Is improved high speed performance following frusemide administration due to diuresis‐induced weight loss or reduced severity of exercise‐induced pulmonary haemorrhage?

Abstract

Prerace administration of frusemide to horses has been linked with a significant improvement in racing performance, but the basis for this improvement is unclear. To test whether improved performance with prerace administration of frusemide is due to the drug's diuresis-induced weight loss rather than its apparent alleviation of exercise-induced pulmonary haemorrhage (EIPH). Eight thoroughbred horses underwent 3 trials in a random order, 2 or 3 weeks apart: control (C), frusemide/unburdened (FU), and frusemide/burdened (FB). None of the horses were known to have exhibited post-exercise epistaxis or endoscopic evidence of EIPH. Endoscope-guided bronchoalveolar lavages (BALs) were performed before and after each horse completed a standardised exercise test (SET) on an inclined treadmill to assess semi-quantitatively the volume of EIPH. For C, horses received an i.v. saline placebo injection (5 ml) and were unburdened while performing the SET. With FU, horses received frusemide (0.5 mg/kg) and were also unburdened. For FB, horses received frusemide and were burdened with weight equal to that lost during the 4 h post frusemide injection period. Erythrocyte number in BAL fluid, mass specific VO2max, time and distance for the entire SET as well as at maximum speed were recorded. A one-way repeated measures analysis of variance was conducted on all results. Mass specific VO2max was significantly higher for the FU than for FB or C. Mass specific VO2max for FB and C were not different. More RBCs were found in BAL samples after C runs than after both FU and FB trial runs. Horses with the frusemide treatment (either burdened or unburdened) produced less EIPH than in the C trial, but their mass specific VO2max values were higher on the FU trial alone. For FU, horses ran longer at 115% VO2max than under C or FB conditions. Improvement of performance in the furosemide trials was due more to the weight-loss related effects of the drug than its apparent alleviation of EIPH. Further research is warranted with the same or similar project design, but with a larger sample size and with horses known to have more severe EIPH.