Ambient temperature and relative humidity influenced packed cell volume, total plasma protein and other variables in horses during an incremental submaximal field exercise test

Thermoregulation may limit exercise performance under hot and humid conditi ons. This study compared heart rate (RR), respiratory rate (RR), rectal tem perature (Tr), packed cell volume (PCV) and total plasma protein concentrat ion (TPP) during a submaximal incremental field exercise test under high vs . Low ambient temperature and relative humidity. Ten horses-were tested 3 t imes in summer (July) and 3 times in autumn (September), Heart rate was. me asured continuously, the other variables at rest and immediately after 4 mi n at 3.5, 4.5 and 7.0 mis, separated by 3 min rest intervals, and after 5 a nd 10 min recovery. Data for all variables were significantly greater durin g exercise and recovery in the hot vs. cool conditions, respectively: after 4 min at 7.0 mis, HR was 135 +/- 1 and 123 +/- 1/min(P<0.0001), Tr was 39. 0 +/- 0.06 and 38.0 +/- 0.05 degrees C (P<0.0001), RR was 99 +/- 3 and 50 /- 3/min (P<0.0001), PCV was 48.8 +/- 0.06 and 42.1 +/- 0.3% (P<0.0001) and TPP was 7.7 +/- 0.14 and 7.6 +/- 0.12 g/l (P = 0.026). These data reflect the thermal burden during submaximal exercise under hot conditions in the f ield. The greater relative PCV increase in the heat probably conferred a th ermoregulatory advantage and reflected a greater circulating red cell volum e increase rather than a decrease of plasma volume. This study illustrates how differences in environmental conditions can affect assessment of exerci se responses and how these factors must be considered in monitoring progres s during fitness and acclimatisation regimes in the field.