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 Standardbred horses were trained to breathe through the apparatus while exercising on a treadmill. Comparisons of arterial blood gas tensions were made at 3 work levels for each horse, without (control), and with the gas collection system present. At the highest work level, the arterial 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 arterial oxygen content remained unchanged. The horses completed a standardized incremental treadmill test on 4 occasions to determine the repeatability of measurements of oxygen consumption (VO2), carbon dioxide production (VCO2), inspired minute ventilation (VI), respiratory exchange ratio (RER), ventilatory equivalent for oxygen (VI/VO2), tidal volume (VT), and ventilatory frequency (VF). All gas exchange and respiratory measurements showed good reproducibility with the mean coefficient of variation of the 4 horses ranging from 3.8 to 12%. We examined the relationship between 3 indices of energy metabolism in horses performing treadmill exercise: respiratory exchange ratio (RER), central venous plasma and muscle lactate concentrations. A relationship between RER and plasma lactate concentration was established. To compare muscle and plasma lactate concentrations, the horses completed a discontinuous exercise test without the gas collection apparatus present. Significant relationships (P < 0.05), between plasma lactate concentration and RER, and between plasma and muscle lactate concentration, were described for each horse. The valved gas collection system produced a measurable but tolerable degree of interference to respiration, and provided reproducible measurements of gas exchange and ventilatory measurements. It was concluded that measurements of both gas exchange and blood lactate may be used to indicate increased glycolytic activity within exercising skeletal muscle.