Use of dew-point hygrometry, direct sweat collection, and measurement of body water losses to determine sweating rates in exercising horses

Objective-To compare dew-point hygrometry, direct sweat collection, and measurement of body water loss as methods for determination of sweating rate (SR) in exercising horses. Animals-8 exercise-trained Thoroughbreds. Procedure-SR was measured in 6 horses exercising at 40% of the speed that elicited maximum oxygen consumption for 45 km, with a 15-minute rest at the end of each 15-km phase. Each horse completed 2 exercise trials. Dew-point hygrometry, as a method of local SR determination, was validated in vitro by measurement of rate of evaporative water loss. During exercise, local SR was determined every 10 minutes by the following 2 methods: (1) dew-point hygrometry on the neck and lateral area of the thorax, and (2) on the basis of the volume of sweat collected from a sealed plastic pouch attached to the lateral area of the thorax. Mean whole body SR was calculated from total body water loss incurred during exercise. Results-Evaporation rate measured by use of dewpoint hygrometry was significantly correlated (r(2) = 0.92) with the actual rate of evaporative water loss. There was a similar pattern of change in SR measured by dew-point hygrometry on the neck and lateral area of the thorax during exercise, with a significantly higher SR on the neck. The SR measured on the thorax by direct sweat collection and by dew-point hygrometry were of similar magnitude, Mean whole body SR calculated from total body water loss was not significantly different from mean whole body SR estimated from direct sweat collection or dew-point hygrometry measurements on the thorax. Conclusions-Dew-point hygrometry and direct sweat collection are useful methods for determination of local SR in horses during prolonged, steady-slate exercise in moderate ambient conditions. Both methods of local SR determination provide an accurate estimate of whole body SR.