High-Sensitivity Troponin in Patients With Coronary Artery Endothelial Dysfunction

Abstract: Background. Coronary endothelial dysfunction (CED) is associated with recurrent ischemia. The role of high-sensitivity cardiac troponin I (hscTnI) levels in patients with CED has not been established. Methods. Patients with suspected ischemia, who underwent clinically indicated coronary angiography and were found to have non-obstructive coronary artery disease, were included in the study. CED was defined as ≤50% increase in coronary blood flow from baseline and/or a decrease in epicardial coronary artery diameter >20% in response to maximal dosages of acetylcholine. HscTnI was measured at the time of the procedure using the Architect hscTnI assay (Abbott). Results. Of 299 patients, 60 had normal endothelial function and 239 patients had abnormal endothelial function. The median age of the population was 52 years (interquartile range [IQR], 45-60 years). Patients with abnormal endothelial function had significantly higher log hscTnI values when compared to patients with normal endothelial function (0.9 ng/L [IQR, 0.7-1.4 ng/L] vs 0.7 ng/L [IQR, 0.7-1.1 ng/L]; P=.04). An hscTnI value >12.5 ng/L was 100% specific for the presence of endothelial dysfunction (100% positive predictive value). There were 39 major adverse cardiovascular events during follow-up. In patients with normal endothelial function, hscTnI levels were significantly higher in patients who developed major adverse cardiac events when compared to patients who did not (1.35 ng/L [IQR, 1.1-2.1 ng/L] vs 0.7 ng/L [IQR, 0.7-1.1 ng/L]; P=.02). Conclusion. Our findings suggest that endothelial dysfunction may be associated with higher baseline hscTnI levels, suggesting increased myocardial injury in this population of patients. Additional studies are necessary to further define the role of hscTnI in risk stratification in this population. 

J INVASIVE CARDIOL 2018;30(11):406-410.

Key words: chest pain, endothelial dysfunction, high-sensitivity troponin

The coronary endothelium plays a major role in modulating vascular tone and hemostasis. Normal endothelial function assists in the maintenance of vasodilation. However, with endothelial dysfunction, impaired nitric oxide production leads to leukocyte and platelet activation, causing release of inflammatory cytokines. This can result in vascular wall damage, proliferation of smooth muscle cells, and eventually atherosclerosis.^1-3 Patients with endothelial dysfunction may develop anginal chest pain due to epicardial coronary vasoconstriction, including (in extreme cases) vasospasm and/or microvascular dysfunction.^1-5 Moreover, coronary endothelial dysfunction is associated with recurrent ischemia.⁵ Cardiac troponin is the preferred biomarker for detection of myocardial ischemia.⁶ Elevated values of cardiac troponin (cTn) are associated with worse cardiovascular outcomes regardless of baseline characteristics.^7,8 Novel high-sensitivity troponin (hscTnI) assays have high analytic precision with the ability to detect subclinical ischemia, and thus may play a role in the diagnosis of endothelial dysfunction, which is characterized by recurrent ischemia.⁵ However, values of hscTnI and their potential diagnostic and prognostic influence have yet to be defined in patients with endothelial dysfunction. Given the increasing availability of hscTnI assays, it is important to define reference values and evaluate the diagnostic utility in coronary endothelial dysfunction. 

The current study aimed to define reference hscTnI values in patients with endothelial dysfunction and to determine whether there is an association between hscTnI levels and coronary endothelial dysfunction. We attempted further to explore the potential prognostic significance of hscTnI.

Methods

The current study is a retrospective, single-center study. All patients provided written and informed consent. The study was approved by the Institutional Review Board of the Mayo Foundation. 

Study protocol. All patients presented with chest pain and underwent cardiac catheterization and comprehensive assessment of coronary vasomotor function, including coronary endothelial function by administration of acetylcholine in an attempt to define the etiology of the chest pain symptoms. After diagnostic angiography using a 7 Fr JL4 guiding catheter, a total of 5000 units of heparin was infused intravenously, and a Doppler guidewire (0.014˝ diameter FloWire; Endosonics Incorporated) within a 2 Fr coronary infusion catheter (Ultrafuse; SciMed Life System) was positioned in the mid-left anterior descending coronary artery. Adenosine (60-72 µg) was infused to achieve maximal hyperemia. Acetylcholine was administered in increasing concentrations (at doses of 10-6, 10-5, and 10-4 mol/L) into the left anterior descending coronary artery for 3 minutes. Angiographic, hemodynamic, and coronary Doppler data, including vessel diameter and flow velocity, were recorded. Both microvascular and epicardial blood flow were assessed. Microvascular coronary endothelial dysfunction was defined as ≤50% increase in coronary blood flow (CBF) from baseline in response to a maximal dose of acetylcholine. Epicardial dysfunction was defined as a decrease in epicardial coronary artery diameter >20% in response to maximal dosage of acetylcholine. Endothelial dysfunction was defined as any patient having epicardial dysfunction and/or microvascular coronary endothelial dysfunction. Blood drawn at the time of the procedure was collected in heparin vacutainers, placed on ice, centrifuged within 2 hours, separated into multiple aliquots, and placed in a freezer at -80 °C. A new, never-thawed aliquot was used for each assay. cTnI measurements were made with the Abbott hscTnI assay using the Architect equipment supplied by Abbott in the hospital clinical laboratory. The limit of detection for this assay is 1.2 ng/L, with a 99th percentile upper limit of normal value of 34 ng/L for men and 16 ng/L for women.

Study population. A total of 300 patients >18 years of age were included. All patients had been referred for coronary angiography for suspected ischemia, but no evidence of acute coronary syndrome and no evidence of obstructive coronary artery disease (CAD) on angiography. The cTn levels were collected before angiography according to standardized protocol. We excluded patients with documented obstructive CAD (>70%) and end-stage renal disease (glomerular filtration rate [GFR] <30). Demographic characteristics were noted and included: documented CAD, hypertension, diabetes mellitus, prior myocardial infarction, chronic obstructive pulmonary disease, prior or active smoking history, cardiovascular drug use, peripheral vascular disease, hyperlipidemia, and absence of normal sinus rhythm. Echocardiographic data were available in all patients; included in the analysis were left ventricular hypertrophy, left atrial enlargement, regional wall-motion abnormalities, valvular dysfunction, ejection fraction <50%, and/or diastolic dysfunction, all of which have been associated with higher hscTnI levels in prior studies.

Patients were grouped into those with normal endothelial function and those with abnormal endothelial dysfunction, defined as epicardial and/or microvascular endothelial dysfunction.

Follow-up. Patients were followed for major adverse cardiovascular event (MACE) outcomes for a median of 7.0 ± 0.3 years by a trained nurse.

Statistical analysis. Descriptive statistics were used to summarize demographic and clinical characteristics. Continuous numeric variables are reported as median and lower/upper quartiles. Categorical data are summarized with number and percentage. Levels of hscTnI were transformed before analysis to their logarithmic values due to skewed distribution. The distribution of hscTnI was examined and compared for these groups using gender-specific normal values as a referent. These were included as candidate variables for the multivariate model to determine the reasons for differences if discernible. Variables assessed in both groups included body mass index, hypertension, smoking history, creatinine, estimated GFR, diabetes, ejection fraction, prior myocardial infarction, CAD, total cholesterol, high-density lipoprotein (HDL) cholesterol, and presence of structural heart disease (defined as presence of left hypertrophy, left atrial enlargement, regional wall-motion abnormalities, valvular dysfunction, ejection fraction <50% as a continuous variable, and/or diastolic dysfunction). Age was examined per 10-year change. Body mass index, creatinine, estimated GFR, ejection fraction, total cholesterol, and HDL cholesterol were assessed as continuous variables. For hypertension, smoking status, diabetes, prior myocardial infarction, CAD, presence of structural heart disease, and total cholesterol, each variable was categorized as abnormal/high or normal.

Subsequently, we aimed to determine whether the hscTnI values predicted MACE, defined as a composite endpoint of death, stroke, and myocardial infarction. A multivariate model of risk predictors was developed to assess whether hscTnI retained independent predictive value after multivariate correction for other variables associated with outcomes at the P=.20 level. Statistical significance was assumed at P<.05. Analysis was performed using JMP and Strata.

Results

Baseline characteristics. There were 60 patients who had normal endothelial function, with neither microvascular nor epicardial endothelial dysfunction, and 239 patients with abnormal endothelial function, defined as those having microvascular and/or epicardial endothelial dysfunction. The median age of the overall population was 52 years (IQR, 45-60 years). There were no significant differences in age, hypertension, hyperlipidemia, or ejection fraction in patients with and without endothelial dysfunction. There was higher aspirin use in patients with endothelial dysfunction (P=.046). Baseline demographics are summarized in Table 1.

High-sensitivity cardiac troponin I values. There was no relationship between the magnitude of endothelial dysfunction and hscTnI. Patients with abnormal endothelial function had significantly higher log hscTnI than patients with normal endothelial function (0.9 ng/L [IQR, 0.7-1.4 ng/L] vs 0.7 ng/L [IQR, 0.7-1.1 ng/L]; P=.04) (Figure 1). In multivariable analysis, adjusting for potential confounders, including age, gender, atrial fibrillation, diabetes, hyperlipidemia and left ventricular hypertrophy, log hscTnI remained significantly associated with abnormal endothelial dysfunction (likelihood ratio, 4.3; P=.04). A value >12.5 ng/L was 100% specific for the diagnosis of endothelial dysfunction (100% positive predictive value). 

Association of endothelial dysfunction with MACE. Thirty-nine MACEs occurred at follow-up. There were no differences in MACE rate between patients with and without endothelial dysfunction (Supplementary Figure S1). Moreover, there was no statistically significant difference in hscTnI levels between those who developed MACE and those who did not (Figure 2). Using a cutoff value based on analysis of gender-specific hscTnI values from our dataset (hscTnI levels >4.7 ng/L for women) and >7.0 ng/L for men), high hscTnI levels were not associated with higher MACE rates (P>.05).

When stratifying patients by the presence or absence of endothelial dysfunction, in those with normal endothelial function, hscTnI levels were significantly higher in patients who developed MACE when compared to patients who did not (1.35 ng/L [IQR, 1.1-2.1 ng/L] vs 0.7 ng/L [IQR, 0.7-1.1 ng/L]; P=.02) (Figure 2). 

Discussion

The current study has two main findings. First, hscTnI levels are independently associated with abnormal endothelial function after adjustment for potential confounders. Second, hscTnI levels were associated with MACE in patients with normal endothelial function, perhaps secondary to the lack of aggressive medical therapy in this group of patients.

In many patients with suspected ischemia, endothelial dysfunction may be partially responsible for symptoms of chest discomfort secondary to recurrent ischemia.^9-11 In fact, multiple studies have shown that symptomatic patients with endothelial dysfunction and non-obstructive coronary disease often have myocardial perfusion defects.^9,11,12 Endothelial dysfunction not only leads to symptoms, but is also associated with unfavorable outcomes. Multiple registries, including the cardiovascular health study and multiethnic study of atherosclerosis, have demonstrated an association between the presence of endothelial dysfunction and adverse cardiovascular outcomes.^13-19 In a prospective study, Suwaidi et al showed that symptomatic patients without obstructive CAD who have severe endothelial dysfunction in response to acetylcholine have a 14% increase in the composite risk of cardiovascular death, MI, heart failure, and stroke.²⁰ Importantly, we and others have demonstrated the ability to mitigate symptoms with specific therapies. These findings necessitate the need for better diagnostic tools with which to identify endothelial dysfunction.

Compared to contemporary assays, high-sensitivity troponin assays have the ability to measure 10-fold lower concentrations of cardiac troponin.^21,22 This potentially enables quantification of almost all individuals in society and allows for detection of subclinical ischemia.⁶ There are compelling data that small increases in hscTnI within the normal range identify those with subclinical myocardial injury from structural heart disease or subclinical ischemia.²³ Indeed, in our study, patients with endothelial dysfunction had higher levels of hscTnI compared to patients with normal endothelial function. 

Although there was significant overlap, a value >12.5 ng/L was highly specific for endothelial dysfunction. These signals are likely due to the flow-demand mismatch associated with endothelial dysfunction.^5,9,11 Endothelial dysfunction causes alterations in vasomotor regulation of the coronary vasculature, which in turn can lead to recurrent ischemia either by epicardial coronary artery vasospasm or an inability of coronary microcirculation to increase blood flow in response to stress. The level of hscTnI thus likely recognizes the subclinical ischemia present in patients with endothelial dysfunction. This is likely the mechanism for the increases in hscTnI we observed. 

These abnormalities can be exacerbated by structural abnormalities such as left ventricular hypertrophy and increased left ventricular wall stress, which is associated with endothelial dysfunction as well as with elevated hscTnI values.²⁴ Accordingly, we adjusted our analyses for structural heart disease but the findings of a higher hscTnI persisted, suggesting that subclinical ischemia is more likely the underlying mechanism for the troponin elevations. This may have important clinical implications as it provides a reference value for this patient subset and can help identify etiology in patients with suspected ischemia.

Previous studies have shown that any increase in hscTnI levels may be associated with substantial increases in the risk of all-cause mortality and cardiovascular death, acute MI, and heart failure.^{25-30,25,32,33} The current study shows that this phenomenon held true in patients with normal endothelial function. 

Study limitations. This was a retrospective observational study to establish an association between hscTnI and endothelial dysfunction. The retrospective design of this study has inherent bias. The findings in this study were based on hscTnI levels drawn at the time of the initial presentation in a cross-sectional fashion; therefore, a temporal association could not be verified with certainty. Samples used in this study were collected between 1992 and 2009 and stored as frozen samples that were then rethawed and processed specially as suggested by the manufacturer for this study. Despite the precautions taken with extra centrifugation given the hscTnI assay, and the fact that troponin concentrations in general remain stable for many years,³¹ there might be degradation and thus under-estimation of troponin levels. The data on outcomes in this study were obtained through a questionnaire-based approach, which could lead to under-reporting of the events in each group. Finally, there was no association between hscTnI values and MACE in patients with endothelial dysfunction, which could be due to the presence of other confounders that were not adjusted for, or due to the aggressive treatment we initiate here at Mayo in these patients.

Conclusion

Endothelial dysfunction was associated with baseline elevated hscTnI likely secondary to recurrent ischemia. An hscTnI value of >12.5 ng/L had good specificity for the presence of endothelial dysfunction and may be helpful in identifying which patients with possible endothelial dysfunction should be more aggressively studied. Further investigation is necessary to better define the role of hscTnI in risk stratification in this population.

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^*Joint first authors.

From the ¹Department of Cardiovascular Diseases and ²Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, Minnesota.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Karon reports non-financial support (instrument/reagents for high-sensitivity troponin I) from Abbott Diagnostics. Dr Jaffe reports consultant income from Beckman-Coulter, Roche, Siemens, Abbott, Sphingotec, Quidel, ET Healthcare, and Novartis. The remaining authors report no conflicts of interest regarding the content herein.

Manuscript submitted July 4, 2018, provisional acceptance given July 12, 2018, final version accepted July 23, 2018.

Address for correspondence: Allan S. Jaffe, MD, Department of Cardiovascular Diseases, Mayo Clinic, 200 First Street SW, Rochester, MN 55905. Email: jaffe.allan@mayo.edu