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ANIMAL PRODUCTION |
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* Department of Animal and Veterinary Science, University of Idaho, Caldwell Research and Extension Center, Caldwell 83607; and
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Department of Animal Sciences and
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Department of Clinical Sciences, Colorado State University, Fort Collins 80523
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: beef cattle • brisket disease • pulmonary arterial pressure
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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). Jensen et al. (1976) indicated that HAD was one of the primary causes of calf morbidity and mortality on western beef cattle ranches above 2,133 m. The cumulative annual disease incidence rate for HAD was 0.43 per 100 cows based on data from 39 cow-calf herds in Colorado (Salman et al., 1990).
In cattle, hypoxia at high elevation causes pulmonary vasoconstriction, increased pulmonary arterial pressure (PAP), right ventricle stress, congestive right heart failure, and hydrothorax in the chest cavity and brisket (Alexander and Jensen, 1959, 1963). No consistently effective treatment or prevention is available other than movement of affected cattle to lower elevation or selection using an indicator trait.
The indicator trait for HAD, the PAP test, has been shown to be moderately (Schimmel, 1981) to highly (LeValley, 1978; Enns et al., 1992) heritable. The PAP test is a chute-side right heart catheterization procedure performed to measure the average PAP. Typically, cattle with PAP values >45 mmHg are culled, resulting in a reduced incidence of HAD in current and subsequent generations (T. Holt, unpublished data).
Although the PAP test is an effective method to select against HAD susceptibility, few seedstock cattle are tested because of the test’s invasive, dangerous, and expensive nature. Availability of an easier, cheaper, and less invasive alternative would encourage more testing and allow for the development of a genetic evaluation for susceptibility to HAD.
Early clinical research suggests that the disease process includes changes to blood parameters (Cueva, 1967; Weir et al., 1974) in addition to vasoconstriction. Therefore, the objective of this experiment was to evaluate the relationship between blood components predicted by 3 new blood evaluation technologies (hemogram, pulse oximetry, and portable clinical analyzer) and PAP score.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Exp. 1
Experimental Design. In the first experiment, 39 purebred yearling Angus bulls born and raised at an elevation of 2,200 m (John E. Rouse Colorado State University Beef Improvement Center, Saratoga, WY) were used in early January to collect the following information: PAP score (mmHg), 10 blood components determined by a hemogram using whole blood [1) packed cell volume (PCV), 2) hemoglobin concentration (HgB), 3) red blood cell (RBC) count, 4) mean cell volume (MCV), 5) mean cell hemoglobin concentration (MCHC), 6) red cell distribution width (RDW), 7) nucleated cells, 8) platelet count, 9) mean corpuscular hemoglobin, and 10) mean platelet volume], and arterial oxygen saturation (SpO2) as predicted by pulse oximetry at 2 anatomical locations. A state-licensed veterinarian collected a PAP score on each bull while it was restrained in a hydraulic squeeze chute. A rope halter was used to safely restrain each animal’s head during PAP testing (procedure outlined subsequently) and blood collection.
PAP Test. The PAP test is a right heart catheterization procedure requiring jugular venipuncture, catheter insertion, and passage of the catheter through the right atrium and ventricle to the pulmonary artery. Once the catheter is inside the pulmonary artery, an average blood pressure (average of systolic and diastolic values) is recorded from the heart monitor, which is attached to the catheter via a transducer. A small number of experienced veterinarians in the Rocky Mountain States perform the PAP test as a service to clients raising beef cattle at high elevation. In Exp. 1, 11 of the 39 bulls evaluated had elevated (>45 mmHg) PAP scores.
Hemogram. The 10 hemogram values specified previously were determined from venous blood collected into nonheparinized Vacutainer tubes (Becton Dickinson Co., Franklin Lakes, NJ) immediately after collection of the PAP score and removal of the catheter. Once collected, the samples were placed on ice for 3 h before being transported to the laboratory for hemogram analysis (Colorado State University Veterinary Teaching Hospital Blood Analysis Laboratory, Fort Collins). A hemogram was completed on all samples within 6 h from the time of collection.
Pulse Oximetry.
A pulse oximeter with a corresponding reflectance probe (Heska Vet/Ox 4404, Heska Corp., Fort Collins, CO) was used to collect chute-side SpO2 values. A transmittance probe is typically applied to an easily accessible portion of an animal that is well vascularized (e.g., tongue) to monitor SpO2 while the animal is under general anesthesia, typically during surgery. In this experiment, animals were not anesthetized; therefore, a transmittance probe could not be used on the tongue. In contrast, a reflectance probe was used at 2 anatomical locations: 1) approximately 15 cm inside the rectum with placement against the ventral side of the sacral region of the vertebral column; and 2) orally against the nonpigmented upper gum and cheek of the restrained animal. According to Shapiro et al. (1989), the pulse oximeter predicts arterial oxygen saturation of hemoglobin using 2 wavelengths of light (near infrared and red light). The red light is absorbed by deoxygenated hemoglobin, whereas the near infrared light is absorbed by the oxygenated hemoglobin. To determine SpO2, the transmittance probe measured the amounts of each light type that were reflected from the tissue. Saturation levels were differentiated between an artery and a vein because of the ability of the pulse oximeter to sense a pulse, which is unique to arteries.
Statistical Analyses.
Statistical analyses of the data included calculation of Pearson correlation coefficients (Proc Corr, SAS Inst. Inc., Cary, NC) among all hemogram or SpO2 values and PAP measurements. A parameter estimate, SE, and partial R2 value were determined for each hemogram value using linear regression (Proc Reg of SAS) and the backwards model selection process with PAP score as the dependent variable. Prediction equations for PAP score were developed using multiple linear regression (Proc Reg of SAS), and all 10 hemogram values were used as independent variables in the stepwise model selection process in which an 
value of 0.05 was required for a variable to enter and remain in the model.
Exp. 2
Experimental Design. In Exp. 2, 62 yearling Angus heifers and 22 yearling Angus bulls (born and raised at the same location as those evaluated in Exp. 1) were used to collect the following information: PAP score (mmHg) and 11 blood components: Na concentration, K concentration, PCV, HgB, pH, partial pressure of oxygen, saturation of oxygen, partial pressure of carbon dioxide, total carbon dioxide concentration, bicarbonate concentration, and base excess concentration. These blood components were predicted in a laboratory adjacent to the squeeze chute by using a portable clinical analyzer (I-Stat portable clinical analyzer, Heska Corp.).
All PAP scores were collected as described in Exp. 1. Seventeen of the 84 animals evaluated in Exp. 2 had elevated (>45 mmHg) PAP scores. Immediately after the completion of the PAP test, blood was collected via jugular venipuncture, carotid arterial puncture, tail venipuncture, or tail arterial puncture using a 10-mL syringe and 20-ga needle. Collection of arterial blood was attempted first because it would provide a better indication of saturation of oxygen and partial pressure of oxygen than venous blood. In cases where an artery could not easily be located, venous blood was collected. Once whole blood was collected, approximately 1 mL was immediately placed into the well of a cartridge (EG6+ I-Stat cartridge, Heska Corp.), and the cartridge was inserted into the portable clinical analyzer for data collection. The results were typically available within 1 to 2 min. Some data points were lost because of difficulties with sample handling, including sample clotting before analysis by the clinical analyzer.
Statistical Analyses. In Exp. 2, all SAS procedures were the same as those described for Exp. 1. Pearson correlation coefficients were calculated among all portable clinical analyzer values and PAP measurements. A parameter estimate, SE, and partial R2 value were determined for each portable clinical analyzer value using linear regression and the backwards model selection process with PAP score as the dependent variable. Prediction equations for PAP score were developed using multiple linear regression, and all 11 portable clinical analyzer values were used as independent variables in the stepwise model selection process.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Hemogram Values.
In HAD-susceptible cattle, high PAP scores are caused by a significant change in the pulmonary vasculature as an animal attempts to compensate for hypoxic conditions. Additionally, cattle also appear to modify some characteristics of their blood, specifically red blood cells, to further compensate for hypoxia (Cueva, 1967). At high elevations, both cattle and humans have increased RBC numbers, circulating HgB concentrations (Cueva, 1967), and HgB affinity for oxygen (Ganong, 2001). For these reasons, the hemogram was included in this experiment to analyze the relationship between hemogram blood components and PAP score.
Of the 10 hemogram values (mean ± SD for each value reported in Table 1), PCV, HgB, and RDW were the only values correlated (P < 0.10) with PAP score (Table 2). When all 10 hemogram values were included in a regression model, the combined R2 value was 0.375 relative to the PAP score (Table 3). When the model was reduced by the stepwise model selection process, only HgB and RDW were included (P < 0.05) in the model. The final reduced multiple regression model had an R2 value of 0.305, which was the largest for any model that included only 2 variables (Table 4).
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). The increased hemoglobin concentration improves oxygen-carrying capacity, and a rightward shift occurs in the dissociation curve for oxygen and hemoglobin. In cattle, most of the research on response of oxygen transport mechanisms to hypoxia was published in the 1960s and 1970s with inconclusive results relative to biological significance. Cueva (1967) found differences in blood components between "susceptible" and "resistant" cattle at high elevation; differences were noted in hemoglobin dissociation curves PCV, MCV, and MCHC. Card (1977) also reported that cattle with increased PAP scores had increased values for PCV, HgB, red cell mass, and erythropoietin titers. The findings of the current experiment agree with previous results for PCV and HgB, but not for the MCV and MCHC differences reported by Cueva (1967).
No mention of a relationship between RDW and PAP score was found in the literature, most likely because of the recent development of the RDW prediction measurement. The RDW component of a hemogram is a red cell measurement that indicates the amount of variation in red blood cell sizes within a sample. In humans, increased RDW values can be evidence of anemia, iron deficiency, or red cell fragmentation (Lee et al., 1999; Beutler et al., 2001).
The negative correlation in the current experiment between RDW and PAP score (r = –0.36; P = 0.03) is difficult to interpret. The RDW values in these bulls appeared to be high (mean = 23.8; range = 19.9 to 26.6) compared with the range (16 to 21) reported by the Colorado State University Veterinary Teaching Hospital Blood Analysis Laboratory (L. Vapp, unpublished data). Greater RDW values in these bulls may be indicative of iron deficiency or RBC fragmentation, which could result from both increased erythropoiesis and/or vasoconstriction of the pulmonary vessels. However, the possibility of an iron deficiency is unlikely based on previous mineral analyses of this cow herd (Stanton et al., 2000). This measurement needs to be examined further, possibly by comparing iron concentrations with PAP score or evaluating blood smears for RBC appearance. The current experiment did not address these issues.
Most notable in the hemogram results were the partial R2 values of RDW and HgB compared with those of the other 8 variables. The R2 value for the 2-variable model was inadequate to accurately predict PAP score because it was only able to explain 30.5% of the variability in PAP score. Comparatively, by adding the other 8 variables, the full model was only able to explain an additional 7% of the variability. Based on this, further research is needed to help identify additional or alternative blood components with comparable or larger partial R2 values to enhance the ability to predict PAP score and ultimately HAD incidence.
Pulse Oximetry.
Use of pulse oximetry to monitor SpO2 during surgery in humans and animals has be come very common. The easy, noninvasive, and accurate method of using a pulse oximeter to collect SpO2 predictions was the primary reason to evaluate the relationship between SpO2 and PAP score in cattle in the current experiment. Changes in HgB affinity in cattle reported by Cueva (1967) indicate that possible changes in SpO2 may be occurring as well. In the current experiment, we evaluated the relationship between the SpO2 prediction via pulse oximetry and PAP score.
Neither of the 2 SpO2 values reported by the pulse oximeter were correlated with PAP score (P > 0.10) or with each other (P > 0.10; Table 5). In this study, pulse oximetry provided a prediction of SpO2 within a short amount of time (1 to 2 min) at both anatomical locations using the reflectance probe; however, neither of these values was useful in predicting PAP score. Although not evaluated in the current experiment, it appears that the SpO2 predictions collected did not accurately predict the true SpO2 of these cattle, possibly because of 1) black skin pigmentation of Angus cattle and/or 2) motion artifact in the restrained and nonanesthetized animals. In a typical production system, anesthetizing animals to use this technology would be both impractical and cost-prohibitive.
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Although no relationship was observed between SpO2 and PAP score, further research using pulse oximetry should be performed once limitations with motion artifact sensitivity are eliminated. Future experiments using nonblack-hided cattle, other anatomical locations, and additional probes (e.g., ear- and nose-clip transmittance probes) would be useful to determine whether this technology may be beneficial in the prediction of PAP score and ultimately HAD susceptibility.
Exp. 2
A portable clinical analyzer was evaluated for its ability to predict PAP score. This relatively new technology allows a veterinarian to observe blood parameter predictions chute-side rather than later in a laboratory. This technology involves use of a portable clinical analyzer instrument, which is about the size of a large television remote control device, and a large array of disposable cartridges to evaluate numerous blood parameters at reasonable cost and accuracy. This new technology was incorporated into Exp. 2 because it is meant to predict blood components that have been shown to be correlated with PAP score, such as PCV and HgB (Cueva, 1967).
None of the 11 blood components predicted by the portable clinical analyzer were correlated with PAP score (P > 0.10; Table 6). When these variables were combined in a multiple regression model, the 11 components explained only 12% of the variation in PAP score. This lack of a relationship with PAP score and the inability to help predict PAP score was probably due to a variety of factors.
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). This variation might have been associated with the difficulty in obtaining arterial blood consistently.
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The inherent susceptibility of beef cattle to HAD is an economic and animal health concern on high elevation ranches in the western United States. However, in the current experiment, only 3 blood parameters predicted by a hemogram were individually correlated with PAP. Use of 2 of these correlated variables in a prediction equation provided a moderate R2 value in the prediction of PAP. Further research that incorporates additional blood components, technologies that can accurately and repeatedly predict blood parameters, other beef breeds, and larger sample sizes will be necessary to develop an accurate alternative to PAP testing. Development of an alternate test could lead to the genetic prediction of the disease in cattle, as well as the study of HAD in humans through the use of cattle as an experimental model.
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Footnotes |
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2 Corresponding author: mark.enns{at}colostate.edu
Received for publication September 23, 2005. Accepted for publication December 21, 2005.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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This article has been cited by other articles:
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  K. L. Shirley, D. W. Beckman, and D. J. Garrick Inheritance of pulmonary arterial pressure in Angus cattle and its correlation with growth J Anim Sci, April 1, 2008; 86(4): 815 - 819. [Abstract] [Full Text] [PDF]
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