Horses undergoing supramaximal fixed-speed treadmill exercise after administration of furosemide experience less severe acidosis and slower onset of fatigue.

OBJECTIVE: To document changes in acid-base and ion balance during exercise following furosemide administration and to relate them to performance. METHODS: 6 fit Thoroughbred horses received 0.5 mg/kg IV furosemide (F) or a placebo (C) in randomized crossover order 4 hours before galloping to fatigue on a treadmill at the speed equated with an oxygen demand of 1.15 X maximum oxygen consumption (V˙o2max). Arterial and mixed venous blood was sampled before treatment, preexercise, after 2-minute warm-up, at fatigue, and 1 minute postexercise. The pH, temperature-corrected Pco2, total CO2, and sodium, potassium, chloride (Cl), bicarbonate (HCO3-), and lactate concentrations were measured, and strong ion difference (SID) was calculated. Run time to fatigue, V˙o2max, and CO2 production (V˙co2) were recorded. Data were analyzed using a linear mixed-effects model, and P ≤ .05 was significant. RESULTS: Furosemide induced a preexercise hypochloremic alkalosis as concentrations of arterial and venous Cl- and Cl- were lower, and pH, SID, HCO3- concentrations and percent mixed venous total CO2 transported as mixed venous HCO3- were higher for all measurements from pre- to postexercise. The mixed venous Pco2 was lower at fatigue and postexercise following F when compared to C. Sodium, potassium, and lactate concentrations increased with exercise, but F and C values were not different. The V˙o2max (F, 73 ± 13 L/min; C, 70 ± 12 L/min) and peak V˙co2 (F, 100 ± 13 L/min; C, 99 ± 16 L/min) were not different. The time to fatigue was significantly longer for F (F - C = 15.4 s; 95% CI, 6.6 to 24.3 s). CONCLUSIONS: Furosemide at 0.5 mg/kg IV 4 hours preexercise was associated with approximately equal to 2% weight loss and a mild alkalinizing effect characterized by higher pH, SID, and HCO3- concentrations and lower Pco2 from preexercise to postexercise. After furosemide administration, horses ran longer before fatiguing. CLINICAL RELEVANCE: Preexercise furosemide administration was associated with improved treadmill performance.