AbstractBackgroundThe addition of calcium to resuscitation fluids is a common practice in horses, but studies evaluating the effects of calcium supplementation are limited. In healthy horses, decreases in heart rate and changes in serum electrolyte concentrations have been reported.HypothesisCalcium gluconate administration at a rate of 0.4 mg/kg/min to eliminated endurance horses with metabolic problems will affect heart rate, gastrointestinal sounds, and serum electrolyte concentrations.AnimalsEndurance horses eliminated from the Tevis Cup 100‐mile (160 km) endurance ride for metabolic problems and requiring IV fluid therapy were eligible.MethodsSixteen horses were randomly assigned to receive 0.4 mg/kg/min of calcium (23% calcium gluconate solution) over 1 hour diluted in 10 L of a non‐calcium containing isotonic crystalloid (CAL group) or 10 L of a non‐calcium containing isotonic crystalloid (CON group). Staff members administering the fluids were blinded to treatment group. Blood samples were collected and physical examinations performed before and after treatment. Heart rates were recorded every 15 min during fluid administration. Data were compared using 2‐way analysis of variance (ANOVA) with repeated measures for continuous variables and Fisher's exact test for categorical variables.ResultsCalcium was associated with lower heart rates 45 min after starting the infusion (P= .002). Gastrointestinal sounds were less likely to improve in the calcium group compared with the control group (P= .005). An increase in plasma phosphorus concentration (P= .03) was associated with calcium administration.ConclusionsIntravenous calcium supplementation to endurance horses eliminated from competition after development of metabolic problems may decrease heart rate but impairs improvement in gastrointestinal sounds.