THE RELATIONSHIP BETWEEN RESPIRATORY EXCHANGE RATIO, PLASMA LACTATE AND MUSCLE LACTATE CONCENTRATIONS IN EXERCISING HORSES USING A VALVED GAS COLLECTION SYSTEM

A valved gas collection system for horses was validated, then used to examine the relationship between the respiratory exchange ratio (RER), and plasma and muscle lactate in exercising horses. Four healthy Stan dardbred horses were trained to breathe through the apparatus while ex ercising on a treadmill. Comparisons of arterial blood gas tensions we re made at 3 work levels for each horse, without (control), and with t he gas collection system present. At the highest work level, the arter ial oxygen tension (PaO2) was significantly lower (P < 0.05), and the arterial carbon dioxide tension (PaCO2) was significantly higher (P < 0.05), than control levels when the apparatus was present; however art erial oxygen content remained unchanged. The horses completed a standa rdized incremental treadmill test on 4 occasions to determine the repe atability of measurements of oxygen consumption (VO2), carbon dioxide production (VCO2), inspired minute ventilation (V-1), respiratory exch ange ratio (RER), ventilatory equivalent for oxygen (V-1/VO2), tidal v olume (V-T), and ventilatory frequency (V-F). All gas exchange and res piratory measurements showed good reproducibility with the mean coeffi cient of variation of the 4 horses ranging from 3.8 to 12%. We examine d the relationship between 3 indices of energy metabolism in horses pe rforming treadmill exercise: respiratory exchange ratio (RER), central venous plasma and muscle lactate concentrations. A relationship betwe en RER and plasma lactate concentration was established. To compare mu scle and plasma lactate concentrations, the horses completed a discont inuous exercise test without the gas collection apparatus present. Sig nificant relationships (P < 0.05), between plasma lactate concentratio n and RER, and between plasma and muscle lactate concentration, were d escribed for each horse. The valved gas collection system produced a m easurable but tolerable degree of interference to respiration, and pro vided reproducible measurements of gas exchange and ventilatory measur ements. It was concluded that measurements of both gas exchange and bl ood lactate may be used to indicate increased glycolytic activity with in exercising skeletal muscle.