The never-ending story of cardiac biomarkers: A further step toward a very early detection of ischemic patients?
The never-ending story of cardiac biomarkers: A further step toward a very early detection of ischemic patients?
Claudio Marcassa 0
0 Reprint requests: Claudio Marcassa, MD, Maugeri Clinical and Scientific Institutes, IRCCS, Cardiology Department, Scientific Institute of Veruno (NO) , Via Per Revislate 13, 28010 Veruno , Italy; J Nucl Cardiol 1071-3581/$34.00 Copyright 2018 American Society of Nuclear Cardiology
1 Maugeri Clinical and Scientific Institutes, IRCCS, Cardiology Department, Scientific Institute of Veruno (NO) , Veruno , Italy
Once upon a time. In 1954, Karmen et al. found
that serum glutamate oxaloacetate transaminase (SGOT)
activity was increased in patients with acute myocardial
infarction, suggesting the possible application of these
biomarkers in patients with acute coronary syndromes.1
Then, in 1963 creatine kinase (CK) was found to rise
rapidly following an acute coronary syndrome (ACS).2
In 1966, the superior cardiac specificity of CK-MB was
demonstrated3; this led to the adoption of CK-MB as the
biomarker of choice for the detection of myocardial
damage until the early 1990s, when cardiac troponins
(cTn) were demonstrated to have both higher sensitivity
and specificity in the diagnosis of patients with ACS.
It is a matter of debate whether cTnI and cTnT are
released into blood only from irreversible
cardiomyocyte damage with complete loss of membrane integrity
such as, for example, AMI, or also during reversible
damage such as transient myocardial ischemia.
Changing the target: from cell death to transient
cell hurt. Some preclinical and clinical studies suggest
that not only necrosis but also transient myocardial
ischemia might play a role in the release of some
biomarkers such as BNP, N-terminal pro B-type
natriuretic peptide (NT-proBNP), or cTn.4–12
In this view, in this issue of the Journal, Pipikos
et al. correlated the behavior of common biomarkers
considered to be associated with a critical and prolonged
reduction in coronary blood flow, such as NT-proBNP
and high sensitivity cardiac troponin T (hs-cTnT) with
the presence of myocardial ischemia as assessed by
exercise radionuclide myocardial perfusion imaging
(MPI).13 They found that only hs-cTnT levels were
significantly different between patients with or without
ischemia, although in this study they did not assess the
severity of ischemia; however, NT-pro BNP was not
able to identify ischemia. The authors made the
hypothesis of two possible mechanisms at the basis of
hs-cTnT released during ischemia: from the fraction
located in the cytosol, even in the absence of cell
death,14 or due to micro-ruptures of non-calcified
plaques, suggesting that hs-cTnT might also be used as a
marker of vulnerable coronary plaque.15 At odds with
most of the other available studies assessing the value of
biomarkers in the detection of ischemia, as determined
by MPI, and that evaluated just cardiac troponins or
BNP, Pipikos et al. also evaluated additional
biomarkers, such as ischemia modified albumin, and
neuropeptide Y; however, like NT-proBNP, these
biomarkers were not associated with ischemia.13
The matter of whether the presence of cTn in the
blood is correlated with the presence or severity of
ischemia was assessed by several trials that used
radionuclide MPI as the reference method for the
demonstration of reversible perfusion defects.
In a preliminary study by Thayapran et al. who
compared CK-MB and cTnI to exercise stress thallium
testing in a small group of 31 patients with known or
suspected of coronary artery disease, there were no
increases in either cTnT or cTnI in blood samples
collected both before and 12 to 24 h after exercise testing in
any of the patients, including those with severe
However, because of limitations of the analytical
performance of troponin assays at very low
concentrations, it was not easy to address adequately
whether there is a quantifiable release of cardiac
troponins in the clinical setting of myocardial ischemia.
Recently, a cardiac troponin assay has been
developed that has a limit of detection that is at least one order
of magnitude lower than current commercial assays (this
is highly sensitive (hs)-cTn). The effect of reversible
myocardial ischemia on hs-cTnT levels was evaluated by
Kurz et al. in a group of 100 patients: reversible ischemia
did not induce significant changes of hs-cTnT, baseline
concentrations of hs-cTnT were not different in patients
with or without perfusion defects, and hs-cTnT levels did
not change after 18 min and 4 h.17 On the other hand, in
120 patients enrolled in the Protein Markers of Ischemia
using Proteomic Testing (PROMPT)—TIMI 35
prospective cohort study, hs-cTnI was measured before,
immediately after, and 2 and 4 h after stress testing, and
correlated with the presence and severity of ischemia as
assessed at MPI. The Authors documented a significant
rise in circulating hs-cTnI levels in response to stress
testing, with a level proportional to the amount of
ischemia.18 Moreover, in another large group of 714 patients,
significantly higher levels of hs-cTnI were documented at
rest in patients with exercise-induced ischemia compared
with those without: using a cut-off value of 1.54 ng/L,
sensitivity, specificity, positive, and negative predictive
values of 95%, 18%, 26%, and 92%, respectively, were
documented. Finally, hs-cTnI was also an independent
predictor of exercise-induced myocardial ischemia in
The finding that cTn does not increase as a response
to reversible myocardial ischemia was also suggested by
the experimental results of Fishbein et al. demonstrating,
by immunohistochemical staining using antibodies to
human cTnT and cTnI, that the loss of cTnT and cTnI by
the cells occurs very early following a prolonged
ischemic injury and precede the histologic evidence of
necrosis, but it does not occur in myocardium that is not
However, the exact mechanism for the discharge of
cTn is still debated. Considering that the transport across
the cell membrane is a prerequisite for the release of
troponin from viable cardiomyocytes, the smaller size of
cTnI (approximately 26 kDa) compared with cTnT (37–
39 kDa) could make cTnI a superior biomarker to detect
reversible ischemia. As a matter of fact, while hs-cTnI
concentrations have been found to be selectively
increased in patients with reversible myocardial
ischemia,18 hs-cTnT does not.21,22
In an experimental animal model, cTnI values after
brief reversible ischemia increased between 3 and 24 hours
following restoration of flow. Although brief ischemia did
not produce any pathologic evidence of necrosis or
infarction, regional apoptosis of single dispersed myocytes
transiently increased at 1 hour and normalized 24 hours
after reperfusion. This fact is in contrast with the pattern of
myocyte necrosis that occurs in myocardial infarction and
provides a possible mechanism by which cTnI is released
into the circulation: cTnI elevations do not reflect the early
release of an exchangeable pool of cTnI from viable
myocytes but arise from delayed programmed myocyte
death from apoptosis.23
Another variable to be taken into account when
assessing molecules as biomarkers of ischemia is the
degree of exercise or the duration of the ischemic event
during a stress test. Cardiovascular biomarkers (cTnT,
hs-cTnT or hs-cTnI, BNP, NT-proBNP, and D-dimer)
that are currently used in clinical diagnosis of ACS and
heart failure are influenced by the presence of strenuous
exercise. For this reason, it is necessary to take the
occurrence of physical exercise into account when a
cardiac emergency is suspected. In a review of 33
studies, including 1045 athletes, the response in cTnT
concentration after strenuous exercise was evaluated: in
51% of subjects cTnT increased above the cut-off value.
The exercise-induced increase in troponins could be due
to the release of cytoplasmic cTnT and cTnI, because
exercise may increase membrane permeability of
cardiomyocytes.24,25 This reversible membrane leakage
might be also due to increased mechanical stress on the
cardiomyocytes, overload with free radicals, increased
body temperature, or prolonged acidosis.24,26 However,
the reason for the release of non-structurally bound cTn
through reversible membrane leakage in apparently
healthy athletes remains controversial.
B-type natriuretic peptide (BNP) is one of the
biomarkers linked to heart failure; its usefulness as a
marker also of ischemia is still controversial. In 63
patients (62% with known coronary disease) undergoing
exercise stress single-photon emission computed
tomography (SPECT), plasma BNP values, at baseline
or after exercise, were not associated with myocardial
ischemia.27 These results are in agreement with those
reported by Pipikos et al. that also documented that
reversible ischemia was not associated with changes in
NT-pro BNP values.13
Many transient cTnI elevations probably occur
without demonstrable ECG changes or chest pain, since
these are insensitive indices of brief ischemia, and might
justify the presence of elevated levels of hs-cTnI in
resting conditions in patients that will manifest
stressinduced ischemia. However, mechanisms other than
repetitive ischemia have been also postulated. In 378
patients with stable angina and unknown CAD and
enrolled in the Evaluation of Integrated Cardiac Imaging
(EVINCI) study, hs-cTnT and NT-proBNP were
measured. All patients underwent stress imaging to detect and
quantify myocardial ischemia and coronary computed
tomographic angiography to define the presence and
characteristics of coronary artery disease. In this group of
patients, the presence and extent of coronary
atherosclerosis was related with circulating levels of hs-cTnT, also
in the absence of ischemia, suggesting an
ischemia-independent mechanism of hs-cTnT release; furthermore,
only patients with CAD and ischemia showed
significantly higher levels of NT-proBNP.28
The article by Pipikos et al. is another piece of the
conundrum about the role of biomarkers, and hs-cTn in
particular, in the diagnosis of ischemic heart disease
other than ACS. The data presented in this study also
continue the debate concerning the different
mechanisms responsible for biomarkers released in response to
transient ischemia vs. prolonged ischemia leading to
necrosis. Understanding the basis for the release of
cardiac marker will have an impact on how these tests
can be used in the clinical setting and for patients’
management. It is now important to assess the
shortterm risk of cardiac events for non-AMI patients who
present to the ED with a history of acute chest pain. In
this case, the use of a cardiac marker that is increased,
also as result of a reversible ischemic event, might be of
help in identifying high-risk subjects. It may be possible
that cardiac troponin can be used for both reversible
ischemia and irreversible necrosis if one accepts the
concept of continuum spectrum for ischemic heart
disease, with low blood concentrations indicating the
former and higher concentrations the latter. Thus, two
cut-off concentrations might be necessary to
differentiate between these two events.
While it seems, according to most reports, that
NTproBNP is not ideal as a marker of ischemia, several
issues about troponins are still matter of debate: What is
the role of elevated hs-cTn at rest (and at which cut-off
point) in identifying patients with coronary artery
disease and prone to more severe ischemia and thus at
higher risk? Is the increase in hs-cTn related to the
amount of ischemia and how long elevated blood levels
are detectable after an ischemic episode? Can a single
sample be used at the emergency department to rule out
patients with suspected IHD? Which hs-cTn (hs-cTnT or
hs-cTnI) performs better?
Other studies are necessary to answer these critical
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