PART II - CARDIAC BIOMARKERS
TABLE OF CONTENTS
BOOKSHOP
Updated on December 17, 2019
PART II

Cardiac biomarkers

Evangelos Giannitsis1, Christian Hamm, Holger Nef, Hugo A. Katus1
1. Medizinische Klinik Ill, Universitatsklinikum Heidelberg, Germany

ADDENDUM: Periprocedural myocardial infarction and injury

Published on October 8, 2021

A report from Piera Capranzano

Several definitions of peri-procedural myocardial infarction (PMI) occurring after coronary bypass grafting (CABG) and percutaneous coronary intervention (PCI) have been adopted (Table). Two PMI definitions (SYNTAX and the Fourth Universal Definition of Myocardial Infarction [UDMI]) combine cardiac biomarkers elevation with evidence of new myocardial damage, whilst the other three definitions (ISCHEMIA, SCAI, and EXCEL) consider only cardiac biomarkers elevation as sufficient condition. The association of each of these definitions with clinical outcomes has been tested in the SYNTAXES (Synergy between PCI with Taxus and Cardiac Surgery Extended Survival) trial1. PMI rates were higher when using SCAI and EXCEL definitions, especially in the CABG arm, but had no association with all-cause mortality after CABG at 1 and 10 years. Differently, PMI defined by SYNTAX, Fourth UDMI, and ISCHEMIA definitions was associated with all-cause mortality at 1 year in the CABG arm. PMI in patients undergoing PCI is significantly correlated with higher all-cause mortality at 1 and 10 years irrespective of the definition.

A recent European consensus suggests that rising values of cardiac biomarkers (cTn elevation >5x 99th percentile Upper Reference Limit) post-PCI must be associated with evidence of new myocardial damage to support the diagnosis of PMI, otherwise the diagnosis of major periprocedural myocardial injury applies2. A recent metanalysis has demonstrated that both PMI and major procedural myocardial injury are associated with 1-year mortality after PCI3.

1. Hara H et al. J Am Coll Cardiol 2020;76:1622-1639. 2. Bulluck H et al. Eur Heart J 2021;42:2630-2642. 3. Silvain J et al. Eur Heart J 2021;42:323-334

Definitions of PMI

Definitions of PMI

Summary

Measurement of serological biomarkers before, during and after percutaneous coronary interventions (PCI) for chronic or acute coronary artery disease (CAD) allows for insight into various complex pathophysiological processes which are involved in the progression of atherosclerosis and complications of PCI. Key to the proper use of biomarkers is an understanding of the fundamental principles of atherosclerosis and the mechanisms potentially involved in the transition from chronic disease to spontaneous plaque disruption, as well as the mechanisms behind mechanically induced vessel wall trauma after balloon angioplasty and coronary stenting.

Cardiac troponins are well established for the diagnosis of spontaneous MI and subtypes of MI, including type 4 MI (post-PCI), although there is still controversy regarding the magnitude of concentration rise that is prognostically relevant. New and more sensitive assay generations have been introduced to facilitate earlier and more accurate diagnosis of non-STEMI. These high sensitive cardiac troponin (hs-cTn) assays raise the question of the need for additional markers of early necrosis, such as myoglobin, ischaemia-modified albumin or heart-type fatty acid binding protein. Other biomarkers confer independent information or add to the diagnostic and prognostic information already provided by cardiac troponins. There are several promising biomarker candidates, but all additional biomarkers need further evaluation to prove their usefulness in clinical practice.

Biomarkers in acs

PATHOPHYSIOLOGY OF ACS

The aetiology of acute coronary syndromes (ACS) is complex and consists of multiple inter-related mechanisms, many of which are not yet fully understood. Our current understanding is that a plaque may rupture or erode in response to inflammation, leading to local occlusive or non-occlusive thrombus [1].

Depending on the degree and reversibility of this dynamic obstruction, the clinical manifestations of ACS comprise a continuous spectrum of risk that progresses from unstable angina (UA) to non-ST-elevation myocardial infarction (non-STEMI) and ST-elevation myocardial infarction (STEMI). Non-STEMI is distinguished from UA by ischaemia sufficiently severe in intensity and duration to cause myocyte necrosis, which is recognised by the detection of troponin. This molecule is considered the preferred biomarker for detection of myocardial infarction due to its absolute cardiospecificity and superior sensitivity compared to cardiac enzymes or protein mass of CKMB [2].

Cardiac troponin is composed of three subunits: T, I, and C, which are the products of different genes. The total mass of the troponin complex is minuscule as compared to the protein mass of other myofibrillar proteins like actin and myosin. However, both troponin T and I are ideally suited for the detection of myocardial damage as they are expressed as cardio-specific isoforms, which are encoded by separate cardiac troponin T and I genes. The vast majority of the troponin complex is immobilised in the sarcomere of striated muscle and only a minor fraction exists as a soluble pool, which eventually represents a precursor pool of sarcomere assembly [3].

In patients with myocardial infarction (MI) distinct release kinetics have been described, with a rapidly appearing first peak resulting from the loosely bound troponin pool and a second long lasting elevation due to degradation of the contractile apparatus [3]. A biphasic pattern has been observed for cTnT whereas a monophasic release has been demonstrated for cTnI [3]. The exact reason for this difference in release kinetics remains elusive. cTnT differs from cTnI with respect to higher molecular weight, with a higher fraction of unbound cTnT and less degradation, whereas cTnI is more frequently found as binary or tertiary complexes in blood ( Figure 1 ). There is evidence that the early appearing pool may give information on the quality of micro-vascular reperfusion, while the concentration of cTn on day 3 or 4 reflects myocardial infarct size [4]. Experimental data strongly suggest that troponin leaks out of the cell only after membrane disruption following myocardial cell death [3]. The detection of a brief rise and subsequent fall of troponin concentration during marathon running [5] and a rise after inducible myocardial ischaemia [6] has cast some doubts on the hypothesis that troponin is released only upon irreversible damage. However, at this time there are neither consistent experimental nor clinical data providing proof of this concept.

Novel biomarkers of inflammation, activation of coagulation, myocyte necrosis, vascular damage and haemodynamic stress enable a more differentiated characterisation of the individual components as well as their contribution at different stages of the disease, giving insights into the local consequences of percutaneous interventions in acute and stable coronary artery disease.

DIAGNOSIS OF MI ( Table 1 )

According to the 4th of the Joint ESC/ACCF/AHA/WHF Task Force universal infarct definition [7], the diagnosis of MI is based on the presence of:

  1. an elevated cardiac troponin, which is considered the preferred biomarker indicating myocyte necrosis,
  2. a dynamic change of biomarker concentration suggesting an acute rather than a chronic myocardial damage, and
  3. the clinical context of acute myocardial ischaemia [7].

CK or CKMB is not longer recommended, neither for the diagnosis of MI nor for the diagnosis of early re-infarction where diagnosis should be based on serial troponin measurements [8, 9].

There is a consensus recommendation [10] on the magnitude that defines a relevant rise and/or fall. If the baseline cTn is still below the upper limit of normal, i.e. the 99th percentile value of a healthy reference population, an acute myocardial injury can be assumed if the second troponin value exceeds the ULN and there is a change of 50% or more of the upper limit of normal. If the baseline value is already above the ULN a change of 20% or more from the baseline value is sufficient to diagnose an acute myocardial injury. The latter recommendation is based on a recommendation from the National Academy of Clinical Biochemistry (NACB) for patients with end-stage renal disease (ESRD) patients, and for all patients who may have baseline elevations of cTn, presenting with possible ACS, dynamic changes in the cTn values of 20% or more should be used to define AMI . Implementation of serial concentration changes is paramount to distinguish acute from stable cTn level. Recent observations, however, have suggested that absolute changes may have greater diagnostic utility [7].

2015 ESC guidelines on NSTE-ACS (9) recommend the use of high sensitivity troponin assays. If available, repeat sampling of hsTn after 3 hours is recommended instead of re-testing after 6 to 9 hours. If a hsTn assay is available that has been validated for this purpose, re-testing after 1 or 2 hours is recommended as an alternative. Additional testing after 3–6 h is indicated if the first two troponin measurements are not conclusive and the clinical condition is still suggestive of ACS. Furthermore, a rule-out strategy using a single low troponin value below the 99th percentile together with a normal copeptin value at baseline has been recommended for rule-out of MI. Beyond the ability to rule-out an MI based on a single blood draw at presentation, this strategy was found to identify low to intermediate risk patients who can be directly discharged home after rule-out without an excess of major cardiac events compared to usual care [11, 12].

Besides definition of type 1, type 2 and type 3 MI, the UDMI provides additional subtypes of MI including procedure-related MI following PCI or CABG ( Table 2)

SPONTANEOUS VERSUS PERIPROCEDURAL MI ( Table 3 )

The universal definition of MI (7) has introduced a specific category, type 4a for periprocedural MI to distinguish it from spontaneous MI, type 1 and type 2.

This discrimination appears increasingly important as recent ACS trials found that PMI occurred more often (>2 times) but was less prognostically important than spontaneously occurring MI after adjustment for baseline and procedural characteristics [13].

The definition of infarct subtypes has changed over time. The most recent 4th version of the Universal MI definition of postprocedural MI has slightly changed compared to the 3rd version of the UDMI [14]. In particular, More details on the definition of type 4a postprocedural MI and the discrimination from postprocedural myocardial injury are provided in subsequent paragraphs.

NON-ACS RELATED TROPONIN ELEVATION

Troponin is released following irreversible myocardial damage and not solely due to myocardial ischaemia. Thus, cardiac troponins (cTn) are tissue specific, indicating myocyte death, but not specific for ACS [15]. Other frequent mechanisms of cardiac troponin release include tachy- and bradyarrhythmia, pulmonary embolism, inflammatory processes such as myocarditis, toxic agents or mechanical trauma ( Table 4 ). A typical reason for chronically elevated cardiac troponin in asymptomatic patients is end-stage renal failure [15].

Patients with elevated cTn values should be followed closely since an elevated cTn indicates an adverse prognosis in almost all non-ACS pathologies. Some of these pathologies may also show a rising and/or falling pattern of cTn values indicating the presence of an acute myocardial injury [7]. Patients without a changing pattern should be labelled as chronic myocardial injury including conditions such as unstable angina with underlying structural heart disease in the presence of acute ischemic symptoms, or stable CAD in the absence of acute or worsening ischemic symptoms, or chronic renal failure, chronic heart failure, and severe left ventricular hypertrophy, irrespective of the presence or absence of symptoms.

The introduction of more sensitive cardiac troponins (hsTn) has the advantage of detecting MI earlier and to identify a higher number of MI at decreasing numbers of unstable angina [16]. However, the most relevant obstacle for clinicians is that clinical specificity decreases as sensitivity increases [15]. This phenomenon leads to an increased prevalence of troponin-positive patients. In this scenario, the clinical challenge is to discriminate ACS from a wide range of differential diagnoses associated with acute or chronic myocardial damage, particularly in the symptomatic patient.

FOCUS BOX 1What does a raised troponin mean?
  • Elevations of cardiac troponin indicate the presence of irreversible myocardial damage and are not specific for acute myocardial infarction where the mechanism is myocardial ischaemia
  • A troponin result should not therefore be interpreted without the clinical context and the kinetic change during serial sampling

CONVENTIONAL cTn ASSAYS

There are many commercially available cTnI assays but, due to patent limitations, only one cTnT assay facilitating standardization of cTnT while there is an unresolved issue with standardization of cTnI.. All available cTnI assays vary with respect to the epitope the detecting antibody is directed against. In addition, there are several differences regarding the molecular weight, the predominant form of release and molecule stability at room temperature and after freezing. For conventional assays true false positives are very rare in the absence of heterophilic antibodies or severe haemolysis which may interfere with the troponin assay to cause either overestimation or underestimation of the true troponin concentration. Very high doses of biotin might also interfere with detection antibodies and result in underestimation of cTn concentrations [17]. In contrast to cTnT, cardiac troponin I is more susceptible to the interference with autoantibodies, and more prone to oxidation, reduction and phosphorylation which may affect epitope recognition and thus blood concentrations [18]. In addition, prevalence of cTnT is considerably higher among patients with endstage renal failure than cTnI. In clinical routine, however, the diagnostic and prognostic performance of cTnT and cTnI are comparable, provided the test quality of the particular cTnI assay complies with quality specifications and has been validated in clinical trials. Among patients with ESRD, cTnT gives stronger prognostic information on outcomes than cTnI and should be preferred for this purpose. Despite enormous efforts to establish a reference material, a major obstacle in clinical routine using cTnI is the lack of standardisation between assays [18].

The method of choice is quantitative measurement of cTn in the central laboratory. However, if results for cTn cannot be reported within 45 to 60 minutes after blood draw, the use of a quantitative point-of-care (POC) test is recommended for rule-in. Unfortunately, most POC test systems lack adequate analytical sensitivity and precision at the low end of the concentration range and bear the potential to miss a considerable number of small infarcts. To date, only one POC Troponin I assay (Mitsubishi Pathfast) has received clearance by the FDA to be labelled a high sensitivity assay [19].

HIGH-SENSITIVITY CARDIAC TROPONINS

For more than 10 years more sensitive, high-sensitive and ultrasensitive cTn assay generations have become available [10]. These assays are characterised by higher analytical sensitivity and improved analytical precision and now allow detection of manifold lower cTn concentrations and meet the precision criteria. These high-sensitivity assays (hsTn) allow measurement of picograms rather than nanograms per litre, and are now able to detect cardiac troponin in most healthy individuals. An updated IFCC/AACC statement [20]based on a previously developed scorecard classification [21] provides criteria that qualify a cTn assay to be designated a hsTn. Accordingly, a hsTn assay must meet the precision criteria of less than 10% imprecision at the 99th percentile value of a reference population and must be able to detect concentrations above the LoD in at least 50% of men and women.

Specificity and “false-positives”

In clinical practice, improved sensitivity has been shown to allow a more accurate and earlier detection of MI, and will increase the number of cases of MI at the expense of unstable angina [8, 10, 16]. However, lowering the diagnostic cut-off will also decrease clinical specificity as more non-ACS related acute, subacute, or chronic cardiac diseases - such as heart failure or cardiomyopathies - will be detected [7, 15]. Frequently, such patients are inappropriately labelled as “false-positives” and diagnostic workup is stopped after a significant coronary artery disease has been ruled out invasively or non-invasively. In order to increase clinical specificity for MI diagnosis, adherence to the clinical criteria of the universal definition of MI is paramount. In addition, dynamic changes of cTn provide some information on acuity of disease and may help to distinguish acute from chronic myocardial damage. Until recently, the magnitude of such a rise and/or fall indicative of AMI in patients with ACS was unsettled. More recently, a consensus group from the ESC acute cardiac care provided recommendations on the magnitude of a relevant rise and/or fall [10].

The magnitude of cTn may give some indirect information on the cause of elevation ( Figure 2 ).

Accordingly, small elevations may be seen with numerous cardiac and extra-cardiac pathologies, whereas very high concentrations are less likely with conditions other than myocardial infarction [22].

Earlier diagnosis and the role of clinical risk scores

Use of more sensitive cTn assays at lower decision cut-offs has been found to reduce the time for the next blood draw from the recommended 6 to 9 hours after presentation to 3 hours, or even within 120 minutes [23, 24, 25, 26, 27, 28], Therefore, the current ESC guidelines on the management of patients with acute coronary syndrome without ST segment elevation have recommended the use of more sensitive cTn assays and a novel algorithm that supports a second blood draw after 3 hours as the standard strategy [9]. Alternatively, an accelerated 1-hour diagnostic rule-out and rule-in algorithm that has been extensively evaluated in observational trials [23, 24, 27, 28] and few randomized studies [25, 26, 29] can be used if validated hsTn assays are available. The rule-out and rule-in categories as well as a grey zone that does not meet either classification is well defined regarding baseline values and corresponding concentration changes at 1 hour. In addition, a 2-hour diagnostic algorithm can be used with [25, 26] or without a clinical score [30] Distinct to other diagnostic pathways, the dual biomarker strategy based on Copeptin and troponin should only be used in pre-selected patients at low-to-intermediate risk based on the GRACE score [11, 12].

FOCUS BOX 2High-sensitivity troponin assays
  • More sensitive troponin assays will detect more patients with acute or chronic myocardial damage, and at earlier stages of disease
  • These small elevations of troponin are associated with an adverse prognosis, regardless of whether troponin is due to ACS or due to one of the numerous differential diagnoses
  • Therefore, the underlying reason should be actively searched for as soon as possible to warrant early start of a specific therapy

OTHER BIOMARKERS FOR EARLIER RULE-IN OR RULE-OUT OF MI (EARLY ISCHAEMIA MARKERS, COPEPTIN

Other biomarkers except Copeptin are not being recommended for the diagnosis of MI [9] although several other biomarkers including CK, CKMB and myoglobin are still used in clinical practice without supportive evidence [31].

Copeptin, the C-terminal part of the vasopressin prohormone, is recommended by the 2015 ESC guidelines on the management of NSTE-ACS [9] but not by the AHA/ACC guidelines [10]. Copeptin has been found to improve sensitivities and NPVs when used together with conventional or contemporary sensitive cTn assays than to high sensitivity assays [32, 33]. The reason for the strikingly different performance of copeptin as compared to biomarkers of myocardial ischaemia is still unsettled. It has been hypothesised that copeptin reflects endogenous, life-threatening stress and haemodynamic changes as copeptin is secreted from the neurohypophysis in response to endogenous stress, different states of shock, and very early after the onset of myocardial ischaemia. However, due to the lack of cardiac specificity, copeptin has to be measured in addition to a cardio-specific troponin. Recently, the Biomarkers-in-Cardiology 8 (BIC-8) study [11], a multicentre trial on 902 patients found that cTn negative patients at presentation randomized to an experimental copeptin arm could be safely discharged earlier, with rates of follow-up events comparable with the reference diagnostic protocol that requires repeat testing of cTn after 6 hours .

FOCUS BOX 3Early rule-out for Myocardial infarction

  • The advent of more sensitive cTn assays which allow use of cTn at lower decision cut-offs with appropriate imprecision (10%CV at the 99th percentile cut-off) obviate the need for additional biomarker that detect myocardial necrosis earlier

    • A faster diagnostic protocol with re-testing of cTn at 0/3 hours is altready recommended as the standard of care when a hsTn assay is available.
    • Faster diagnostic protocols with re-testing of cTn are recommended as an alternative if validated hsTn assays are being used.

As an additional option, copeptin combined with a conventional cTn may rule-out of MI based on a single blood draw at presentation. This strategy could also help to guide safe discharge of patients after rule-out.

BIOMARKERS FOR DETECTION OF COMPLICATIONS OF MI (REINFARCTION, MVO, INFARCT SIZE)

Reinfarction

The term re-infarction is used clinically for an acute MI that occurs within 28 days of an incident or recurrent MI [7].

In patients in whom recurrent myocardial infarction is suspected from clinical signs or symptoms following the initial infarction, an immediate measurement of cardiac troponin is recommended. A second sample should be obtained 3–6 h later or earlier with more sensitive cTn assays. If the cTn concentration is elevated, but stable or decreasing at the time of suspected re-infarction, the diagnosis of re-infarction requires a > 20% increase of the cTn value in the second sample. If the initial cTn concentration is normal, the criteria for new acute MI apply. Previously, CK-MB was recommended for detection of early reinfarction when the concentration of cardiac troponin is still rising [14].

Microvascular obstruction

Microvascular obstruction (MVO) is thought to be related to peripheral embolisation of platelet microaggregates, intimal oedema, vasoconstriction, and/or leukocyte sticking. Irrespective of cause, experimental and clinical studies clearly demonstrate that MVO is associated with a greater degree of myocardial damage, more severely depressed LV function and a higher mortality [34]. Accordingly, significantly higher concentrations of cardiac cTn have been found in the presence of MVO, and the levels of early cTn release correlate with transmurality of infarction, infarct size and magnitude of MVO [35].

Infarct size

Animal experiments and human studies using thallium SPECT and contrast-enhanced MRI demonstrated an excellent correlation between infarct size and cTn concentrations. Any single cTn measurement between 24 and 96 hours after the onset of symptoms may be used to estimate infarct size [4, 4, 36, 37]. However, it should be noted that correlation between infarct size and level of cTnT or cTnI is excellent for large STEMI but is less impressive for the heterogeneous group of non-STEMI and for small infarcts [37]. Although serial measurements performed as effective as single one point protocols, the latter may have a better acceptance for clinical practice as a simple algorithm is more convenient and more cost-effective than serial measurements.

The use of late troponin re-testing for estimation of infarct size is being endorsed by the 2014 ACC/AHA guidelines on NSTE-ACS [10] stating that it may be reasonable to re-measure troponin once on day 3 or day 4 in patients with MI as an index of infarct size and dynamics of necrosis.

FOCUS BOX 4Troponin and infarct size
  • The cumulative release of cardiac constituents reflects the total amount of myocardial necrosis
  • More conveniently, infarct size may be estimated by single time-point measurement of cTn
  • There is good correlation between infarct size and cTn at any time beyond the first 24 hours after onset of symptoms
  • Correlation between cTn and infarct size is better for STEMI or larger infarct than for non-STEMI or more small infarcts
  • 2014 ACC/AHA guidelines state that it may be reasonable to remeasure troponin once on day 3 or day 4 in patients with MI as an index of infarct size

PROGNOSIS

The role of cardiac troponins for risk assessment in suspected ACS is well established. The presence of elevated cardiac troponin in blood indicates the antecedent occurrence of plaque rupture with subsequent embolisation of platelet micro- or macro- aggregates from the ruptured plaque to the periphery causing obstruction of the capillaries with ensuing myocardial necrosis. As a mirror of platelet activation and representative for a prothrombotic milieu, an elevated troponin predicts a higher acute (thrombotic) risk for (re-)infarction and death. In addition, the presence of elevated cTn has been linked to more severe atherosclerotic disease, more prevalent triple coronary artery disease, left main disease, and more severely depressed left ventricular function [38]. Therefore, the prognostic significance of a cTn elevation during the index event persists for years reflecting the persistence of vulnerable plaque. Other biomarkers which indicate persistence of a chronic underlying inflammatory process such as fibrinogen and C-reactive protein have also been found to predict an adverse long-term outcome. These markers confer additive and independent prognostic information to cardiac troponin [39, 40].

A large body of evidence has also confirmed that concentrations of natriuretic peptides (BNP, NT-proBNP) is elevated across the entire spectrum of ACS and that the magnitude of this increase closely correlates with outcomes [41]. The reasons for natriuretic peptides increase following ACS are multiple, including haemodynamic stress and volume overload due to acute heart failure. However, there is also evidence from experimental studies showing that expression of B-type natriuretic peptides (BNP and NT-proBNP) may be stimulated directly or indirectly by myocardial ischaemia [42].

Natriuretic peptides were found to predict adverse outcomes independently and add prognostic information to established risk predictors such as renal function, levels of C-reactive protein and cTn [41]. Eventually, the combination of NT-pro BNP and creatinine clearance was found to be the strongest predictor of 1-year mortality. The measurement of natriuretic peptides appears meaningful in a multimarker strategy as natriuretic peptides may refine risk particularly among patients negative for cTn.

At present, practice guidelines on the management of NSTE-ACS explicitely recommend natriuretic peptides and cardiac troponin for risk stratification [9, 10]. 2014 ACC/AHA guidelines on NSTE-ACS [10] endorse the measurement of cTn as the presence and magnitude of troponin elevations are useful for short- and long-term prognosis. The measurement of natriuretic peptides may be reasonable to provide additional prognostic information in patients with suspected ACS [10]. There are other newer biomarkers that may provide complementary prognostic information, although their role is less well established.

Growth differentiation factor 15 (GDF-15) is a member of the transforming growth factor cytokine superfamily and is induced in the myocardium after ischaemia and reperfusion injury, pressure overload, and heart failure, possibly via proinflammatory cytokine and oxidative stress-dependent signalling pathways. In patients with chest pain, NSTE-ACS and STEMI elevated level of GDF-15 on presentation were associated with higher risk of death and MI, and was found independent of ECG changes, cTn and NP concentrations [43]. In addition, patients with elevated GDF-15 have been shown to benefit from early invasive strategy whereas patients with concentrations in the lower tertile derive no therapeutic benefit, even if cTn is elevated [44]. So far, GDF-15 is a promising new biomarker but further evaluation is mandatory before routine use may be recommended.

Myeloperoxidase (MPO) is abundantly expressed in the azurophilic granules of neutrophils and monocytes, is released in a state of inflammation, and catalyses the formation of several reactive species, including hypochlorous acid. Several clinical studies indicate that measurement of MPO in patients presenting with acute chest pain provides clinically relevant information in addition to and independent from cTn, and thus appears to allow a refined risk stratification [39]. However, it should be noted that comparison of the diagnostic and prognostic performance between MPO and troponin may have been influenced by inappropriate use of troponins with respect to cut-offs, sampling protocols, and outdated infarct definitions. In addition, there are some preanalytical and analytical issues regarding the measurement of MPO that limit its general acceptance for clinical routine. The concentrations of MPO are sensitive to storage conditions at room temperature; depending on the type of specimen, i.e., serum or plasma; and are markedly increased by bolus injection of heparin which mobilises MPO from vascular compartments. The comparability of trials is limited by the heterogeneous use of assay procedures (mass or activity) and differing reports of MPO concentrations on a molar or weight basis, or as MPO/protein ratios [45].

Lipoprotein-associated phospholipase A2 (Lp-PLA2) is a specific marker for vascular inflammation and has gained some interest since expression of Lp-PLA2 indicates the transition from vulnerable plaque into plaque rupture. Several studies demonstrated that Lp-PLA2 is an independent risk predictor for cardiovascular outcomes post ACS. Preliminary and controversial data have also suggested that suppression of Lp-PLA2 using darapladib during the acute phase may stabilise the vulnerable plaque [46].

Impaired renal function is associated with higher mortality in patients with ACS [47]. It is believed that creatinine clearance indicates the cumulative extent of vascular damage caused by hypertension, dyslipidaemia, and diabetes. The knowledge of renal function is important for risk assessment and for dose adjustment of anticoagulation and antiplatelet therapies, as patients with renal failure are prone to excess bleeding due to overdosing. Glomerular filtration rate (GFR) estimates based on creatinine levels can be an unreliable indicator of renal function, as creatinine serum concentrations may be affected by tubular secretion, age, sex, muscle mass, physical activity, and diet. There is accumulating evidence that plasma cystatin C level is an accurate marker of renal function and an independent predictor of mortality in patients with coronary artery disease (CAD), but only few studies have evaluated the prognostic role of cystatin C specifically in patients with ACS. In a study on 726 patients with suspected ACS, cystatin C had better discrimination power than creatinine clearance or creatinine and this suggests that its measurement can improve early risk stratification [48].

OTHER BIOMARKERS

For every stage in the evolution of atherosclerosis, and for the transition of a vulnerable plaque into plaque rupture or erosion, numerous representative biomarkers are currently under investigation. Some of them have already provided promising results regarding prognostication of plaque rupture and adverse outcomes. A list of potential candidates is presented in Table 4.

FOCUS BOX 5Biomarkers and ACS risk stratification
Numerous biomarkers have been found useful for risk stratification of ACS
  • Most evidence is available for cardiac troponins, inflammation proteins such as C-reactive protein, fibrinogen, natriuretic peptides, and markers of renal dysfunction
  • Whilst elevation of cardiac troponin indicates the risk for acute (thrombotic) events such as death, MI during the first 30 days, inflammatory markers, natriuretic peptides and impaired renal function are associated with long-term outcomes
  • So far, few biomarkers including cardiac troponins and natriuretic peptides are being recommended by practice guidelines

GUIDANCE OF THERAPY

Benefits of adjunctive pharmacological therapy

It has been shown consistently that cardiac troponin may be used for objective guidance of more aggressive anticoagulation with dalteparin or enoxaparin [49, 50], and antiplatelet therapies with abciximab, tirofiban, lamifiban [51, 52, 53], or triple antiplatelet therapy [54] followed by early coronary intervention were mostly restricted to patients with elevated troponins.

Benefits from early invasive strategy and timing of early invasive therapy (ESC guidelines)

The ESC guidelines recommend a staged approach to the decision and the timing of coronary angiography. An immediate invasive approach within 120 minutes after initial presentation is recommended in patients being in a haemodynamically unstable condition, manifest signs or symptoms of acute heart failure or malignant tachyarrhythmias, continue to experience symptoms despite adequate anti-ischaemic treatment, or show dynamic ST-changes on 12-lead ECG [9].

An invasive strategy within 24 hours is advocated in patients with a rise and/or fall of cardiac troponin, a GRACE Score > 140 points, or dynamic ST-T changes with or without symptoms. Elevated cardiac troponin with a rising or falling pattern indicates a higher risk that such an acute coronary event is imminent due to the fact that troponin is closely linked to increased pro-thrombotic activity. Angioscopic and angiographic evidence suggests a clear relationship between the presence and concentration of cTn and the presence and magnitude of intracoronary thrombus [55]. In addition, elevated troponin indicates the development of cardiac events during mid-term and long-term follow-up due to its relation to disease severity and persistent inflammatory activity of the atherosclerotic plaque.

An invasive strategy within 72 hours is indicated for individuals who carry a higher acute risk for death or MI including an intermediate GRACE score, post infarct angina, previous PCI or CABG, LV-EF < 40% or congestive heart failure, diabetes mellitus, renal insufficiency defined as eGFR < 60 ml/min/1.73m2 A lselectively invasive strategy invasive strategy is indicated in patients without a risk indicator who remain asymptomatic. In these cases, the decision to perform a coronary angiography should be based on recurrency of symptoms despite optimal medical therapy or evidence of myocardial ischemia based on stress testing, preferably stress imaging (9).

FOCUS BOX 6Troponin and therapy for ACS
  • Whether a patient will benefit from more effective antithrombotic or antiplatelet therapy, or will derive benefits from early invasive vs. selected invasive therapy strongly depends on risk assessment
  • Benefits of therapy are almost exclusively restricted to patients with high risk, or intermediate-to-high risk profile.
  • Traditionally, cardiac troponins have been used for guidance of therapy and selection of invasive strategy
  • Global risk can also be estimated by clinical multivariable scores such as the GRACE score

Biomarkers following elective PCI

In the setting of PCI, biomarkers may be helpful for prediction of procedural success and complications such as PMI. The assessment of the relative contribution of underlying CAD and the additional impact of PCI is almost impossible as the pathophysiology is too complex and the reasons for concentration changes of biomarkers are multifactorial. Generally, PCI leads to mechanical injury of the endothelial layer and plaque and may cause acute release of chemokines, reactive oxygen species, pro-inflammatory and pro-coagulant molecules, and may instigate a pro-inflammatory milieu that may promote adverse vascular remodelling and lead to restenosis, subacute or late stent thrombosis and poor outcomes. Biomarkers that mirror specific pathophysiological aspects may be helpful to identify patients at risk or to develop drugs alleviating adverse sequelae of PCI. In addition, increase of biomarkers before PCI may give information on the presence of vulnerable plaque and hence help to identify patients who require more intense adjunctive treatment during PCI and optimal secondary prevention.

DIAGNOSIS OF PERIPROCEDURAL MI

The diagnosis of periprocedural myocardial infarction following PCI is relatively simple as the mechanism of PCI-related myocardial necrosis is ischaemia [56]. Consistently, Johansen et al were able to demonstrate that the majority of cTnT elevations after PCI persisted for at least 96 hours indicating ongoing release of cTnT from the contractile apparatus reflecting irreversible myocardial injury [57]. In contrast-enhanced cardiac MRI areas of myocardial infarction were identified as the substrate of minor serum marker elevations [58, 59].

There is currently no solid scientific basis for defining a biomarker threshold for the diagnosis of periprocedural myocardial infarction. In the the 3rd version of the universal definition of myocardial infarction [14], by arbitrary convention, it was suggested to label increases of more than five times the 99th percentile as PCI-related myocardial infarction.

After publication of the 3rd version of UDMI (14), new findings on patients undergoing elective PCI in whom baseline values were disregarded showed that a 5-fold increase of hsTn conferred a similar prognostic relevance as the previous UDMI criteria with or without ischemic symptoms (60). As a consequence, the presence of symptoms was removed from the list of diagnostic criteria for postprocedural MI in the 4th version of the UDMI [7]. Patients with either a rise of cTn concentration below the diagnostic cutoff, or without at least 1 additional feature are labelled acute postprocedural myocardial injury but not Type 4a MI. The UDMI criteria are discordant with criteria recommended by SCAI organization [61]. Despite academic interactions, an agreement for harmonization of definitions could not be reached. According to SCAI, a rise of peak CKMB, the preferred biomarker for diagnosis of procedure-related MI, measured within 48 hours >10x local laboratory ULN, or >5x ULN in the presence of Q-waves in ≥2 contiguous leads or new persistent left bundle branch block allows diagnosis of postprocedural MI. In the absence of CMBM measurements and if cTn is normal at baseline, a postprocedural increase of cTn peak levels measured within 48 hours >70x the local laboratory ULN, or >35x ULN with new pathologic Q-waves in ≥2 contiguous leads, or new persistent LBBB represent the diagnostic criteria for type 4a MI.

If the values are stable or falling, criteria for re-infarction by further measurement of biomarkers together with ECG criteria can be applied. A diagnostic dilemma arises if cardiac troponin is elevated before the procedure and not stable for at least two samples 6 hours apart. In this situation, there are insufficient data to recommend biomarker criteria for the diagnosis of post-procedural myocardial infarction.

Other PCI-related MI subtypes

Stent/scaffold thrombosis,i.e. type 4b MI, represents another subcategory of PCI-related MI, documented by angiography or autopsy is diagnosed using the same criteria utilized for type 1 MI [7].

A Type 4c MI is diagnosed if in-stent restenosis, or restenosis following balloon angioplasty in the infarct territory is the only angiographic explanation since no other culprit lesion or thrombus can be identified. Again, the same criteria utilized for type 1 MI apply [7].

In general, two major pathological mechanisms have been proposed ( Figure 3 , adapted from Prasad A [62]):

  1. Type 1 or proximal type of periprocedural myocardial injury (25-50% of PMI). The major reason for myocardial injury arises from occlusion of a side branch during angioplasty. The risk increases when the side branch arises from within the target lesion of the native coronary artery or in the presence of ostial stenosis of the side branch. Responsible mechanisms of side branch occlusion include plaque shift, dissection of the dilated coronary artery extending to the side branch, thrombus formation, or plaque embolisation, and side branch spasm.
  2. Type 2 or distal type of periprocedural myocardial injury (50% to 75% of PMI). Atherosclerotic plaque rupture and local vessel trauma lead to distal perfusion deficits in the distal perfusion territory. The mechanisms involved include distal embolisation of atheromatous debris and thrombotic material, leucocyte and platelet activation leading to microvascular plugging of platelets and neutrophils, neurohumoral activation and modulation of vascular and myocardial function, and oxidative stress and inflammation.

Given the above pathomechanisms, biomarker can be used to identify and monitor pathophysiological processes and may be useful in the future to develop cardio-protective drugs that prevent microvascular obstruction, improve distal territory perfusion, limit inflammation and thereby reduce PMI and later native coronary artery or stent stenosis or thrombosis. Representative biomarkers of platelet activation and activated thrombosis in blood include measurement of soluble CD40 ligand (sCD40L) and other granule secretion products including platelet factor 4 (PF4). Measurement of high residual platelet reactivity or degree of platelet inhibition by several aggregometry based methods has emerged as an interesting technique to monitor the efficacy of dual antiplatelet therapy, with most interest on clopidogrel therapy due to higher prevalence of low or no-response, potential interaction with proton pump inhibitors and propensity to CYP2C19 loss of function. The measurement of haemostatic proteins may include tissue factor, thrombin-antithrombin-complex (TAT), prothrombin F1+2, and von Willebrand factor (vWF). The great obstacle of measuring haemostatic proteins are, however, mostly pre-analytical issues [39].

Biomarkers that indicate neurohumoral activation include serotonin and endothelin while natriuretic peptides including BNP and NT-proBNP may increase after severe myocardial ischaemia or haemodynamic stress during PCI. Increases of C-reactive protein and IL-6 have been demonstrated after PCI, particularly among those with significant PMI as indicated by troponin level.

Oxidative stress is rather represented by increases of IMA suggesting oxidative modification of albumin by reactive oxygen species after vascular trauma or neutrophil activation [39].

The most thoroughly investigated biomarkers are those indicating myocardial necrosis. For the latter, enzyme activity and protein mass of creatine kinase or its MB fraction (CK-MB) have been used traditionally. However, for more than two decades cardiac troponins have been recommended as the preferred markers of necrosis due to their absolute tissue specificity, high signal-to-noise ratio and superior sensitivity.

FOCUS BOX 7Periprocedural MI
  • Diagnosis of a type 4 MI after elective PCI should be confirmed if the troponin was normal before PCI and rises to >5 times 99th percentile value within 24 hours post PCI and only in the presence of additional signs or symptoms of myocardial ischemia such as symptoms, ECG changes, new wall motion abnormalities on echo, or loss of viable myocardium. Otherwise, any postprocedural cTn elevation should be labelled as myocardial injury and not as type IV MI.
  • In ACS type 4 MI is difficult to assess unless cTn before PCI is negative in two consecutive samples taken at least 6 hours apart
  • Reinfarction in the first hours after PCI (as long as cTn is rising) is difficult to assess
  • In cases with falling cTn values, an increase of 20% or more suggests re-infarction
  • For the diagnosis, the clinical picture and ECG changes should ideally be considered

PROGNOSTIC RELEVANCE OF CARDIAC BIOMARKER ELEVATION BEFORE AND AFTER ELECTIVE PCI

Cardiac troponin elevation post-PCI

Troponin release is a more sensitive indicator of myocardial necrosis than elevations of CK-MB. Accordingly, elevations of cardiac troponin have been reported following PCI in up to 69% of cases. Nevertheless, the prevalence of Type 4a PMI using the 3rd version of the Universal MI definition ranges between 3.1 and 7% only (14). Two recent meta-analyses on more than 20,000 patients consistently demonstrate increased mortality rates associated with elevated cTn post-PCI [63, 64]. In contrast, the findings of individual studies on the prognostic relevance of cTn rise post-PCI were less consistent. Studies did not systematically exclude patients with acute coronary syndromes and did not measure preprocedural troponin which could have been abnormal at baseline. In addition, cut-off levels for normal were higher than the currently recommended 99th percentile value. Other reasons that may explain inconsistencies regarding the relationship between postprocedural troponin level and outcomes include heterogeneity of inclusion criteria, use of different assays, different sample sizes and duration of follow-up. More recently, it appears that preprocedural elevation of cardiac troponin is a more powerful and independent predictor of short-and long-term mortality than postprocedural troponin rise [64, 65, 66, 67, 68]. More recently, it becomes apparent that troponin thresholds indicating adjusted probabilities of mortality at 3-months [69] and 1-year [70] that is prognostically equivalent to a spontaneous MI are between 20 to 70-times the upper limit of normal of the individual cTn assay.

Pre-procedural cTn elevation

Interestingly, a substantial proportion of patients admitted for elective PCI demonstrate elevated cTn concentrations prior to PCI. In the EVENT registry on 7,592 patients undergoing elective PCI, Jeremias et al found that baseline elevation of cTn was relatively common among patients with stable coronary artery disease and was an independent prognostic indicator of ischaemic complications [65]. Of the 2,382 patients who underwent PCI for stable CAD, 6.0% had a baseline cTn level above the ULN. These patients were found to have a 2.1-fold higher adjusted risk for the composite of death or MI at hospital discharge and a 2-fold higher adjusted risk at 1-year. This composite endpoint was driven by significantly higher rates of MI and only a trend for higher in-hospital mortality.

More recently, Prasad et al investigated a mixed cohort of 5,487 patients undergoing PCI for either stable coronary artery disease or acute coronary syndromes [67]. The authors found that 37% of all patients had preprocedural elevation of cTnT using the 99th percentile as cut-point. This percentage was surprisingly high given that CK-MB values prior to PCIs were in the normal range in the majority of patients with pre-procedural cTnT values up to 0.1 ng/mL. These low level preprocedural but not postprocedural elevations of cTnT were independently predictive of long-term adverse outcomes. In addition, CK-MB fraction elevation of more than 5-fold to 8-fold after PCI was extremely infrequent in patients with normal preprocedural cTnT. Consistently, Kizer and colleagues confirmed the predictive value on long-term adverse events associated with preprocedural but not postprocedural cTnT elevation in a small study with 212 patients. However, a much less sensitive cut-off > 0.1 ng/mL was used to diagnose myonecrosis [62].

Finally, Ndrepepa et al (68) studied 5,626 patients undergoing elective PCI who had baseline and peak post-procedural hs-TnT measurements available (). The study demonstrated that peak post-procedural hs-TnT increases were not associated with mortality in patients with non-elevated baseline hs-TnT levels. Thus, it is tempting to speculate that preprocedural troponin elevation is more important than postprocedural elevation as these patients have a greater atherosclerotic burden, complex coronary anatomy, difficult-to-treat lesions, higher rates of intra-coronary thrombi and more unstable disease than patients with normal baseline troponin level. In contrast, postprocedural troponin elevation is also a marker of atherosclerotic burden and procedural complexity but seems less prognostically relevant unless large periprocedural infarcts occur with CK-MB elevations of more than 5 times the upper limit of normal, or new Q-waves, or in the presence of visible procedural complications.

An algorithm for the management of PMI according to preprocedural and postprocedural biomarker elevation has been proposed by Prasad et al [62] ( Figure 4 , adapted from Prasad et al, [62].

OTHER BIOMARKERS

Natriuretic peptides

Recent studies enrolling large numbers of patients demonstrated the prognostic value of both NT-pro-BNP and BNP in patients with stable coronary artery disease. There is some evidence that increased plasma BNP levels at baseline are associated with more complex lesions, which may play a role for chronic or repetitive ischaemia. Several investigators found that tissue hypoxia alone can trigger release of BNP, even in the absence of left ventricular dysfunction [42]. Whether elective PCI may cause significant alterations in natriuretic peptide level is unknown and findings are conflicting. In the JUMBO-TIMI 26 trial NT-pro BNP level were measured in 747 patients before and after elective or urgent PCI [72]. Patients with procedural MI had significantly higher levels of NT-pro-BNP at 12 to 24 hours (405 vs. 146 pg/ml, p <0.001). The magnitude of increase in NT-pro-BNP correlated strongly with the extent of myocardial injury, even in those with very low-level troponin increases. Thus, these findings suggest that periprocedural myocardial injury, even at low levels, during PCI is associated with increased haemodynamic stress as measured by increasing NT-pro-BNP. In two small studies, circulating BNP levels increased after PCI of stable CAD patients, but findings diverged largely in regard to the timing of BNP peak after the procedure. Finally, in one study on 103 patients, balloon inflations during elective PCI did not cause significant alterations in plasma BNP levels [73]. It was speculated that short balloon inflations did not cause ischaemia severe enough to elevate BNP levels. In summary, baseline elevations of natriuretic peptides indicate adverse long-term prognosis independent of left ventricular dysfunction. However, the relevance of increases of BNP following elective PCI remains unsettled.

Inflammatory markers

A number of studies suggest an association between baseline level of inflammatory markers including proinflammatory cytokines, metalloproteinases, selectins and adhesion molecules [39]. Among these inflammatory markers, CRP has been investigated extensively and an association between CRP levels and adverse outcome has been reported after elective and emergent PCI. However, study results focusing on the association between CRP level and events occurring after PCI is still controversial. Possible mechanisms for the association with post-procedural troponin increase and for the adverse outcomes include a relation between CRP and diffuse atherosclerotic disease, vulnerability of the atheromatous lesion that predispose to micro-embolisation or to a pro-thrombotic milieu at the site of angioplasty-induced vessel wall injury. Effective clopidogrel administration before PCI has been associated with less inflammation in response to vascular injury, as indicated by an attenuated increase in post-procedural CRP. Systemic inflammation also provides a link between PCI-associated infarctions and long-term events as previous studies have shown that elevated CRP predicts future events in patients with stable angina [63]. In addition, there is some evidence that statin therapy may reduce cardiac event rates following PCI [75]. It has been speculated that statins strongly attenuate inflammation, and thus may improve long-term outcomes after PCI and prevent restenosis, particularly among stable patients with persistently elevated CRP who are not under statin therapy.

Conclusions

Biomarkers may help to identify pathophysiological processes of stable disease and potentially the transition from stable plaque to vulnerable plaque and plaque rupture. Representative biomarkers include markers of inflammation, activation of coagulation, oxidative stress, myocardial ischaemia and myocardial necrosis.

In clinical practice, cardiac troponins have been recommended as the preferred biomarkers for detection of myocardial necrosis due to high diagnostic sensitivity and their absolute cardiospecificity. Therefore, cardiac troponins represent the reference standard for diagnosis of myocardial infarction.

According to the Universal MI definition, spontaneous MI is diagnosed if troponin is elevated above the 99th percentile value of a reference population and shows a distinct rise and/or fall, together with signs or symptoms of myocardial ischaemia.

In order to distinguish spontaneous MI from more prevalent, but less prognostically relevant post-interventional MI, the universal definition of myocardial infarction introduced a specific category. Type 4 postprocedural MI following PCI is defined as a rise of cardiac troponin three times above the 99th percentile value. The clinical context of myocardial ischaemia is given by the nature of the intervention.

Regarding the prognostic relevance of post-procedural elevation of CKMB or cardiac troponin there are several issues and inconsistencies.

Accumulating evidence suggests that the measurement of a baseline cardiac troponin level with the use of a high-sensitivity assay, together with clinical and angiographic characteristics represents a powerful surrogate for the preprocedural risk of a patient.

Postprocedural elevations of cardiac troponin or CK-MB are less frequently observed in patients without preprocedural elevations. However, relevant elevations above 5 times ULN may indicate short-term and long-term mortality. It appears that preprocedural rather than postprocedural biomarker level predict short-term and long-term outcomes.

New algorithms for the management of patients undergoing elective PCI taking into account pre- and postprocedural biomarker level may help to identify patients at risk who might require prolonged observation, more intense secondary prevention, and repeat angiography, if clinically indicated, to improve short-term and long-term survival.

Personal perspective – Hugo A. Katus

Among biomarkers, measurement of cardiac troponins is widely accepted as the biochemical standard for detection of myocardial damage and has been incorporated as the standard for diagnosis of MI. Cardiac troponins also belong to the most versatile biomarkers as they allow risk stratification, guidance of therapy, detection of per-procedural MI, re-infarction and estimation of infarct size. More sensitive troponins have now closed the gap related to the relatively late appearance of troponins in blood after myocardial necrosis and have almost abolished the need to measure other, earlier biomarkers of necrosis. Along with more accurate detection of smaller infarcts and improved risk stratification, the higher analytical sensitivity translates into earlier detection of myocardial infarction. As a consequence, a more rapid diagnostic protocol with measurement of hsTn at presentation and retesting between after 3 hours has replaced the former recommendation to re-test after 6 to 9 hours [9], provided that a hsTn is being used. Further acceleration with re-testing of cTn within 2 hours (1 hour or 2 hours) is feasible and safe, with or without the need to use of clinical risk score, provided that validated hsTn assays are used [76].

Currently, clinical research is focused on whether low risk patients may be safely discharged home or need to be hospitalized. For this purpose, prospective registries reflecting real world evidence are providing supportive evidence until confirmatory evidence becomes available from randomized clinical trials [77, 78, 79].

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  • KANHA VIJAY SINGRU
    KANHA VIJAY SINGRU
    21 May 2015, 08:10

    OTHER BIOMARKERS : Biomarkers of Stent restenosis (ACEI gene polymorphism & others) BNP levels & future roles in cardiomyopathy/ADHF, aortic stenosis & other non-coronary conditions Biomarkers of Myocyte stress, Markers of extracellular Matrix remodelling & Markers of Clinical outcomes in HFpEF like Galectin-3, GDF, cystatin, Resistin etc Their current signicficance & future roles in clinical practice needs further elaboration

     
    • Rodney de Palma
      Rodney de Palma
      29 May 2015, 12:37

      Thank you for your helpful comments. The chapter and it's first update have concentrated on biomarkers relevant to coronary intervention but we shall be including details relating to non-coronary intervention in future updates.