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      Measuring O 2 in hypoxic and hyperoxic conditions using dynamic gas mixing with a flow-through indirect calorimeter

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          ABSTRACT

          Measurements of gas exchange while breathing gases of different O 2 concentrations are useful in respiratory and exercise physiology. High bias flows required in flow-through indirect calorimetry systems for large animals like exercising horses necessitate the use of inconveniently large reservoirs of mixed gases for making such measurements and can limit the amount of equilibration time that is adequate for steady-state measurements. We obviated the need to use a pre-mixed reservoir of gas in a semi-open flow-through indirect calorimeter by dynamically mixing gases and verified the theoretical accuracy and utility of making such measurements using the mass-balance N 2-dilution method. We evaluated the accuracy of the technique at different inspired oxygen fractions by measuring exercising oxygen consumption ( O 2) at two fully aerobic submaximal exercise intensities in Thoroughbred horses. Horses exercised at 24% and 50% maximum oxygen consumption ( O 2 max) of each horse while breathing different O 2 concentrations (19.5%, 21% and 25% O 2). The N 2-dilution technique was used to calculate O 2. Repeated-measures ANOVA was used to tested for differences in O 2 between different inspired O 2 concentrations. The specific O 2 of the horses trotting at 24% O 2max and cantering at 50% O 2max were not significantly different among the three different inspired oxygen fractions. These findings demonstrate that reliable measurements of O 2 can be obtained at various inspired oxygen fractions using dynamic gas mixing and the N 2-dilution technique to calibrate semi-open-circuit gas flow systems.

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          One-step N2-dilution technique for calibrating open-circuit VO2 measuring systems.

          A simple one-step procedure that eliminates the need to calibrate the O2 analyzer or measure the flow past the animal is described for calibrating an open-flow respirometry system. The technique is particularly useful for situations of high ambient humidity and for large or active animals where a mask is employed to capture expired gases. A measured N2 flow is used to calibrate the system. The equations describing the technique are given, and the accuracy of the method is discussed in detail. The errors associated with the technique are compared with those of more conventional procedures and are usually smaller.
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            Oxygen transport during exercise in large mammals. I. Adaptive variation in oxygen demand.

            This study investigated mechanisms used by horses and steers to increase O2 uptake and delivery (VO2) from resting to maximal rates and identified the mechanisms that enable horses to achieve higher maximal rates of O2 consumption (VO2max) than steers. VO2 and circulatory variables were measured while Standardbred trotting horses and steers (450-kg body mass) stood quietly and ran on a treadmill at speeds up to those eliciting VO2max. As VO2 increased in both species, heart rate and circulating hemoglobin (Hb) concentration increased, thereby increasing O2 delivery by the circulation, while cardiac stroke volume remained unchanged. At VO2max arterial PCO2 increased from its resting value in horses but was unchanged in steers, and arterial PO2 decreased in both species. Although the horses hypoventilated and were hypoxemic at VO2max, no significant decrease in arterial Hb saturation occurred. VO2max of the horses was 2.6 times higher than that of the steers and was associated with a 100% larger cardiac output, 100% larger stroke volume, and 40% higher Hb concentration, whereas heart rates at VO2max were identical in the two species. The higher cardiac output of the horses at VO2max resulted from a 1.2-fold higher mean arterial pressure and 1.6-fold lower peripheral tissue resistance (associated with a larger skeletal muscle capillary bed). Both the magnitude of the difference in VO2max between horses and steers and the mechanisms used to achieve it are the same as observed in smaller pairs of mammalian species with large variation in aerobic capacity.
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              Design of the mammalian respiratory system. II. Measuring maximum aerobic capacity.

              We describe a simple and reproducible procedure for measuring maximal O2 consumption (Vo2max) that can be used for mammals spanning a wide range of body mass (Mb). Vo2 of trained animals was measured as a function of speed while they ran on a treadmill. Lactate concentration of the blood was determined at the beginning and end of the run. We considered that an animal had achieved Vo2max when Vo2 no longer increased with increasing tread-speed and the additional energy consumed by the muscles could be accounted for by anaerobic glycolysis. We found that Vo2 increased linearly with tread-speed up to a maximal rate and then remained unchanged with further increases in speed in nine species. When the animals ran at speeds faster than that where Vo2max was reached, they refused to continue when blood lactate concentrations reached 18-28 mmol . kg-1. Values for Vo2max obtained with this procedure were found to be about 20% greater than values obtained with the 'cold exposure' procedure for determining Vo2max. We conclude that our treadmill procedure is valid for determining the allometric relationship between Vo2max and Mb.
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                Author and article information

                Journal
                J Equine Sci
                J Equine Sci
                JES
                Journal of Equine Science
                The Japanese Society of Equine Science
                1340-3516
                1347-7501
                18 December 2019
                2019
                : 30
                : 4
                : 87-92
                Affiliations
                [1 ] Equine Sports Medicine Consultants, DE 19711, U.S.A.
                [2 ] Sports Science Division, Equine Research Institute, Japan Racing Association, Tochigi 329-0412, Japan
                [3 ] Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, U.S.A.
                Author notes
                *Corresponding author. e-mail: hajime@ 123456equinst.go.jp
                Article
                1918
                10.1294/jes.30.87
                6920056
                31871410
                43ab20d4-9e9c-4d60-9098-93162a40749c
                ©2019 The Japanese Society of Equine Science

                This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (by-nc-nd) License. (CC-BY-NC-ND 4.0: https://creativecommons.org/licenses/by-nc-nd/4.0/ )

                History
                : 24 June 2019
                : 30 October 2019
                Categories
                Full Paper

                horse,n2 dilution,oxygen consumption,training
                horse, n2 dilution, oxygen consumption, training

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