Biomarkers and myocardial injury
In this exploratory study we demonstrated significant relationships between MR-proADM/CT-proET-1
and myocardial injury. The relationship was strongest and most consistent with CT-proET-1.
This finding supports a biologically plausible relationship as both pro-hormones are
strongly vasoactive and may play key roles in sepsis-associated myocardial injury.
Indeed, we demonstrated significant associations between both pro-hormones and hsTNT
and echocardiographic markers of LV systolic dysfunction.
In epidemiological studies, increased MR-proADM has been associated with poor cardiovascular
outcomes 22], 25], 26]. In sepsis there is upregulation of ADM expression 27], 28] and ADM seems to be important for the initiation and continuation of hyperdynamic
shock in animal models 4], 5], 29]–31]. Importantly, the administration of anti-ADM antibodies prevents the hyperdynamic
response 27] and seems beneficial to survival 32], 33], while exogenous ADM prevents and reverses hypodynamic circulation and pulmonary
hypertension, and reduces endothelial hyperpermeability in experimental models of
septic shock 4], 5], 30], 34], suggesting possibilities for therapeutic intervention. In this study we found only
moderate correlation between MR-proADM and two of four echocardiographic markers of
reduced LV systolic function. Despite this there was strongly significant correlation
between proADM and hsTNT concentrations, which could suggest a role of this pro-hormone
in cardiac injury.
Experimental and clinical studies link increased ET-1 levels to decreased cardiac
performance 17], 19], 35]–38]. This is supported by our findings of highly significant correlation between CT-proET-1
levels and all echocardiographic markers of reduced LV systolic function, and hsTNT.
The results of these studies appear paradoxical to earlier experimental data showing
positive inotropic effects of ET-1 39], 40]. Thus, the role of ET-1 is still unclear and seems related to the balance between
receptor types.
Antagonism of endothelin pathways has been explored in a number of experimental settings,
and its effects during septic shock are areas worth exploring 35]–37], 41]–43]. To our knowledge, there is only one other study investigating the relationship between
cardiac function and CT-proET-1 in patients with septic shock. Furian et al. 17] demonstrated significant association between CT-proET-1 and echocardiographic markers
of left and right ventricular dysfunction, but did not describe biochemical markers
of myocardial injury. Our findings highlight the importance of CT-proET-1 in cardiac
dysfunction measured using echocardiography and cardiac troponins, and in mortality.
Importantly, the LR- of 0.25 indicates that CT-proET-1 is useful for ruling out myocardial
injury. Taken together, our results indicate that the combination of MR-proADM and
CT-proET-1 might be a useful supplement for the diagnosis of myocardial injury, as
shown by a LR+ of 12.
Biomarkers and mortality
We have shown that increased concentrations of MR-proADM, CT-proET-1, and hsTNT are
increased in non-survivors of septic shock, supporting the results of earlier studies
7], 11], 16], 19], 44]–46]. MR-proADM and hsTNT seem to be more important determinants of both short-term and
longer-term outcome, whereas CT-proET-1 seems to be most significant for longer-term
mortality with higher concentrations detected in non-survivors on days 2–5 (Fig. 1b). When considered as a pair, CT-proET-1 and MR-proADM increased the odds for mortality
twofold to fivefold. When a combined panel of all three biomarkers were positive,
the posttest odds for mortality increased 13-fold to 20-fold.
ProADM and proET-1 are especially attractive biomarkers in septic shock because they
are both endothelium-derived pro-hormones and their end products have important vasoregulatory
opposing effects. As suggested by Scheutz and colleagues 45] it is plausible that the net balance between the hormones is of significance for
clinical outcome. Increased concentrations of ADM and ET-1 have been described in
patients with systemic inflammatory response syndrome (SIRS) 47] and septic shock 6], 7], 17], 29], 44]–46], 48], and appear to be related to severity and mortality, but dynamic evaluations and
their significance for short-term and long-term mortality in patients with shock are
poorly investigated. Herein we demonstrated that concentrations of both pro-hormones
are higher in non-survivors, particularly during the first 3 days of ICU admission
(see Fig. 1).
In line with our results, Guignant et al. reported higher initial levels of proADM
among non-survivors of septic shock. Further, the combination of proADM with a vasoconstrictor
biomarker, pro-vasopressin, was better for prediction of 28-day mortality when assessed
at day 1–2 than the SOFA score and simplified acute physiology score (SAPS) II 7]. Similarly, in a cohort of critically ill patients with sepsis, Christ-Crain et al.
found a significantly higher level of proADM among intensive care unit (ICU) non-survivors
46]. They reported an optimal cutoff value of 3.9 nmol/L for MR-proADM, resembling the
optimal cutoff of 3.5 nmol/L identified in this study for 28-day mortality. The optimal
cutoff identified by Guignant et al. was also in this range (5 nmol/L) 7]. Taken together, these findings support proADM as a useful predictor of mortality.
Our results for ET-1 are different to those reported previously, where no differences
between survivors and non-survivors were shown 45], 49]. There may be several explanations for this. First, our patients were severely ill
with higher illness severity scores than in previous studies. The median day 1 SOFA
and APACHE II scores were 12 and 24, respectively, and all 53 patients were in shock
despite fluid resuscitation. Second, we used 7-day and 28-day mortality as outcome
parameters, in contrast to in-hospital mortality as used in some of the other studies.
Third, we collected blood 6-hourly in the first 24 hours, and used average daily values
in an attempt to capture average values for each patient every day. In comparison,
Scheutz et al. collected a single sample within 24 hours of ICU admission. Guignant
et al. collected a single sample within 48 hours of ICU admission and had a substantial
number of missing values. These reflect difficulties in the conduct of clinical studies
but may be of significance, as measuring biomarker levels at an early stage, i.e.,
when the patient is most unstable, may reveal important information about the state
of the cardiovascular system. It also allows the possibility of early intervention
and disease staging.
Although elevated cTn is most commonly used for the diagnosis of MI 50], increased cTns are commonly seen in patients with septic shock without MI and are
independent predictors of mortality 11]–15]. Recent studies suggest that high-sensitivity assays may add to risk assessment and
prediction models 11], 16]. Our study confirms the importance of hsTNT for the outcome of patients with septic
shock. When used as an indicator of injury along with echocardiographic parameters,
it may potentially be used to stratify risk and monitor treatment. Both alone, but
especially when used in a biomarker panel with MR-proADM and CT-proET-1, hsTNT increased
the posttest odds ratio of mortality by 13-fold to 20-fold.
It remains to be seen whether this biomarker panel ultimately improves current risk
prediction models in critical care. Another potential area of investigation is the
use of these biomarkers as a basis for selection of patients for interventional studies,
or as pharmacodynamic markers for cardiac dysfunction.
Limitations
This paper has several limitations. This study was designed to be exploratory in nature
and the findings here confirm associations between biomarkers and outcome, and refrains
from any conclusions on causality. The limited number of outcome events does not allow
adequate power for multivariate analysis. As a rule-of-thumb 10 outcome events would
be required for each multivariate variable 51], thus, future studies investigating the prognostic potential of these biomarkers
should be planned with this in mind. While we realize the limitations of this type
of monocenter investigation, in particular the risk of overestimation of effect size,
we believe that our study contributes new information to a hitherto under-investigated
area. Second, although we defined ICU admission as a starting point for this study,
patients have had variable times to presentation, different degrees of shock and variable
responses to fluid resuscitation, making the material potentially heterogeneous. As
dynamic changes in biomarker levels may be important, particularly early in the course
of septic shock, we attempted to capture these changes by measuring up to four times
during the first 24 hours, and twice daily during ensuing days. Closer sampling times
may have revealed different results. We have no data on right ventricular echocardiographic
parameters. As almost all components of the endothelin system are upregulated in pulmonary
hypertension 8], and right ventricular dysfunction is common in septic shock, it is plausible that
high levels of CT-proET-1 could correlate with right ventricular dysfunction. Because
of the lack of a universal definition of myocardial injury, our definition was arbitrary
but chosen on the basis of previous studies 23], 52]–56]. As premorbid echocardiographic data were not available, we cannot exclude that some
patients suffered from co-existing myocardial dysfunction that was unrelated to sepsis.