cardiac markers into the new millennium

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Marita du Plessis
Oktober 2000
1
CARDIAC MARKERS INTO THE NEW
MILLENNIUM
INTRODUCTION
The past years has seen the publication of a number of landmark papers on interventions in
acute coronary syndromes (ACS) (eg TIMI, GUSTO and FRISC trials) and the relationship of
these interventions to cardiac markers. Two consensus documents on the use of cardiac
markers for the investigation of patients with ACS have recently been produced, one by the
National Academy of Clinical Biochemists (NACB) [1] and one by the International Federation
of Clinical Chemists (IFCC) [2].
The 1959 World Health Organisation (WHO) has defined the diagnosis of AMI as a triad. Two
of which must be present for diagnosis:
a) History: the history is typical if acute, severe (resistance to nitroglycerol if taken
during the attack) and prolonged (> 20 minutes) chest pain is present;
b) Electrocardiogram (ECG): unequivocal changes are the development of abnormal,
persistent Q-waves, or equivalents, in at least 2 contiguous leads of the standard
ECG, lasting longer than one day;
c) Serum enzymes: unequivocal change consists of serial enzyme change with an initial
rise and subsequent fall of the serum concentrations, which must be properly related
to the time window of the particular enzyme and the delay between onset of
symptoms and blood sampling.
Comments on WHO criteria:
- Third criterion of the WHO definition of AMI should be expanded to include the use of
serial biochemical markers (cTnT, cTnI and myoglobin) and not be limited to enzyme
changes [1].
- Defining MI in accordance with WHO criteria is difficult:
- History can be non-specific in up to one-third of patients, particularly in diabetics and
in the elderly who frequently present with atypical symptoms of ischemia.
- Initial ECG is diagnostic of AMI in slightly more than 50% of AMI patients
- The representation of cardiac ischemia as binary by the WHO criteria (dividing patients
into 2 groups: AMI or not) may be viewed as an anachronism.
It is now widely accepted that ischemic heart disease is comprised of a pathological
continuum that involves erosion and rupture of unstable coronary artery plaques, activation of
platelets, and intramural thrombosis, with/without vasospasm.
This continuum is collectively termed the “acute coronary syndromes” (ACS) and ranges from
unstable angina, frequently associated with minor myocardial damage (MMD)
(incomplete/reversible occlusion), non Q-wave myocardial infarction (without ST-segment
elevation) and Q-wave myocardial infarction (ST-segment elevation) MI with complete
occlusion and extensive necrosis.
Identifying where an individual patient’s disease lies in the continuum of acute coronary
syndromes has biological implications regarding the reversibility of injury and quantity of
ischemic cell injury, as well as the patient’s relative risk for an adverse outcome [3].
Figure 1 [4]: The WHO, NEW and REALITY classification of acute coronary syndromes.
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Figure 2 [5]: Pathophysiologic events culminating in the Acute Coronary Syndromes (ACS)
Patients admitted with suspicion of an ACS constitute a diagnostic, prognostic and
therapeutic challenge:
- Patients with AMI who are mistakenly discharged from the ED have short-term mortality
rates of about 25 % (twice mortality rate of admitted patients) – resulting in legal costs
from malpractice litigation.
- On the contrary, the admission of a patient with chest pain who is at low risk for AMI costs
an average of $2 000 and $5 000 at many institutions and can lead to unnecessary tests
and procedures, with their attendant costs and complications [6].
- In the USA over 5 million patients are seen annually with chest pain at ED’s. Only about
10% of these patients are subsequently confirmed to have AMI.
- While most of the patients have chest pain of cardiac cause due to either angina or that of
the unstable acute syndromes, a significant percentage in the range from 20-30% is from
non-cardiac causes [7].
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Oktober 2000
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The initial ECG is diagnostic of AMI in slightly more than 50% of AMI patients, the typical
initial finding being ST-segment elevation, followed by T-wave inversion and finally, an
enlarged Q-wave.
In patients presenting with chest pain and typical ST-segment elevation (> 1 mm in 2 or more
leads):
- No need for biochemical confirmation of the diagnosis.
- The ST-segment elevation reflects transmural myocardial ischemia caused by an
occlusive thrombosis
- Goal of treatment is to re-open the occluded coronary artery as soon as possible by
thrombolysis or acute angioplasty.
Patients presenting with chest pain and without ST-segment elevation:
- Normal ECG/ischemic changes (flat or down-sloping ST-segment depression/T-wave
inversions
- Broad spectrum of diagnoses:
- relatively large MI’s with severe prognosis (biochemical markers indicate AMI)
- minor MI’s or
- unstable angina with/without minor myocardial damage (MMD)
- chest pain of noncardiac causes.
- Collectively termed unstable coronary artery diseases (UCAD):
- associated with a lower initial mortality but a higher risk of later MI or cardiac death –
in the period of 30 days to 3 years (9-10% of all unstable angina patients).
- patients with unstable angina and non-Q-wave MI often present in a similar manner
- distinction are arbitrary and can be made only many hours or days later, when the
results of biochemical cardiac marker tests become available:
- non-Q wave MI: biochemical changes in the MI range (irreversible cardiac
damage),
- MMD of unstable angina: elevation of specific cardiac markers (troponins) above
the normal reference range, but lower than the MI cut-off (reversible cardiac
damage).
- Evaluation of patients with non-ST segment elevation is a key issue and aimed at
diagnosis of the high-risk group (associated with increased cardiac troponin) that would
benefit from new treatments such as LMW heparins, platelet glycoprotein IIb/IIIa
antagonists, direct thrombin inhibitors, and coronary intervention in the acute as well as
the subacute phase.
- Thrombolysis is only indicated if LBBB is present on ECG.
Table 1 [8]: Simplified schematic presentation of the acute coronary syndromes
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Figure 3 [1]: Plot of the appearance of cardiac markers in blood vs time after onset of
symptoms (peak B: cardiac troponin after AMI, peak D: cardiac troponin after MMD of
unstable angina)
During the last 3 decades the in-hospital mortality of AMI has decreased from 30-50% to 810%. This reduction correlates with improved management of patients with AMI together with
the introduction of specific therapies namely thrombolytic therapy and PTCA.
The motivation for thrombolysis was the discovery that MI evolved over an interval of a few
hours (5-7 hours) during which time restoration of flow could limit the amount of cardiac
damage and decrease mortality and morbidity. AMI treated within the first hour of onset of
symptoms is associated with a mortality of 1-2% as opposed to that treated at 6 h of 10-12%
[7].
An appreciation of the importance of time led to improved public education with respect to the
urgency to get to the hospital once chest pain developed. The mean time of patients
presenting to the emergency room has changed from 10-12 hours after onset of symptoms to
around 6 hours.
There is therefore increased pressure for early and accurate biochemical diagnosis of AMI
with shorter TAT’s and early risk stratification for appropriate selection of treatment.
CARDIAC MARKERS
GENERAL OVERVIEW
Table 2 [9]: Characteristics of commonly available cardiac markers and time course following
onset of myocardial infarction
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It is convenient to class the biochemical markers in terms of their time to positivity from the
onset of symptoms:
- early markers (within 1-6 hours):
o Myoglobin
o CK-MB isoforms
o Heart-type fatty acid binding protein (FABP)
o Glycogen phosphorylase isoenzyme BB (GPBB)
-
middle markers (6-12 hours):
o CK-MB mass
o Cardiac troponins (T and I)
-
late markers (more than 12 hours):
o CK-MB mass
o Cardiac troponins (T and I)
MYOGLOBIN:
-
-
-
-
Sensitive marker (90-100%) for AMI in early time period, rising above the reference
interval as early as 1 hour after AMI, with peak activity in the range of 4-12 hours
(reduced sensitivity after 12 hours), preceding the release of CK-MB by 2 to 5 hours.
Major limitation: lack of specificity (60-95%) – false positives due to chronic renal
disease, skeletal muscle and neuromuscular disorders (including several toxins and
drugs intake).
Increased specificity:
o Combined measurement of myoglobin and skeletal specific marker (carbonic
anhydrase III) or a cardiac specific marker (troponin – highest efficacy when
the 2 parameters are used in series)
o Myoglobin on serial samples (a doubling of or increase in the rate of
myoglobin in a 1-2 hour interval, increases specificity by up to 98%)
Best used as negative predictor of AMI: if myoglobin concentrations remain
unchanged and within the reference interval on multiple, early samplings within 3 to 6
hours after onset of chest pain, there is 100% certainty that muscle (either cardiac or
skeletal) injury has not occurred recently.
CK-MB ISOFORMS
-
-
-
REP (Rapid electrophoretic prototype) available, high voltage electrophoresis, fully
automated, 25 min for analysis
Advantages:
o Each patient’s MB-CK activity serves as its own control (ratio of tissue to
plasma form)
o ??? Early diagnosis of AMI
Conflicting evidence:
o Roberts: sensitivity of CK-2 isoforms in detecting AMI within 6 h after onset of
symptoms was 97.5%
o ROC analysis of CK-2 isoform and CK-2 mass concentration showed that
these tests were comparable and neither was sensitive within 4 hours after
onset of symptoms
Other shortcomings limiting routine use:
o Optimized cutoff levels and decision thresholds have not been clearly
established
o A study has demonstrated that CK-2 isoforms are increased in most patients
with acute skeletal muscle injury
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GLYCOGEN PHOSPHORYLASE ISOENZYME BB (GPBB): [10]
-
-
Not yet commercially available
Future challenge is the development of a rapid assay suitable for “stat” use in the
routine laboratory
Marker of tissue ischemia (with onset of tissue hypoxia GPBB is converted from a
structurally bound to a cytoplasmic form)
Not cardiac specific (tissue concentrations of GPBB in heart and brain are
comparable, possibility of re-expression in chronically stressed skeletal muscle eg
Duchenne muscular dystrophy)
Positive GPBB should be confirmed by specific marker (cTn)
Although preliminary results suggest that GPBB is the most sensitive marker for the
diagnosis of AMI within 4 hours after onset of chest pain (sensitivity 77% for GPBB
compared to 47% for myoglobin), the results will have to be confirmed in a larger
number of patients.
Figure 4 [10] GPBB, CK-MB mass, myoglobin and cTnT time courses in a patient with a small
non-Q wave MI.
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FATTY ACID-BINDING PROTEIN (human heart specific) (FABP): [11]
-
-
Not yet commercially available
Rapid and sensitive immunochemical assay systems in development
Plasma kinetics closely resemble those of myoglobin
Diagnostic sensitivity for diagnosis of AMI within 6 hours after onset of symptoms was
significantly greater for FABP (78%) than for myoglobin (53%).
Differences in contents of myoglobin and FABP in heart and skeletal muscle and
simultaneous release upon muscle injury allow the plasma ratio of myoglobin/FABP
to be applied for discrimination of myocardial (ratio 4-5) from skeletal muscle injury
(ratio 20-70).
Cardiac specificity has not yet been studied.
Potential drawback: renal clearance – plasma concentrations markedly increased
with chronic renal failure.
Figure 5 [11]
Table 3 [11]: Biochemical markers in first blood samples from 83 patients with confirmed AMI
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OTHER POTENTIAL EARLY MARKERS: markers of ischemia (before necrosis): [3]
- inflammatory markers of unstable plaque – CRP and SAA
- indicators of intracoronary thrombosis o platelet activation: P-selectin (adhesion molecules)
o thrombus formation: soluble fibrin and fibrin degradation products
- the above markers could possibly be combined with markers of necrosis
(conventional cardiac markers), clinical indicators, ECG and imaging studies to form
an integrated combined model for optimum assessment of a patient’s position on the
ACS-continuum and risk.
CREATINE KINASE-2
-
-
-
-
“Gold standard” for diagnosis of AMI
Mass measurements has similar sensitivity to that of enzymatic activity, neither
having adequate sensitivity (> 90%) to exclude AMI during the first 6 hours after
onset of chest pain – see table 4 [12]
Sensitivity after 10-12 hours approach 90-100% (figure 6:ROC curve) [9]
Some authors have proposed assessment of the CK-MB mass slope as a means of
improving the diagnostic sensitivity in the early time-interval (0-12h) after admission:
o 3 early CK-MB mass measurements – at 0,2 and 4 h after admission (fig 7)
[12]
o claims 100% sensitivity 4 h after admission, similar to a single CK-MB mass
concentration in their study
o NB: times referenced to admission time (0-8 h), not onset of symptoms (1216h)/important to select patients with increasing CK-concentrations/low
specificity/small study (73 patients) [13]
Serial measurement > single measurement
CK-MB is not perfectly cardiac specific:
o Skeletal muscle contains small but significant amounts of CK-MB (1-3%)
o Increased skeletal muscle CK-MB content observed following inflammatory
muscle disorders and dystrophies due to fetal reexpression of the CK-B gene
o CK-MB can also be increased due to chronic renal failure
Use of the percent relative index (%RI; %RI = CK-2 mass/total CK-activity x 100%)
has been proposed to increase the cardiac specificity, with values > 2.5% (assayspecific) pointing towards a myocardial source of MB-isoenzyme.
o False negative results: skeletal damage can cause a large increase in total
CK and resultant reduction in %RI, obscuring the diagnosis of myocardial
damage in the presence of severe skeletal injury
o False positive results: Total CK below or in the lower range of the reference
interval
Table 4: Diagnostic sensitivity for AMI at different time intervals for the CK-MB mass assay
[12]
Figure 6: ROC curves for total CK and CK-2 activities after MI [9]
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Figure 7: Diagnostic strategy for the early diagnosis of AMI by assessment of CK-MB mass
slope
TROPONINS:
-
-
-
-
Troponin is a complex of 3 protein subunits:
o Troponin C – the calcium binding component
o Troponin I - the inhibitory component
o Troponin T – the tropomyosin-binding component
The subunits exist in a number of isoforms, and cardiac-specific troponin T (cTnT)
and cardiac-specific troponin I (cTnI) isoforms have been identified
CTnI is absolutely cardiac specific
However, during human fetal development, in regenerating rat skeletal muscle, and in
diseased human skeletal muscle (eg muscular dystrophy, polymyositis, uremic
myositis in chronic renal disease), small amounts of cTnT are expressed as one of 4
identified isoforms in skeletal muscle.
Troponin is localised primarily in the myofibrils (94-97%), with a smaller free
cytoplasmic component estimated at 6-8% for cTnT and 3-4% for cTnI.
Troponins are released in a biphasic pattern, with the initial peak due to release of the
cytosolic pool, and the second peak caused by degradation of the structural
elements.
Following AMI, troponin is released into blood as a ternary complex of cTnT-I-C, a
binary complex of cTnI-C and free subunits.
The early release kinetics of both cTnI and cTnT are similar to those of CK-2 after
AMI: increases above the upper reference limit are seen at 4-8 hours (figure 8)[9],
and clinical sensitivity is similar to CK-2 during the first 48-72 hours.
Figure 8: Serial CK-MB, cTnI and cTnT profiles after AMI [9]
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After 72-96 hours, the troponins have improved and high clinical sensitivity (>90%) for
late diagnosis of AMI.
Like CK-2, the troponins are insufficient for effective early diagnosis, with a sensitivity
of 50% at 4 h, 70% at 6 h and 90% at 12 hours.
Adequate sensitivity (>90%) for reliably excluding AMI using troponins is only
reached at about 16 hours after AMI.
Figure 9: ROC curves for CK-MB and cTnI [9]
-
It is postulated that the troponins are released following reversible and irreversible
ischemia:
o Reversible ischemia causes release of only the cytosolic pool
o Irreversible ischemia and necrosis shows the typical biphasic pattern, with
release of the cytosolic fraction followed by prolonged release from the
contractile apparatus.
o This is in contrast to the release of large molecular weight enzyme markers
(such as CK, CK-MB or LD) which do not leak across membranes unless
myocytes are irreversibly damaged.
Figure 10: Release of cardiac markers following injury [14]
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Because cTnT and cTnI are released into the circulation in situations of both reversible and
irreversible ischemia, and do not significantly circulate in the blood of healthy persons, two
cutoff concentrations can be used for interpreting cardiac troponin results [14]:
-
First cutoff is set at the upper reference limit (URL)(on a population of healthy
individuals, using the 97.5 percentile of results): enables the determination of MMD
(detection of reversible ischemia)
-
Second AMI cutoff (standardised ROC analysis of results from a population of
consecutive chest pain patients presenting to the ED for AMI rule-out, AMI dx
according to WHO criteria, independent of experimental cardiac marker being tested):
differentiate between unstable angina and AMI (detection of irreversible ischemia),
consistent with cutoff used for less specific markers such as CK-MB [1].
In less specific cardiac markers (myoglobin, CK, CK-MB), the use of a lower cutoff (at URL)
will result in an increase of false positive results in patients with skeletal muscle injury or
disease. Subsequently only the AMI-cutoff can be used.
Figure 11: Selection of cutoff concentrations [14]
Diagnosis of MMD:
- Cardiac troponin above normal reference limit, but below AMI cutoff
- Other cardiac markers (CK-MB) below AMI-cutoff
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TROPONIN I:
-
Advantage: Because Troponin I is absolutely cardiac-specific it can be used to
eliminate false clinical impression of AMI in patients with increased CK-2
concentrations because of:
o Acute skeletal muscle injury following marathon racing
o Chronic myopathy of Duchenne’s muscular dystrophy
o Chronic renal failure requiring dialysis
o Cocaine-induced chest pain
o Blunt chest trauma
o Critically ill patients
o Perioperative MI
-
Major drawback concerning determination of cTnI is that absolute concentrations of
cTnI obtained in different clinical assays can not be compared due to the following:
o Several manufacturers have developed monoclonal-based diagnostic assays
for the measurement of cTnI (including Dade Behring, Beckman, Abbott,
Bayer) – different monoclonal antibodies directed against different epitopes
o currently no primary reference cTnI material available for standardization of
assays by different manufacturers
o CTnI is present in the circulation in 3 forms, either free, bound as a two-unit
complex or bound as a three-unit complex, resulting in conformational
changes.
o Additionally, cTnI can be released as both oxidized (intramolecular disulfide
formation of 2 cysteines) and reduced forms, and can be phosphorylated on
serine groups
o Heterogeneity in the cross-reactivities of antibodies to different troponin I
forms.
Standardization is currently underway – IFCC C-SMCD: six candidate materials have
been tested and characterized (recruitment phase) and the first study phase is
scheduled for late 1999.
Until appropriate standardization is attained, comparisons must use changes relative
to each assay’s respective upper reference limit.
-
-
TROPONIN T
-
-
-
In contrast to cTnI, assays for determination of cTnT are only patented and marketed
by 1 company (Roche diagnostics) and therefore no standardization bias exist for
cTnT.
However, antibodies used in the first generation ELISA cTnT assay showed some
cross-reactivity with skeletal muscle TnT, and caused falsely increased cTnT in 3050% of severely uremic patients in the absence of increased cTnI and evidence of MI.
2nd generation cTnT assay has been developed with monoclonal antibodies against
cTnT that do not show any cross-reactivity against skeletal muscle TnT, but uremic
patients still show a 12-17% false positive rate. Possible explanations of these
findings include:
o lower increases seen with 2nd generation assays suggest that part of the
previous elevation was caused by release of skeletal muscle TnT by uremic
myositis
o additionally the fetal gene for cTnT may be re-expressed in uremic skeletal
muscle myositis (this does not occur for cTnI)
o cTnT might also be increased due to uremic cardiomyositis and poor renal
clearance causing accumulation of cTnT, but not of the smaller cTnI
o possibility of true positive cTnT and minor myocardial damage can not be
ruled out (correlations between cTnT and endothelin-I might indicate
subclinical myocardial damage in such patients).
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TROPONIN I VERSUS TROPONIN T
cTnI is the preferred cardiac marker to use in patients with renal disease.
Few direct comparison studies between cTnI and cTnT for the detection of AMI have been
published: recently 2 prospective studies showed:
- no differences in clinical sensitivity of AMI dx between cTnT and cTnI,
- both troponins can be used for identification of AMI more than 6 hours after
presentation,
- elevations of either marker within 6 hours predicted an increased risk of complications
and need for interventions [9].
Figure 12: Clinical sensitivities for CK-MB, myoglobin,cTnI and cTnT for AMI as a function of
time from presentation at the ED.
RECOMMENDATIONS FOR THE USE OF CARDIAC MARKERS
IN CORONARY ARTERY DISEASE [NACB, IFCC S-SMCD]
For routine clinical practice, blood collections should be referenced relative to the time of
presentation to the ED and (when available) the reported time of chest pain onset.
The ideal biochemical marker:
- high clinical sensitivity and specificity
- appears early after AMI to facilitate early diagnosis
- remains abnormal for several days after AMI
- can be assayed with a rapid turnaround time.
Because there currently is no single marker that meets all of these criteria, and because the
interval between onset of pain and ED presentation varies form patient to patient, a
multianalyte approach has the most merit.
Ruling out AMI requires a test with high early (within 6 h) diagnostic sensitivity of > 90%
(because close to 90% of patients presenting with chest pain will not have infarction) –
myoglobin is the current marker that most effectively fits the role as an early marker.
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Ruling in AMI requires a test with high diagnostic specificity – the cardiac troponins are
currently the best markers for definitive AMI diagnosis and also fulfil the requirement for late
markers, remaining abnormal for several days after onset.
In patients with a diagnostic ECG on presentation (ST-segment elevations, presence of Qwaves or LBBB in 2 or more contiguous leads):
- Diagnosis of AMI can be made and acute treatment initiated without results of acute
cardiac marker testing.
- Biochemical marker testing at a reduced frequency (eg twice per day) is valuable for
confirmation of diagnosis, to qualitatively estimate the size of the infarction (from the
peak concentration of a cardiac marker), and to detect the presence of complications
such as reinfarction.
Detection of reinfarction:
- Occurs in approximately 17% of AMI patients, between 7 and 14 days after the initial
event.
- Use cardiac markers that return to baseline early (within 24 hours), such as
myoglobin, and CK-isoforms, and CK-MB mass (returns to baseline reasonably early
– after 3-4 days).
Cardiac markers play an essential diagnostic role for AMI rule-out of patients who have
equivocal ECG changes:
- Rule-out of AMI require serial collection and testing of blood for cardiac markers
according to the following proposed schedule:
Marker
Early (< 6 h)
Late (> 6 h)
Admission
X
X
4 h (2-4 h)
X
X
8 h (8-12)
X
X
12-24 h
(X)
(X)
(X) indicates optional determinations
-
-
When an early marker such as myoglobin is used, acute myocardial necrosis can be
effectively ruled out within 6-9 hours after ED presentation.
On the other hand, for AMI rule-in, a single positive result for either cTnT or cTnI
would trigger a diagnosis of AMI and triage of the patient to the appropriate level of
care.
A blood collection at 12-24 h may be useful for detection of reinfarction or myocardial
extension or for risk stratification of patients with unstable angina.
Two decision limits are needed for the optimum use of sensitive and specific cardiac markers
such as cTnT and cTnI:
- Patients with results between the URL and decision limit for AMI should be labelled
as having minor myocardial damage (MMD)
- Patients with results above the decision limit for AMI are diagnosed as AMI.
A single cut-off concentration can be used alternatively if set at the lower of the 2 decision
limits:
- To simplify diagnosis
- Detection of any myocardial injury is important
- Therapeutic approaches for patients with unstable angina and non-Q-wave AMI are
identical and that differentiation between these 2 groups is therefore unnecessary.
CTnT or cTnI should be used for the detection of periooperative AMI in patients undergoing
noncardiac surgical procedures. The same AMI decision limit should be used.
Myocardial infarct sizing involves serial collection of cardiac markers and integrating the area
under the curve of a plot of enzyme activity or protein concentration vs time.
- Because current cardiac markers exhibit the washout phenomenon, they are
inaccurate for infarct sizing in the presence of spontaneous, pharmacologic or
surgical reperfusion.
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Other markers that are not sensitive to reperfusion states, such as myosin heavy
chains, may provide more accurate estimates – however, commercial assays are not
readily available.
Early in the process of new assay development, manufacturers should seek assistance and
provide support to professional organisations such as the AACC or IFCC to develop
committees for the standardization of new analytes.
Although CK-MB has long been considered the biochemical standard for the laboratory
diagnosis of AMI, the development of cTnT and cTnI seriously challenge the role of CK-MB.
cTnT and cTnI appear in the blood at or near the same time as CK-MB, but remain abnormal
for 4-10 days. It is therefore suggested that cardiac troponin (T or I) become the new standard
for diagnosis of AMI and detection of MMD, replacing CK-MB. The NACB committee
recognizes that it is unrealistic for a hospital or medical centre to completely change over to
cardiac troponin without a “transition period”, during which both CK-MB and cardiac troponin
assays are offered.
The laboratory should perform stat cardiac marker testing on a continuous random-access
basis, with a target turnaround time (TAT) of 1 hour or less (TAT is defined as the time from
blood collection to the reporting of results).
Institutions that cannot consistently deliver cardiac marker TATs of ~ 1 h should implement
POCT devices. The cutoff concentrations of these devices should be set at the 97.5% URL so
that the devices detect the first presence of true myocardial injury (MMD).
Among other tasks, laboratory personnel must be involved in the selection of devices, training
of individuals to perform the analysis, maintenance of POC equipment, verification of the
proficiency of operators on a regular basis, and the compliance of documentation with
requirements by regulatory agencies. Quality-assurance and quality control programs must be
instituted and fully documented on a regular basis.
The total precision required for a particular assay is dependent on the biological variation of
the analyte, which has been established as < 5.6% for myoglobin and < 9.3% for CK-MB. The
biologic variation for cardiac troponin has not been established and was arbitrarily set at 10%.
Assays for cardiac markers should have an imprecision (CV) < 10% at the AMI decision limits
and an assay TAT of < 30 min. Before launch, assays must be characterised with respect to
potentially interfering substances.
Plasma or anticoagulated whole blood are the specimens of choice for the stat analysis of
cardiac markers, because it will eliminate the extra time needed for clotting and/or
centrifugation, thereby reducing the overall preanalytical TATs.
With EDTA tubes, troponin complexes will degrade to free subunits, because ionised calcium
is needed to maintain the complex. Troponin assays that do not exhibit an equimolar
response between complexed and free subunits will produce significant biases between
serum and EDTA plasma. Heparin does not disrupt complexes, and no change in results
between serum and plasma are expected. The laboratory must follow the recommendations
for acceptable specimen types listed in manufacturers’ package inserts and should use a
reference interval specific to the sample type.
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CLINICAL UTILITY OF CARDIAC MARKERS IN
DETECTION OF MINOR MYOCARDIAL INJURY
AND RISK STRATIFICATION OF ACS
Increased cTnT concentrations were shown to be a powerful independent risk marker (for MI
and death) within 30 days in patients who presented with myocardial ischemia. Recent
findings regarding early risk assessment have also demonstrated that:
- risk of cardiac events increases in patients with unstable angina who have increasing
maximal concentrations of cTnT within the initial 24h
- increased cTnT identifies a subgroup of patients with unstable angina in whom
prolonged antithrombotic therapy is beneficial
- in patients with AMI, the presence of an increased cTnT on admission defines a
subgroup at increased risk of subsequent cardiac events who may benefit from early,
alternative management strategies.
Data are just becoming available on the use of cTnI in unstable angina patients, and
preliminary findings suggest that cTnI appears to be similar to cTnT as prognostic indicator in
unstable angina patients without AMI [15].
Several large landmark clinical studies have been conducted that had a significant impact on
the practice of cardiology, and the role of serum cardiac markers:
- The Thrombolysis in Myocardial Infarction (TIMI) trial began in 1987 and compared
intravenous streptokinase to tissue plasminogen activator.
- The Global Use of Strategies to Open Occluded Coronary Arteries (GUSTO) trial
began in 1993 and examined multiple thrombolytic strategies.
- From these beginnings there have been several other TIMI and GUSTO trials that
have addressed continuing issues. Although cardiac markers were not the main focus
of these clinical trials, blood was collected from these patients and subsequent side
studies conducted on these samples.
GUSTO II a trial [16]:
-
Included 854 patients, who had symptoms of cardiac ischemia within 12 hours of
enrolment and an abnormal ECG
Higher cTnT at presentation was associated with higher 30-day mortality
Patients who tested positive for cTnT had a 3-fold increase in morbidity compared
with patients who tested negative
Table 5:
o cTnT was the most powerful predictor of death in the 30 days after clinical
presentation
o Among the ECG, cTnT and CK-MB, cTnT added the most information
regarding risk of 30-day mortality
o CK-MB provided no added value beyond that provided by the ECG and cTnT.
Table 5 [16]: Relative value of cTnT, CK-MB and the ECG in prediction of 30-day mortality
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FRISC study [17]:
-
-
Included 976 patients with unstable coronary artery disease (unstable angina or nonQ-wave infarction) participating in a randomised study of LMW-heparin
Peak cTnT (24 h period after presentation) was correlated with cardiac death and MI
over the following 150 days
Risk of an adverse cardiac outcome increased as cTnT increased – Table 6 and
figure 13
Conclusion: cTnT within first 24 h provided valuable prognostic information over the
following 5 months, independent of age, hypertension, number of antianginal drugs,
and ECG changes.
An extension of the FRISC study examined whether cTnT can be used to identify
patients who might benefit from therapeutic intervention [18]
Patients with cTnT  0.1 ug/l treated with dalteparin showed significant reduction in
the incidence of death and/or MI compared to placebo (7.4% vs 14.2%, p<0.01) –
figure 14.
Table 6 [17]: cTnT concentrations and 150-day outcomes from the FRISC study
Figure 13 [17]: Cumulative risk and time of occurrence of cardiac death in groups based on
quintiles of maximal cTnT concentrations.
Figure 14 [18]: Cumulative hazard curves for death or MI in patients with and without
dalteparin treatment and with and without elevation of cTnT.
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TIMI III b [19]:
-
-
cTnI was compared with CK-MB mass in 1404 patients with unstable coronary artery
disease (unstable angina or non-Q-wave infarction)
cTnI concentrations  0.4 ug/l (Dade Stratus) were associated with significant higher
mortality at 42 days (risk ratio 3.1) even in patients with normal CK-MB
concentrations – figure 15 and 16
cTnI was an independent predictor of short-term mortality after adjustment for baseline characteristics that were independently predictive of mortality (age 65 years and
the presence of ST-segment depression)
Figure 15 [19]: mortality rates at 42 days according to the time from the onset of pain to study
enrolment and the base-line cTnI levels
Figure 16 [19]: Mortality at 42 days according to the level of cardiac troponin I measured at
enrolment
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Do we need to measure both cTnT and cTnI? [20]
-
-
-
-
-
An extension of GUSTO II a was performed in 755 patients to directly compare cTnT
and cTnI.
Although 90% of the results were concordant using positive/negative cut-offs from the
respective package inserts, a significantly greater number of patients were cTnTpositive but cTnI-negative, than the other way around – cTnT measured at enrolment
appeared to be more useful for predicting 30-day mortality
ROC curves were plotted for cTnT and cTnI to evaluate the relative performance of
assays independent of cutoff – using 30-day mortality as the outcome, the area of the
ROC curve for cTnT was significantly larger at 0.68 than that for cTnI at 0.64
(p=0.002).
Results of logistic regression modelling used to examine cTnT, cTnI and the ECG as
predictive variables, also showed that cTnT provided the most information regarding
prediction of 30-day mortality – table 7.
It must be noted that the characteristics of either the cTnT or cTnI results may be
method-dependent, as are those for CK-MB. Thus, use of different or more sensitive
cTnT or cTnI assays may indicate different results [3].
Also in favour of similar performance for risk stratification using either cTnT or cTnI is
the observation that although the GUSTO II a population included was large, only ~
10% (n=74) of the patients showed discordant cTnT and cTnI results.
Table 7 [20]: Relative value of cTnT, cTnI, and the ECG in the prediction of 30-day mortality
CLINICAL UTILITY OF CARDIAC MARKERS IN
MONITORING REPERFUSION FOLLOWING
THROMBOLYTIC THERAPY
The clinical goal of reperfusion is to salvage myocardium in the early time period after acute
coronary artery occlusion.
Characteristics of the ideal thrombolytic agent would include:
- rapid reperfusion (< 10 minutes)
- 100% efficacy
- low rate of intracranial haemorrhage
- specific for recent thrombi
- no antigenecity
- sustained long-term patency
- acceptable cost
Patient demographics qualifying for thrombolytics:
- age < 75 years
- ST-segment elevation/ LBBB with a history suggestive of AMI
- Presenting within 12 hours after the onset of chest pain.
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In accordance with convention, patency in the infarct-related artery is graded by angiography
according to the Thrombolysis in Myocardial Infarction (TIMI) criteria, in which:
- TIMI 0 is no perfusion past the occlusion
- TIMI 1 is penetration past the occlusion without perfusion
- TIMI 2 is partial perfusion past the occlusion
- TIMI 3 is complete perfusion.
Although angiography is considered the gold standard procedure for assessing patency, this
method is associated with high cost, limited availability and increased morbidity when
performed acutely.
Clinical indicators such as detection of reperfusion arrhytmia and cessation of pain are
unreliable indicators of patency.
The objective of non-invasive assessment of reperfusion is rapid identification of the 20-25%
of patients in whom the occlusion persists (TIMI 0 or 1 flow) in the 90-120 min after
thrombolytic therapy, associated with increased mortality.
Because early reperfusion causes an earlier increase of cardiac markers above the upper
reference range and an earlier and greater peak after reperfusion (washout phenomenon),
cardiac markers could contribute valuable information in assessing thrombolytic success.
Figure 17: Serial CK and CK-MB values for 2 patients following AMI; one patient was
successfully reperfused after plasminogen activator therapy (boxes) and one patient without
reperfusion (circles) illustrating the “washout phenomenon”.
The rapid increase of total CK, CK-MB, myoglobin, cTnT and cTnI after successful
thrombolytic therapy induced reperfusion, follow similar early appearance kinetics.
Figure 18 [21]: Time course of serum myoglobin, CK-MB mass, cTnI and cTnT concentrations
after initiation of thrombolytic therapy in patients with TIMI grade 3 flow.
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For assessment of reperfusion status following thrombolytic therapy, at least 2 blood samples
are collected and marker concentrations compared: time = 0 defined as just before initiation
of therapy, and time = 1, defined as 90 minutes after the start. From these values, any of the
following determinations can be used as discriminating factor between successful and
unsuccessful reperfusion:
- slope value [(marker t=1 – marker t=0)/90 minutes]
- absolute value of marker at 90 minutes
- ratio of marker t=1/ marker t=0
Review of several studies demonstrates that early monitoring of myoglobin, cTnI, cTnT, and
CK-MB mass provides greater than 80% sensitivity and specificity for detecting reperfusion
within 90 minutes following the initiation of therapy [21].
Table 8 [21]: Representative guidelines studies using biochemical markers for detection of
reperfusion
It has also been shown that a myoglobin to total CK activity ratio of greater than 5.0 was
indicative of reperfusion (sensitivity, specificity and accuracy of 75%, 96% and 92%
respectively) – a single sample at admission might be used to assess spontaneous
reperfusion.
In summary, there appears to be promise in the preliminary findings for using cardiac markers
to differentiate TIMI 2,3 from TIMI 0,1 patients following thrombolytic therapy in AMI patients.
Most important, TIMI 3 flow patients cannot be differentiated from TIMI 2 flow patients.
CONCLUSION
In today’s environment of preventive and evidence-based medicine, the use of cTnI or cTnT
measured once at presentation and again at 12-24 h in patients with IHD will allow clinicians
to use markers both as exclusionary and prognostic indicators.
The results will assist in determining who is more at risk for ischemic progression, AMI and
death, and thereby determine who may benefit from early medical or surgical intervention.
This should decrease the morbidity and mortality from progression of CAD.
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