the United States with an acute coronary syndrome (ACS). Approximately 700,000 patients suffer from an initial AMI annually and another 500,000 from a recurrent AMI. Coronary heart disease causes 20% of all deaths and cardiovascular diseases up to 40%. Historically, most deaths caused by ischemic heart disease have been acute, but as our therapeutic abilities have improved, the disease is slowly becoming a more chronic one.

In many patients with AMI, no precipitating factor can be identified. The clinical history remains of substantial value in establishing a diagnosis. A prodromal history of angina can be elicited in 40% to 50% of patients with AMI. Of the patients with AMI presenting with prodromal symptoms, approximately one third have had symptoms from 1 to 4 weeks before hospitalization; in the remaining two thirds, symptoms predate admission by a week or less, with one third having had symptoms for 24 hours or less. The pain of AMI is variable in intensity, and the discomfort is described as a squeezing, choking, vise-like, or heavy pain. It may also be characterized as a stabbing, knife-like, boring, or burning discomfort. Often the pain radiates down the left arm. In some instances, the pain of AMI may begin in the epigastrium and simulate a variety of abdominal disorders, which often causes AMI to be misdiagnosed as indigestion. In other patients, the discomfort of AMI radiates to the shoulders, upper extremities, neck, and jaw, again usually favoring the left side.

Diagnosis

The ideal biomarker of myocardial injury should 1) provide early detection of myocardial injury, 2) provide rapid, sensitive, and specific diagnosis for an AMI, 3) serve as a risk stratification tool in ACS patients, 4) assess the success of reperfusion after thrombolytic therapy, 5) detect reocclusion and reinfarction, 6) determine the timing of an infarction and infarct size, and 7) detect procedural-related perioperative MI during cardiac or noncardiac surgery. In reality, no 1 biomarker is able to 100% cover all these areas. However, cardiac troponin (cTn) does provide the power to be utilized in the majority of these clinical areas. Ruling in AMI requires a test with high diagnostic sensitivity (preferred by the ER physician in the urgent care, emergency setting as to not send anyone home with an AMI), whereas ruling out AMI requires a test with high diagnostic specificity (preferred by the cardiologist following admission to avoid excessive and costly diagnostic evaluations in the non-AMI patient). It is the function of the laboratory to provide advice to physicians about cardiac biomarker/troponin characteristics.

Until 2000, the diagnosis of AMI established by the World Health Organization (WHO) required at least 2 of the following criteria: 1) a history of chest pain, 2) evolutionary changes on the ECG, and/or 3) elevations of serial cardiac biomarkers (total creatine kinase [CK] and CKMB). It was rare for a diagnosis of AMI to be made in the absence of biochemical evidence of myocardial injury. A 2000 ESC/ACC consensus conference, updated in 2007 and most recently in 2012 by the “Global Task Force for the Third Universal Definition of MI,” has codified the role of biomarkers. The advocate that the diagnosis be made from evidence of myocardial injury based on biomarkers of cardiac damage, preferably cTnI or cTnT, in the appropriate clinical situation of ischemic symptoms (Table 9–1). This guideline does not suggest that all increases of cTn should elicit a diagnosis of AMI, but only those associated with the appropriate clinical, ECG, and imaging findings. When cTn increases that are not caused by acute ischemia occur, the clinician is obligated to search for another etiology for the elevation, a number of which are shown in Table 9–2. The initial ECG is diagnostic of AMI in about 30% of AMI patients. Further, the universal classification of different types of myocardial infarction is highlighted in Table 9-3.

cTn testing is most useful when patients are having nondiagnostic ECG tracings. Patients with AMI can be categorized into several groups based on time of presentation. First, there is the group of patients who present very early to the emergency department (ED), within 0 to 4 hours after the onset of ischemic symptoms that include chest pain, without diagnostic ECG evidence of AMI. For laboratory tests to be clinically useful, biomarkers (cTn) of MI must be released rapidly from the heart into the circulation to provide sensitive and tissue-specific diagnostic information. Further the analytical assays using serum, plasma, or whole blood specimens must be rapid and sensitive enough to distinguish small changes within the reference interval. The second group includes those presenting 4 to 48 hours after the onset of ischemic symptoms, without evidence of AMI on ECG. In this group of patients, the diagnosis of AMI requires serial monitoring of both cTn and ECG changes. The third group presents more than 48 hours after the onset of symptoms of ischemia with nonspecific ECG changes. The ideal biomarker, again cTn, of myocardial injury in this group would persist in the circulation for several days to provide a late diagnostic time window. The shortfall of such a marker might be its inability to distinguish recurrent injury from old injury, thus the importance of following a rising or falling pattern. The fourth group includes those who present to the ED at any time after the onset of ischemic symptoms with clear ECG evidence of AMI, either a STEMI or Q-wave MI. In this group, detection with biomarkers of myocardial injury is theoretically not necessary.

Cardiac Troponin

The contractile proteins of the myofibril include the regulatory protein troponin. Troponin is a complex of 3 protein subunits, troponin C (the calcium-binding component), troponin I (the inhibitory component), and troponin T (the tropomyosin-binding component) (TIC). The subunits exist in isoforms distributed between cardiac muscle and slow and fast twitch skeletal muscle. Troponin is localized primarily in the myofibrils (94%–97%), with a smaller cytoplasmic fraction (3%–6%). cTn subunits I and T have different amino acid sequences encoded by different genes allowing for their cardiac tissue specificity. Following myocardial injury, multiple forms are elaborated both in tissue and in blood. The multiple forms of cTnI include the T–I–C ternary complex, IC binary complex, and free I. Multiple chemical modifications of these 3 forms can occur, involving oxidation, reduction, phosphorylation and dephosphorylation, and both C- and N-terminal degradation. The conclusions from these observations are that cTn immunoassays need to be developed in which the antibodies recognize epitopes in the stable region of cTnI and, ideally, demonstrate an equimolar response to the different cTnI forms that circulate in the blood.

Analytical Methods for Measuring Cardiac Troponin

Over the past 20 years, numerous manufacturers have developed monoclonal antibody-based diagnostic immunoassays for the sensitive measurement of cTnI and cTnT. Assay times range from 5 to 30 minutes. Table 9–4 shows analytical characteristics of representative assays approved by the FDA for patient testing. In clinical practice, 2 obstacles limit the ease for switching from 1 cTnI assay to another. First, there is currently no primary reference cTnI material available for manufacturers to use for standardizing assays. Second, concentrations fail to agree because of the different epitopes recognized by the multiple, different antibodies used in different assays. Therefore, standardization of cTnI assays remains elusive. For cTnT, there is only 1 manufacturer. Therefore, there are no standardization problems. In 2012, the IFCC Task Force on Clinical Applications of Cardiac Biomarkers readdressed quality specification aspects for cTn assays. These specifications were intended for use by the manufacturers of commercial assays and by clinical laboratories using cTn assays to establish uniform criteria so that all assays could be evaluated objectively for their analytical qualities and clinical performance. Factors addressed included: antibody selection, calibration materials, imprecision characteristics at clinical decision values, effects of storage time and temperature, glass versus plastic tubes versus gel separator tubes, the influence of anticoagulants, and whole blood measurements.

99th Percentile Reference Value as a Cutoff for Diagnosis of Acute Myocardial Infarction

The Global Task Force's 2012 “Third Universal Definition of Myocardial Infarction” guideline was predicated on cTn monitoring, with detection of a rising and/or falling cTn, and with at least 1 value above the 99th percentile value. Using the 99th percentile value (compared with the older WHO criteria) has demonstrated an increase in the number of MIs in day-to-day clinical practice, EDs, epidemiologic studies, and clinical trials. The data suggest that the more analytically sensitive cTn tests result in greater rates of MI diagnosis and greater rates of cTn positivity compared with the older biomarker CKMB. Milder and smaller MIs are detected. Clinical cases prior to 2007 that were earlier classified as unstable angina are given a diagnosis of MI because of an increased and rising cTn. Further, procedure-related troponin increases (ie, following angioplasty) will be labeled MI (Table 9–3). The importance of small troponin increases has been confirmed by their association with a poor prognosis.

Several biomarkers should no longer be used to evaluate cardiac disease. They include aspartate aminotransaminase (AST), total CK activity, CKMB isoforms, myoglobin, total lactate dehydrogenase (LD), and LD isoenzymes. These markers have poor specificity for the detection of cardiac injury because of their wide tissue distribution. Further, CKMB is no longer a recommended biomarker, and is suggested for clinical use only when cTn assays are not available. CKMB offers no additional diagnostic value to aid in the timing of the onset of myocardial injury, infarct sizing, or determination of reinfarction. There is no evidence to support dual testing for cTn and CKMB.

Role of Cardiac Troponin for Risk Outcomes Assessment

Patients With Ischemia

In the environment of preventive and evidence-based medicine, the use of cTnI or cTnT measured in patients with ischemia will allow clinicians to use biomarkers as both exclusionary and prognostic indicators. The results will assist in determining who is more at risk for AMI and death, and thereby determine who may benefit from early medical or surgical intervention. Such patients benefit from the use of anticoagulant therapy and the use of platelet antagonists, and an early invasive strategy. The goal of monitoring cardiac biomarkers in patients suggestive of ACS with and without AMI would be to effectively identify patients with unstable coronary disease and triage them to an appropriate therapeutic regimen. Optimal use of this strategy requires at least 2 blood samples for cTn measurement. General population screening of hospitalized patients with cTnI or cTnT is not recommended at present.

Patients With Nonischemic Presentations

Clinicians are often confronted with a clinical history of a patient without overt CAD and a low probability of myocardial ischemia. However, as a precautionary measure, serial cTns are ordered. A typical serial order set to rule in or rule out an AMI would include blood draws at 0 hour (presentation), 3, 6, and 9 to 12 hours. When 1 or 2 of the serial cTn concentrations are found to be increased, the clinician would likely be confronted with the following concerns: 1) What does the increase mean in the clinical setting of a nonischemic patient? 2) Is the increase a false-positive finding resulting from an analytical error? 3) Why was the test ordered in the first place? As cTn assays with increasing low-end analytical sensitivity (high-sensitivity [hs] cTn assays) have been developed (currently not FDA cleared for use in the United States), the ability to detect minor degrees of myocardial injury in a variety of clinical conditions has widened and has led to a better understanding that cTn is not just a biomarker for MI, but a sensitive biomarker for myocardial injury. The 20% of suspected ACS patients who clinically do not rule in for MI, but display an increased cTn, represents patients with nonischemic pathologies (Table 9–2) in whom the mechanisms of injury are well defined (such as myocarditis, blunt chest trauma, and chemotherapeutic agents), and patients with increased cTn, in whom the mechanism of injury is not clear.

Orders for Serial Cardiac Troponin Testing

Blood samples should be drawn at presentation (0 hour) to the hospital (often this is hours after the index clinical symptom onset) and at least once more at 6 to 9 hours later. As noted, a typical serial order set to rule in or rule out an AMI would include blood draws at 0 hour (presentation), 3, 6, and 9 to 12 hours. Occasionally a patient may require a 12- to 24-hour sample, if the earlier measurements are normal, but the clinical suspicion of AMI is high. As the cTn concentration may remain increased 3 to 12 days after an AMI, after 2 positive values with a rising pattern, it does not appear cost-effective to continually monitor cTn once a diagnosis is established. In patients where recurrent MI is suspected from clinical signs or symptoms following the initial MI, an immediate remeasurement at the time of a suspicious new event (0) and 3-, 6-, and 9-hour serial blood samples are recommended. It is reasonable to suspect recurrent infarction if there is a >20% increase in the second value as long as it exceeds the 99th percentile.

High-sensitivity Cardiac Troponin Assays

It is important to understand that the term “high sensitivity” (hs) reflects the assay's characteristics and does not refer to a difference in the form of cTn being measured. There is a need for a consensus on defining what nomenclature should be used for an hs assay. Several names have been used in the literature for these assays, but the term “high sensitivity” has been recommended by expert opinion. This term, however, begs the question: how does one define an hs assay? In a scorecard concept (Table 9–5), an assay is designated hs if it meets 2 basic criteria. First, the total imprecision (CV) at the 99th percentile value should be ≤10%. Second, measurable concentrations below the 99th percentile (the upper limit of normal) should be attainable with an assay at a concentration value above the assay's limit of detection for at least 50% of healthy individuals. None of the current US-marketed assays for both central laboratory and point-of-care testing meet the 2-fold hs criteria. Concentrations for hs assays are expressed in nanograms per liter instead of the commonly published units of micrograms per liter.

For deriving normal reference 99th percentile cutoffs for cTn assays, it is recommended that inclusion criteria be based on data obtained from an interview for a history of medications and known underlying disease, as well as a blood measurement of a natriuretic peptide (NP; N-terminal pro–B-type natriuretic peptide [NT-proBNP] or B-type natriuretic peptide [BNP]), interpreted vis-à-vis a cutoff value for the exclusion of ventricular dysfunction to serve as a surrogate biomarker for underlying myocardial dysfunction, and an estimated GFR to exclude renal disease. In addition, the groups should be split equally by sex and include a diverse racial and ethnic mix. Literature now supports reporting of gender-specific cutoffs for hs assays, as men demonstrate approximately 2-fold higher 99th percentiles compared with women.

With improved analytical sensitivity, hs assays have been shown to provide an earlier diagnosis, with the ability to rule in and rule out by 3 hours instead of 6 hours with contemporary assays. However, with increased clinical sensitivity, with the ability to detect smaller myocardial injuries from multiple, pathological etiologies, decreased clinical specificity, below 80%, occurs. Early studies have now demonstrated that the use of a delta change in cTn concentration over a 0- to 3-hour serial time window allows for the ability to separate acute injury, that is, AMI, from a chronic injury, such as heart failure, with improved clinical specificity up to 95%. This will be an important tool to use for clinical care when hs assays are cleared for use in the United States some time in 2014.

Description

CHF is a condition in which there is ineffective pumping of the heart leading to an accumulation of fluid in the lungs. Typically, it results from a loss of cardiac tissue and subsequent function. It is defined as the pathophysiological condition in which an abnormality of cardiac function is responsible for the failure of the heart to pump sufficient blood to satisfy the requirements of the metabolizing tissues. In the United States, CHF is the only cardiovascular disease with an increasing incidence. The National Heart, Lung, and Blood Institute estimates that current prevalence is about 5 million Americans with CHF, with an incidence of approximately 400,000 new cases each year. CHF is the leading cause of hospitalization in individuals 65 years and older. Current prognosis is dependent on disease severity, but overall it is poor. The 5-year mortality is approximately 10% in mild CHF, 20% to 30% in moderate CHF, and up to 80% in end-stage disease.

Diagnosis

Natriuretic Peptides in Monitoring CHF

Two biomarkers have been well studied to assist in these clinical settings: BNP (pharmacologically active hormone) and NT-proBNP (not pharmacologically active peptide). Both blood peptides are derived from cleavage of the myocardial proBNP peptide following myocardial stress and/or fluid overload. In general, the clinical evidence for utilization of either biomarker is very similar, but each has subtle analytical and physiological differences, depending on the pathophysiology of an individual patient. In the course of this chapter, both NPs are used interchangeably unless specifically noted.

The ACC/AHA practice guidelines for the evaluation and management of CHF indicate that the role of NP in the identification of CHF patients remains to be clarified. In contrast, the ESC has incorporated monitoring NPs into their practice algorithm at the time of patient presentation alongside the clinical history, physical examination, ECG, and chest x-ray. An abnormal NP finding would trigger an echocardiogram or other imaging modality. NP concentrations in patients diagnosed with CHF are substantially increased (>1000 ng/L for BNP or >1800 NT-proBNP) when compared with patients who have minor increases (<300 ng/L) because of left ventricular (LV) dysfunction without acute CHF. CHF is more common in patients with advanced chronic renal disease. BNP and NT-proBNP are secreted in a pulsatile fashion from cardiac ventricles with an approximate half-life for BNP of 22 minutes in blood, with the NT-proBNP half-life on the order of 2 hours. While 1 mechanism of BNP clearance involves the renal parenchyma, the kidney is not thought to be the primary site for BNP clearance. The kidney more specifically affects NT-proBNP clearance. Thus, increases in BNP in hemodialysis patients are thought to represent both regulatory responses from the cardiac ventricle, resulting from increased wall tension, and a lack of renal clearance.

Importantly, NPs are not 100% specific for CHF. Increases have been described for other non-CHF etiologies involving filling pressure defects, including LV hypertrophy, inflammatory cardiac diseases, systemic arterial hypertension, pulmonary hypertension, acute and chronic renal failure, liver cirrhosis, and several endocrine disorders (eg, hyperaldosteronism and Cushing syndrome). In CHF patients presenting to the ED, patients admitted tend to have higher BNP concentrations (>500 ng/L) versus those who are discharged (mean <300 ng/L) at triage. Linear relationships with increasing BNP/NT-proBNP levels and the severity of CHF (NYHA classification I-IV) have been described. The largest prospective trial to date to evaluate the diagnostic value of BNP is “The Breathing Not Properly Multicenter Study,” from which the level of BNP was found to be an independent predictor of CHF. Using a blood BNP cutoff concentration of 100 ng/L for CHF, there was a 90% clinical sensitivity and 75% clinical specificity, with an 81% accuracy. Without BNP monitoring, clinical judgment and traditional diagnostic methods demonstrated a diagnostic accuracy of only 74%. The knowledge of BNP reduced the proportion of patients in whom the clinician was uncertain of the diagnosis from 43% to 11%. Plasma BNP monitoring in the ED improved the treatment and evaluation of patients with early dyspnea, reducing the time to discharge and total cost of treatment. Similar data have been shown for the alternate biomarker NT-proBNP. After an AMI, NP increases in proportion to the size of the infarction, prompting investigators to explore the role of screening BNP for detection of LV dysfunction. In post-MI patients, BNP concentrations are inversely correlated with LV ejection fraction. However, there is inconclusive evidence for the role of BNP screening for asymptomatic LV dysfunction in the general population. In general, there does not appear to be a distinct advantage to use 1 biomarker (BNP or NT-proBNP) over the other in clinical practice.

Blood NP monitoring can be valuable in the diagnostic setting, where it will possibly improve the performance of nonspecialist clinicians in diagnosing CHF. In clinical practice, NP monitoring can best be used as a “rule-out” test for suspected cases of new CHF. It is not a stand-alone test and should not be a replacement for a full clinical assessment, including an echocardiogram when indicated. In the presence of a normal BNP or NT-proBNP, a diagnosis of CHF is highly unlikely if concentrations are <100 ng/L for BNP or <300 ng/L for NT-proBNP. Monitoring NP may be useful in 1) guiding therapy, 2) monitoring the course of the disease, and 3) providing useful risk stratification information. NPs have been shown to be an independent predictor of cardiovascular mortality in patients with both CHF and ACS over a 1-year period. Further, BNP or NT-proBNP monitoring may assist in identifying patients with a lower risk of readmission within the next 30 days before discharge.

Analytical Methods for Measuring Natriuretic Peptides

Table 9–6 shows the current FDA-approved assays for BNP or NT-proBNP. The commercial assays differ in standardization of measurements and antibodies used in the assay. Assays that use an antibody that recognizes the N-terminus labile region of BNP (eg, Biosite, Beckman, and Abbott) demonstrate less analyte stability at room temperature (24 hours) than assays that use 1 of their antibodies recognizing the C-terminus (eg, Siemens [Bayer]). The Roche NT-proBNP antibody configuration allows for 72 hours of sample stability at room temperature.

Reference Intervals: Medical Decision Cutoff Values

A number of clinical factors affect the BNP and NT-proBNP concentrations, most importantly age, gender, obesity, and renal function. Significant differences are observed between men and women (higher), and there are increasing concentrations with age by decade. For BNP and NT-proBNP, the significance of the results for these assays in relation to the degree of left ventricle dysfunction remains a debate. For both analytes, there is an inverse relationship between values and body mass index. For NT-proBNP, establishing reference intervals has been challenging. Review of both the FDA-approved US package insert and the European assay package insert reveals substantial differences in what concentrations are considered normal by age and sex. For BNP, a cutoff of 100 ng/L has been endorsed as demonstrating optimal sensitivity and specificity. For NT-proBNP, the FDA-approved package insert describes a 2-tier cutoff by age at <75 years: 125 ng/L and >75 years: 450 ng/L. However, more evidence-based cutoffs have been derived from the PRIDE/ICON studies based on age and renal function, and are recommended as follows—age <50 years: >450 ng/L; age ≥50 years: >900 ng/L; all ages: best negative predictive value <300 ng/L; age <50 years and eGFR >60 mL/min: 450 pg/mL, and eGFR ≤60 mL/min: 1800 ng/L; age ≥50 years and eGFR >60 mL/min: 900 pg/mL, and eGFR ≤60 mL/min: 1800 ng/L.

Implications for Therapy Using Test Results for Natriuretic Peptides

The utility of serial measurements of NPs in guiding therapy for chronic heart failure has been the subject of numerous randomized controlled trials reported in the literature since 2000. The existing trial data suggest that adjustment of treatment in chronic heart failure according to NP measurements, used in conjunction with established clinical treatments, is likely to reduce cardiac mortality and hospital admissions with heart failure, at least in patients with systolic heart failure who are younger than 75 years and relatively free of comorbidities.

Biological Variability

As BNP and NT-proBNP become more widely used to monitor CHF patients following therapy, questions have addressed the usefulness of serial monitoring in assisting the success of drug therapy. In a study of 11 patients with CHF, the biological variation for BNP and NT-proBNP was evaluated using 4 different assays. The findings indicated that a change of 130% for BNP and 90% for NT-proBNP is necessary before results of serially collected data can be considered clinically and statistically significant. For example, these findings imply that a decrease from approximately 500 to 250 ng/L would be necessary for a clinician to conclude that therapy was successful in improving CHF features. Clinicians without this knowledge may inappropriately assume that a decrease from an admission BNP value of 500 ng/L to a 24-hour post-admission value of 400 ng/L may have been a result of successful patient management. It has been suggested that following the admission BNP value, a second BNP value be obtained within 24 hours of discharge to optimize the diagnostic utility of BNP in the overall assessment of patients with CHF.

Novel Biomarkers for Heart Failure Risk Assessment

In addition to the advances in the understanding of established NP biomarkers in HF, there is an increased study of the elucidation of novel biomarkers potentially useful for the evaluation and management of patients with HF, and the growing understanding of important and relevant comorbidities in HF. Literature on candidate biomarkers from a number of classes will be growing over the next several years and include: a) myocyte stretch (with assays for ST2, GDF-15), b) myocyte necrosis (with hs-cTn assays), c) systemic inflammation (with assays for LP-PLA2), d) oxidative stress (with assays for MPO), e) extracellular matrix turnover (with assays for collagen propeptides), f) neurohormones (with assays for chromogranin A), and g) biomarkers of extracardiac processes, such as renal function (with assays for NGAL).