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Keywords:

  • Cardiac biomarker;
  • Cattle;
  • Noncardiac disease

Abstract

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. References

Background

Increased cTnI concentrations are associated with adverse outcomes in humans and animals. Limited information is available on the prognostic value of cTnI in cows.

Objective

To measure cTnI in cows with noncardiac diseases and evaluate the association of cTnI concentration with adverse outcomes such as death or early removal from the herd.

Animals

Thirty control and 53 diseased cows.

Methods

Serum cTnI concentrations were determined with a point-of-care immunoassay. Cows were diagnosed ante- or postmortem with metritis (n = 6), mastitis (n = 4), peritonitis (n = 6), LDA (n = 14), LDA and metritis (n = 4), pneumonia (n = 6), dystocia requiring cesarean section (n = 5), and downer cow syndrome (n = 8). Animal survival was determined for up to 2 months after presentation.

Results

The immunoassay showed reliability for the detection of bovine cTnI. Cows with LDA and metritis (P < .05), peritonitis (P < .05), LDA (P < .001), dystocia requiring cesarean section (P < .01), and downer cow syndrome (P < .001) had higher cTnI concentrations than control cows. The odds of a negative outcome (death or culling) for cows with cTnI concentrations of ≥0.05, ≥0.1, ≥0.2, and ≥0.5 ng/mL were 2.4, 2.9, 4.8, and 6.2, respectively.

Conclusion

Cows with noncardiac diseases may have some degree of myocardial injury. The magnitude of cTnI increased may assist clinicians in evaluating the risk of an adverse outcome and help guide decision-making regarding treatment and prognosis.

Abbreviation
cTnI

cardiac troponin I

The use of cardiac troponin I (cTnI) as a minimally invasive, cost-effective clinical diagnostic test for myocardial injury in both large and small animal patients has increased in recent years.[1-6] In cattle, myocardial damage leading to increased circulating serum cTnI concentrations has been reported in cases of pericarditis,[7] endocarditis,[8] foot-and-mouth disease,[9] downer cow syndrome,[10] and umbilical abscess.[10] A positive correlation between the magnitude of cTnI concentration and the severity of histologic evidence of myocardial injury was reported in cattle with experimentally induced monensin toxicosis.[11] Increased cTnI concentrations have been identified in people with noncardiac diseases[12] such as sepsis[12, 13] or septic shock,[12, 14] pulmonary embolism[15] and chronic obstructive pulmonary disease.[16] Similar findings have been reported in septic foals,[17] calves with experimentally induced endotoxemia,[18] laboratory animals after chemotherapy,[19] dogs with renal failure[20, 21] and noncardiac systemic disease,[21] gastric dilatation-volvulus[22] and in horses with surgical colic.[23]

Increased concentrations of cTnI in noncardiac diseases can have an ischemic, toxic, or inflammatory origin. Ischemic damage may occur because of increased oxygen consumption, decreased perfusion pressure, and decreased oxygen delivery to cardiac muscle[24] caused by hypotension, tachycardia, hypoxemia, and anemia.[24, 25] In cases of sepsis or systemic inflammatory response syndrome (SIRS), the mechanism of cTnI increase is not completely understood. It is assumed that cytokines and endotoxin are released leading to myocardial depression and ventricular dysfunction.[24, 26] Myocardial depressants such as tumor necrosis factor-α (TNF-α) and interleukins (IL) are released in sepsis and trauma patients.[26, 27] Exposure to TNF-α can markedly increase the permeability of endothelial monolayers to macromolecules and lower molecular weight solutes.[28] Similar permeability changes may occur at the level of myocardial cells leading to leakage of cytoplasmic cTnI without cardiomyocyte necrosis. In 1 study of critically ill human patients, increased troponin concentration was associated with significantly higher median TNF-α concentration.[29] Decreased left ventricular ejection fraction also was significantly correlated with increased cTnI concentrations in that study.[29] Increased troponin in cases of sepsis or SIRS may be caused by incomplete cardiomyocyte apoptosis, without irreversible cardiomyocyte damage caused by hypoxia or activated clotting factors.[30, 31]

In humans, the presence of detectable cTnI has been associated with increased adverse event rate and increased mortality rate, which was proportional to the magnitude of cardiac troponin release.[32] In dogs with dilated cardiomyopathy, cTnI concentration correlated well with mortality.[33] Dogs that died after diagnosis of GDV and concurrent complications such as shock, ischemia, and reperfusion injury had significantly higher cTnI concentrations compared to dogs that survived GDV.[22] Cardiac TnI concentration also correlated well with severity of disease and was associated with an adverse outcome in dogs with babesiosis.[34] Increased cTnI and T concentrations also were observed in septic neonatal foals, but no significant difference was found between surviving and nonsurviving foals in that study.[17]

Cattle with primary cardiac diseases such as endocarditis and pericarditis as well as noncardiac intrathoracic disorders such as mediastinal abscesses, thymic lymphoma, caudal vena cava syndrome, and chronic suppurative pneumonia have been reported to have increased cTnI concentrations.[35] However, that study reported cTnI concentrations in cattle with a definitive diagnosis made at necropsy, and no conclusions were drawn regarding the magnitude of the cTnI concentration as a predictor for survival.[35] In 1 study of horses, postoperative increases in cTnI concentration were associated with death in horses with surgical colic.[23]

To the authors' knowledge, no studies have reported serum cTnI concentration in cows with noncardiac diseases and its association with mortality. The objective of this study was to evaluate the diagnostic value of cTnI as measured with the i-STAT point-of-care immunoassay as a minimally invasive test to predict survival in cows with common production diseases. We hypothesized that (1) the i-STAT point-of-care immunoassay is an accurate test for measuring cTnI in cows, (2) measured serum cTnI concentration is nondetectable or very low in healthy cows when using the i-STAT immunoassay, (3) serum cTnI is increased in cows with systemic, noncardiac diseases that potentially compromise cardiovascular function, and (4) measurable increased serum cTnI concentration is associated with adverse outcomes such as death, euthanasia, or early removal from a herd.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. References

Study Population

The study group was comprised of 43 Holstein dairy cows and 10 female beef cows (1 Shorthorn, 1 Beefmaster, 1 polled Hereford, 2 Main-Anjou, and 5 Angus) that presented to the William R. Pritchard Veterinary Medical Teaching Hospital, University of California-Davis or that were examined during routine herd health visits by one of the investigators (T.W.G.) between November 2008 and May 2009. The animals ranged in age from 11 months to 10 years. Animals used in this study were diagnosed ante- or postmortem with metritis (n = 6), mastitis (n = 4), peritonitis (n = 6), LDA (n = 14), LDA with metritis (n = 4), pneumonia (n = 6), dystocia requiring cesarean section (n = 5), and downer cow syndrome (n = 8). The case definition of metritis was presence of foul smelling, reddish-brown colored and watery vaginal discharge with decreased uterine tone and abnormal uterine involution as determined by rectal palpation. Mastitis was defined as an acute onset of swelling, heat, and inflammation of 1 or more quarter, a positive California Mastitis Test, and positive bacteriologic milk culture. Cows with chronic mastitis were excluded from the study. Diagnostic criteria for peritonitis were evidence of abdominal pain, accumulation of peritoneal fluid in the abdominal cavity, and evidence of fibrin in the abdomen as determined on ultrasonographic examination or evidence of abdominal adhesions with or without a penetrating foreign body in the reticulum at the time of exploratory surgery, radiographic examination for detection of a metallic foreign body or necropsy. Cases in which a foreign body penetrated the diaphragm were excluded from the study. A displaced abomasum was clinically diagnosed by auscultation of a left-sided “ping” and was confirmed in all cases during corrective omentopexy surgery via right flank laparotomy. Pneumonia was clinically diagnosed on the basis of thoracic auscultation findings of crackles, wheezes or absent bronchovesicular sounds and confirmed at either necropsy or during thoracic radiography that demonstrated air bronchograms or consolidation, or ultrasound examination that demonstrated consolidation with presence of a disrupted lung surface with comet tails. Cows experiencing dystocia were presented with fetal-maternal mismatch (n = 3), abnormal presentation, position or posture (n = 1) of the calf or with a partially dilated cervix (n = 1), making manual calf removal or fetotomy impossible. After vaginal examination, all such cows were surgically prepared and underwent cesarean section. Cows were diagnosed with downer cow syndrome when presented in sternal or lateral recumbency with inability to rise despite medical treatment for underlying electrolyte disturbances.

A telephone interview was conducted 2 months after presentation to determine the disposition of cattle included in the study. Hospitalized cows that were euthanized or died during hospitalization underwent necropsy examinations; necropsies were not performed on nonhospitalized cows. Examination of the myocardium was only performed in cases in which a pathologist determined, based on gross examination of the heart, that further examination was warranted. Myocardial evaluations were not standardized and no specific sections of the heart were evaluated.

cTnI Assay

Blood (10 mL) was collected at presentation either from the external jugular or coccygeal vein into a vacutainer blood tube.1 The sample was allowed to clot at room temperature for 30 minutes and then centrifuged for 15 minutes at 2,800 rpm. Serum was separated and frozen at −20°C until analyzed. All samples were tested within 2 weeks after collection. Serum samples were assayed for cTnI concentration on an i-STAT immunosystem2 cTnI cartridge. This assay uses a 2-site ELISA method. The manufacturer reported a measurable range for human cTnI from 0.00 to >50.00 ng/mL. Concentrations ≤0.02 ng/mL cannot be differentiated, but the analyzer provides a specific point estimate of either 0.00, 0.01 or 0.02 ng/mL.[36] This assay has been shown to be a sensitive and specific assay to monitor cTnI concentrations in humans; similar assay characteristics are anticipated when used in cows because of the 96.4% amino acid sequence homology between human and bovine cTnI.[37]

Validation Study

To assess the performance of the i-STAT immunoassay with bovine serum, a validation study was performed. Serum was collected from a healthy adult Holstein blood donor cow. Duplicate cTnI determinations were performed with the i-STAT cTnI assay and no detectable concentration of cTnI was identified. This serum was considered free of cTnI.

To determine within-lot precision of the i-STAT assay, cTnI-free serum was spiked with purified bovine cTnI3 to final cTnI concentrations of 0.5, 1.0, 2.0, 8, and 32 ng/mL. These samples were analyzed in triplicate on the same day. Between-day precision was determined in duplicate with the same cTnI-spiked serum samples on 2 subsequent days. Assay linearity was evaluated using spiked serum samples containing 4 different concentrations of cTnI (0.5, 2.0, 8.0, 32.0 ng/mL). These were each diluted with cTnI-negative serum to 80, 60, 40, 20, and 10% of the original concentration. Percentage test recovery was determined for each dilution. Linearity samples were tested in duplicate. The limit of detection of the i-STAT cTnI assay for bovine cTnI was determined in triplicate using serum samples spiked with bovine cTnI at 0.1, 0.05, 0.01, and 0.005 ng/mL; the lowest measurable value was reported.

To determine a bovine cTnI reference interval, the i-STAT immunoassay was used to measure cTnI concentration in serum from 30 dairy cows (26 Holstein, 4 Jersey) determined to be clinically healthy based on physical examination that included cardiac and pulmonary auscultation. The same animal population has been used previously for a cTnI validation study performed by one of the authors (A.V.).[38] Serum samples for this study were taken at the same time point.

Statistical Analysis

Statistical Analysis was performed using commercially available software.4 Precision was calculated as coefficient of variation (CV%) = (SD/average) × 100%. Linearity of serial dilutions at different cTnI concentrations was determined by linear regression analysis. Linearity of the assay was assumed when the correlation coefficient was >0.95 as previously recommended.[39] Recovery (%) was calculated as (obtained cTnI concentration/expected cTnI concentration) × 100%.

The Kolmogorov–Smirnov test was used to assess normal distribution of residuals. A Kruskal–Wallis ANOVA test on rank was used to test differences among cattle groups within different diagnosis categories. Dunn's multiple comparison test was used when significant differences were observed between these groups. The Chi-square test or Fisher's exact test (when a cell in the 2 × 2 table had ≤5 animals) was used for assessment of statistical differences between cows surviving up to 60 days after presentation and cows that died were euthanized or sold for slaughter 2 months postpresentation. Cows that died were sold or euthanized were grouped together for statistical analysis as a negative outcome group. The Mann–Whitney sum test was used for evaluation of statistical differences between dead cows with histopathologic evidence of myocardial injury and cows without observed histopathologic lesions. The association between high serum cTnI concentration and outcome (negative versus positive outcome) was evaluated by calculating the odds ratio using a 2 × 2 table. Test positivity of the immunoassay at each cutoff point (0.05, 0.1, 0.2, 0.5 ng/mL) was defined as the conditional probability that the test result on the immunoassay was positive for cTnI given the presence of a disease condition (eg, metritis). Test negativity of the immunoassay at each cutoff point (0.05, 0.1, 0.2, 0.5 ng/mL) was defined as the conditional probability that the test result on the immunoassay was negative for cTnI given the presence of a disease condition.

All cTnI concentrations in this study were reported as median and interquartile range (25th and 75th percentiles) unless stated otherwise. Statistical significance was defined as P < .05.

Results

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. References

The mean within-day assay and the mean between-day assay precision of cTnI concentrations from 0.5 to 32 ng/mL were 11.8 ± 6.1 and 13 ± 4.9%. Regression analysis showed good linearity for all sets of serial cTnI dilutions except at 0.5 ng/mL (Fig 1). The slope of the regression line ranged from 0.93 to 1.5 and the correlation coefficient was from 0.95 to 0.98 for obtained versus expected cTnI concentrations at 2.0, 8.0 and 32.0 ng cTnI/mL. At a concentration of 0.5 ng/mL, the slope was 0.89 and the correlation coefficient decreased to 0.44 (Fig 1). The average percentage recovery for 0.5, 2.0, 8.0, and 32.0 ng/mL was 145, 93, 121, and 91%, respectively, with a mean percentage recovery of 112 ± 39% for all dilutions. Assessment of the lower limit of detection of the immunoassay indicated undetectable cTnI concentrations for all samples with an estimated concentration of 0.05 ng/mL. The lowest measureable cTnI concentration was 0.01 ng/mL.

image

Figure 1. Plots of observed versus expected concentrations of cardiac troponin I (cTnI) after serial dilution. (A) baseline cTnI concentration 0.5 ng/mL; (B) baseline cTnI concentration 2 ng/mL; (C) baseline cTnI concentration 8 ng/mL; and (D) baseline cTnI concentration 32 ng/mL. Samples were run in duplicate.

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Serum cTnI concentrations for healthy cows and cows with metritis, mastitis, peritonitis, left-displaced abomasum (LDA), LDA and concurrent metritis, cows in dystocia requiring cesarean section, and downer cows are shown in Figure 2. Culture results of the mastitis cows identified Klebsiella spp. (n = 2), E. coli (n = 1), and Mannheimia hemolytica (n = 1). Four cows had dead calves delivered by cesarean section. One live calf was delivered by cesarean section from 1 cow. The minimum, median, and maximum cTnI concentrations in the diseased and the healthy control animals are shown in Table 1.

Table 1. Minimum, maximum, and median cardiac troponin I (cTnI) concentration in cows with different diseases and in healthy control animals.
 nn Sold for Slaughtern Died/Euthanizedn Necropsy PerformedCardiac LesionsMinimum cTnI (ng/mL) ConcentrationMedian cTnI (ng/mL) ConcentrationMaximum cTnI (ng/mL) Concentration
  1. n.a., not applicable.

  2. a

    n = 1 lost for follow-up.

Healthy3000n.a.n.a.000.01
Metritis6a031Subendocardial and subepicardial hemorrhage (n = 1)00.011.90
Mastitis4a022Not evaluated (n = 2)0.010.030.15
Peritonitis6244No lesions (n = 2), myocardial necrosis (n = 1), left ventricle fibrosis and myocarditis (n = 1), foreign bodies (n = 2)0.010.050.18
LDA14210n.a.0.010.040.20
LDA & metritis4210n.a.0.020.090.29
Pneumonia6111No lesion (n = 1)0.000.030.05
Dystocia requiring C-section5a011Not evaluated (n = 1)0.040.081.20
Downer cow syndrome8074Epicarditis and endocarditits (n = 1), lymphoplasmatic and neutrophilic myocarditis (n = 1), only gross necropsy (n = 1), no lesion (n = 1)0.151.7027.00
image

Figure 2. Scatter plot showing serum cardiac troponin I (cTnI; ng/mL) concentration in cattle with noncardiac diseases such as metritis, mastitis, peritonitis, left-displaced abomasum (LDA), pneumonia, dystocia requiring C-section, downer cow syndrome, LDA with concurrent metritis, and healthy control cows. Horizontal bar indicates the median value. Statistical significance when compared to healthy control group indicated by asterisk *P < .05, **P < .01, ***P < .001.

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Differences were found between the median serum cTnI concentrations of each of the 8 different clinical diagnoses and healthy control population (P < .0001). Cows with LDA and concurrent metritis (P < .05), peritonitis (P < .05), LDA (P < .001), cows that had cesarean section performed (P < .01), and downer cows (P < .001) had significantly higher cTnI concentrations than healthy cows. Differences were not found between the control population and cows with metritis, mastitis, and pneumonia (P > .05).

All control cows had cTnI concentrations below the detection limit of the assay (n = 30). Using 0.02 ng/mL cTnI as a cutoff value for healthy cows, 43 (81.1%) cows of the study population had increased cTnI concentrations, whereas 10 animals had cTnI concentrations below the detection limit of the assay. Cows in dystocia requiring cesarean section, cows with LDA and concurrent metritis and downer cows had increased cTnI concentrations. Two of 6 cows with metritis, 3 of 4 with mastitis, 4 of 6 with pneumonia, 5 of 6 with peritonitis, and 12 of 14 with LDA had serum cTnI concentrations ≥0.02 ng/mL.

Within 2 months after presentation, 20 (37.7%) cows died or were euthanized, 7 (13.2%) cows were sold and 3 (5.7%) animals were lost to follow-up. A total of 23 (43.4%) animals were alive and with no concurrent health issues.

Necropsy was performed in 13 cows. In 8 of these animals, histopathologic examination of the heart was performed; 5 animals showed myocardial changes. The main findings in these cows were myocarditis, endocarditis, epicarditis, subendocardial and subepicardial hemorrhage, myocardial necrosis, and ventricular fibrosis. No histopathologic evidence of myocardial necrosis or inflammation was found in the remaining histopathologically examined hearts. The median cTnI concentration of cows with histologic evidence of cardiac injury (n = 8) was 0.28 ng/mL (interquartile range, 0.1 ng/mL and 4.7 ng/mL). Cows without evidence of myocardial injury (n = 3) had cTnI concentrations of 0.04, 0.04, and 0.92 ng/mL. No statistical difference in the cTnI concentration was observed between these 2 groups (P = .571). Because of the small sample size of necropsies performed and the presence of several combinations of histopathologic lesions reported for each examined heart, no statistical analysis was performed related to heart muscle lesions.

In 7 animals, no necropsy was performed for reasons of death or euthanasia at the farm (n = 6) or nonconsent for necropsy by the owner (n = 1). Animals in the negative outcome group (n = 27) had a median serum cTnI concentration of 0.08 ng/mL (interquartile range, 0.03 to 0.29 ng/mL) with a maximum concentration of 26.8 ng/mL. Cows with a positive outcome (n = 23) had a median cTnI concentration of 0.03 ng/mL (interquartile range, 0.01 to 0.08) with a maximum cTnI of 2.48 ng/mL. Nonsurviving cows had significantly higher median cTnI concentrations than surviving cows (P < .0303). Three animals were lost to follow-up, thus only 50 animals were included in the calculation of the odds ratio. The overall odds of having a negative outcome (nonsurviving animal) in cows with a cTnI concentration >0.05 ng/mL was 2.4 times higher than in cows with cTnI concentration <0.02 ng/mL (healthy cows); test positivity and test negativity was 0.65 and 0.67, respectively. When using a higher cutoff value, test positivity was decreased, but test negativity increased, and the odds of having a negative outcome increased to >6 times. The odds of having a negative outcome at different cTnI cutoff values are summarized in Table 2.

Table 2. Test positivity and test negativity of the immunoassay (i-STAT), and the odds ratios for a negative outcome when using different cutoff values for cardiac troponin I (cTnI) in 50 cows.
Cutoff Concentration cTnI (ng/mL)Test PositivityTest NegativityOdds Ratio (95% CI)
  1. Test positivity, the conditional probability that the test result on the immunoassay was positive for cTnI given the presence of a disease condition; Test negativity, the conditional probability that the test result on the immunoassay was negative for cTnI given the presence of a disease condition; 95% CI, 95% confidence interval.

0.050.650.672.4 (1.1; 7.3)
0.10.430.832.9 (1.8; 10.4)
0.20.300.924.8 (1.9; 25.9)
0.50.220.966.1 (1.7; 22.4)

Discussion

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. References

To the authors' knowledge, this is the first prospective clinical study that measured serum cTnI concentration in cows with various noncardiac diseases using a commercial point-of-care analyzer. We evaluated the use of cTnI as a prognostic indicator for short-term survival of cows by calculating the odds ratio in cows with increased cTnI concentrations and a negative outcome.

This study showed acceptable analytical performance for the detection of bovine serum cTnI concentration when measured with the i-STAT immunoassay. The i-STAT handheld analyzer is relatively easy to use and test results are provided within 10 minutes. The immunoassay can be performed on heparinized and nonheparinized whole blood or plasma samples. As demonstrated by the validation study reported here, cTnI concentrations in serum samples also can be determined with the i-STAT immunoassay. Serum samples were chosen to avoid potential interference of heparin with the cTnI concentration as previously reported.[40-42] It is assumed that negatively charged heparin molecules bind to positively charged troponin complexes and decrease immunoreactivity of cTnI.[42] Furthermore, heparin binds with different affinities to different troponin forms (free versus complexed), which may lead to differences in analytical performance, depending on the analyzer used because different antibodies in different assays bind to different epitopes on cTnI.[40] In addition, the brand of blood collection tube, the concentration of heparin, and the concentration of cTnI in the sample may influence the recovery of measurable cTnI.[33-35] Studies in people have reported significantly decreased cTnI concentration in heparinized plasma compared to serum samples.[40, 42]

Serum and heparinized plasma samples should not be used interchangeably in the same patient for determination of cTnI concentrations.[42]

In healthy cows, the measureable serum cTnI concentration was <0.02 ng/mL. We concluded that in healthy cows serum cTnI concentrations should not be detectable or only present in trace amounts when measured with the i-STAT immunoassay. Our results are consistent with findings of other studies in cattle and horses in which only an extremely low concentration of cTnI was detected in healthy individuals.[9, 18, 38, 43, 44]

The general recommendation has been to establish a reference range for each individual immunoassay, because of a wide array of assays using different antibodies and a lack of standardization.[45] Complete standardization can only be achieved if all manufacturers utilize the same antibody.[45, 46] Variations in the measured cTnI concentration occur when the same samples are analyzed with different immunoassays.[46] The absolute measured cTnI concentration is not comparable among different assays. Therefore, the results of this study are only valid for measurement of bovine serum cTnI concentrations with the i-STAT immunoassay and cannot be compared to other immunoassays.

Regression analysis showed good correlation between measured and expected serum cTnI in almost all concentrations when measured with the i-STAT immunoassay. At a cTnI concentration of 0.5 ng/mL, greater variability was observed and the imprecision was higher than previously reported for this immunoassay.[47] For acceptance of clinical test validity in humans, the International Federation of Clinical Chemistry and Laboratory Medicine recommends a coefficient of variation <10%.[48] In our study the CV was below 20% in all evaluated concentrations, which is in accordance with a recent study that evaluated the performance of the i-STAT on bovine plasma samples.[49] We noticed a moderate difference in between-day CV, especially at a concentration of 32 ng/mL. At higher concentration, this difference could occur because of inaccuracy of the assay. The manufacturer has not evaluated the performance characteristics of samples with cTnI concentrations >35 ng/mL. It is unclear why at lower cTnI concentration a moderate difference in CV is measured. The previously mentioned study[49] did not evaluate the between-day performance of the assay, therefore no conclusions can be drawn in our study as to whether the sample type chosen could have influenced the recovery of cTnI. Other reasons for the variation in CV such as multiple freeze thaw cycles and inaccurate storage and handling of the samples and cartridges were excluded because the samples were handled appropriately.

The recommendation for a low CV has been made to establish accurate risk classifications for people with acute myocardial infarction. High assay precision is especially important if low cutoff values are used for the detection of myocardial injury, thus providing an earlier identification of at-risk patients. The importance of low cutoff values in the diagnostic, therapeutic or prognostic use in cattle is currently unknown.

The majority of diseased cows in this study had higher serum cTnI concentrations than the control animals. Physical examination of each patient included thorough cardiac auscultation, but no additional diagnostic procedures (eg, electrocardiography, echocardiography) were performed. Therefore, presence of early myocardial dysfunction or pre-existing cardiac disease cannot be excluded. Additionally, cardiac auscultation alone cannot distinguish acute myocardial injury and pre-existing cardiac disease before presentation.

Cardiac troponin I has 2 different release patterns. The transient release pattern liberates cTnI from the cytosolic pool with increasing membrane permeability because of reversible oxygen deficits in the myocyte, as in inflammation or toxic damage to the heart.[31] Approximately 2.8–8.3% of troponin I is unbound within the cytoplasm in humans.[50] The persistent release of cTnI is associated with nonreversible cardiac damage. Loss of myocyte integrity caused by ischemic necrosis will lead to a prolonged and sustained release of myofibril-bound cTnI with the maximum concentration of cTnI 5–12 times that of cytosolic cTnI.[50, 51] In this study, only 1 serum sample was evaluated for cTnI. Therefore, we are unable to differentiate between transient or persistent cTnI release patterns. Collection of multiple samples over time could have been beneficial in evaluating cTnI release patterns associated with ongoing cardiac damage. We assume that a slight increase in cTnI concentration represents a reversible cardiac insult, but no cutoff values for cattle have been established for the i-STAT immunoassay defining when myocardial necrosis occurs. Additional clinical cases will need to be evaluated to establish clinical cutoff values.

Increased serum cTnI concentrations in cattle with noncardiac diseases possibly are explained by indirect effects on the heart because of ischemia and release of cTnI because of metabolic derangements, fluid shifts, tachycardia, and increased sympathetic tone.[52] A possible influence of ketone bodies on bovine cardiac myocytes cannot be excluded, but was not evaluated in this study.

Other disease processes in the study population may have influenced the measured cTnI concentration. Human patients with renal insufficiency can have measurable cTnI concentrations, even in the absence of clinically suspected ischemia.[53] Cardiac troponin I has been correlated with renal function in patients with chronic kidney disease.[54] The cause of increased cTnI concentration in renal disease is assumed to be multi-factorial. Clinically silent myocardial necrosis, left ventricular hypertrophy in end-stage renal disease, and decreased clearance of cTnI by the kidney may play a role.[14] Kidney disease in our cattle population cannot be excluded and may have led to increased troponin concentrations in some cases. Sepsis also can increase circulating cTnI concentration[55, 56] and was associated with impaired left ventricular function and increased mortality in human patients.[57, 58]

Increased cTnI concentrations were reported in women with pregnancy-induced hypertension, indicating some degree of cardiac damage and dysfunction.[59] The increased cTnI concentrations in the cows presented in dystocia might be explained by prolonged, unsuccessful labor, tachycardia, hypertension, cardiovascular compromise, and infection because of a dead fetus, leading to oxidative stress and ischemia. One study showed that over half of does with prolonged birth associated with cesarean sections had significantly higher cTnI increases compared with does with normal birth.[60] Furthermore, dystocia leads to fetal distress, hypoxia, and subsequently to fetal myocardial ischemia. Fetal cTnI can cross the placental barrier, and positive correlations between maternal and fetal cTnI concentrations in women have been documented.[61] We hypothesize that cTnI concentration in the cows was increased because of prolonged birth and fetal cTnI also may have contributed to the increase.

The number of cattle in our study was small and the population was biased because of the patient distribution presented to a teaching hospital and those evaluated in a conventional private practice. The decision to euthanize or sell a cow could have been influenced by other confounding factors such as age, value of the animal, and size of the dairy. These factors were not accounted for in our study. Furthermore, our selection criteria especially for metritis and mastitis cases could have been better defined, but we had only a very small number of cows included in each group and evaluation for statistical differences would have been problematic. No bulls or steers were enrolled in the study, and the majority of animals were Holstein dairy cows. In humans, some studies have reported slightly higher serum troponin concentrations in healthy men than in healthy women.[62] It is currently unknown if similar observations are true in cattle.

The highest cTnI concentrations were found in downer cows, suggesting that these animals have greater myocardial injury possibly resulting from increased ventricular wall stress, relative imbalance between myocardial oxygen supply and demand or both. This mismatch can lead to ischemic myocardial conditions that facilitate release of cTnI. Studies in hypocalcemic postparturient cows that were not responsive to medical treatment have shown evidence of myocardial necrosis.[63, 64] Associations between duration of recumbency and influence of inflammatory processes because of acute mastitis or metritis, however, have not been evaluated, and it is unknown to what extent they can produce myocardial lesions in cows. These effects should be investigated in future studies. A relationship between the amount of cTnI released and the severity of myocardial injury, however, has been established by histopathologic examination to demonstrate myocardial necrosis.[65, 66] Semiquantitative histopathologic evaluation of the myocardium in cattle with monensin toxicity has shown similar findings.[11] Marked myocardial cell necrosis was detected histopathologically in cows with cTnI concentrations >1.04 ng/mL when measured using a different immunoassay.[11]

All except 1 Holstein heifer in the downer cow group died. That heifer underwent intensive care after it was diagnosed with calving paralysis postvaginal delivery of a dead calf. Its cTnI concentration at presentation was 2.48 ng/mL. Follow-up 1 year later indicated that the animal had completely recovered. The moderate increase of cTnI in this animal might be explained by only a transient release of cTnI without permanent damage of cardiomyocytes. It also demonstrates that an increased cTnI concentration is not always associated with a negative outcome, and it is advised that clinicians do not make euthanasia decisions based on a single measurement of cTnI.

The goal of this study was to evaluate cTnI in cows diagnosed with noncardiac conditions rather than to determine the extent of myocardial damage. In 5 of 8 histopathologically examined hearts, myocardial lesions were found. These assessments were not standardized and only a small portion of the heart was evaluated. Changes such as edema, mitochondrial vacuolation, and myocardial necrosis might have been missed because standardized examination criteria were not used by all pathologists.

This study demonstrated that cTnI concentration can be increased in cows with noncardiac disease and that increased cTnI concentration was associated with adverse outcome in cows. However, cutoff values for cTnI concentrations need to be evaluated further to assess the use of this test as a prognostic indicator in cattle. The clinical relevance and long-term effects of increased cTnI concentration in cows should be further evaluated with larger sample sizes in specific disease categories.

Acknowledgment

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. References

Conflict of Interest: Authors disclose no conflict of interest.

Footnotes
  1. 1

    Monoject, no additive, Tyco Healthcare Group LP, Mansfield, MA

  2. 2

    i-STAT 1, Heska Corporation, Loveland, CO

  3. 3

    Bovine cTnI, Life Diagnostics, Inc, West Chester, PA

  4. 4

    Prism 4, GraphPad Software Inc, La Jolla, CA

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  3. Materials and Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. References
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