The effects of furosemide on pulmonary transmural pressure and exercise-induced pulmonary hemorrhage in supramaximally exercising thoroughbred racehorses.

BACKGROUND

Maximal intensity exercise in equine athletes induces pulmonary capillary stress failure and exercise-induced pulmonary hemorrhage (EIPH) secondary to excessive transmural pressure (Ptm). Furosemide decreases EIPH severity and pulmonary arterial (PAP) and pulmonary wedge (PAW) pressures.

HYPOTHESIS/

OBJECTIVES

Assess the effects of furosemide on Ptm and EIPH during supramaximal exercise.

ANIMALS: Six fit retired Thoroughbreds with consistent EIPH Grade ≥ 2.

METHODS

Horses performed a controlled crossover design protocol, completing standardized treadmill tests (120%V̇O2max) before and 4 h after intravenous (IV) furosemide injection (0.5 mg/kg). Vascular catheters measured PAP and PAW. An esophageal catheter measured intrapleural pressure (Ppl). Pulmonary capillary pressure (Pcap) was estimated (Pcap = [PAP + PAW]/2) to calculate mean and peak Ptm (Ptm = Pcap-Ppl). The percentage of time that Ptm exceeded a stress failure threshold of 75 mm Hg was calculated. Blood and plasma volumes (BV, PV) were measured. Tracheoendoscopic EIPH score and the difference between pre- and post-exercise bronchoalveolar lavage red blood cell count (ΔBALRBC) were determined 1 h post-exercise. Treatment-related differences were compared using a paired t-test (P < .05).

RESULTS

Furosemide decreased mean PAP (P = .03), PAW (P = .04), and Pcap (P = .03), but did not affect Ppl. The outcome was decreased maximum (P < .01) and minimum Ptm (P = .04). Transmural pressure exceeded 75 mm Hg for 57% ± 8% of the time in the control run and 31% ± 13% after furosemide, reflecting an approximately 45% decrease (P < .01). Furosemide (4 h post-treatment) did not affect BV or PV, but significantly lowered EIPH score (P = .03) and ΔBALRBC (P = .03).

CONCLUSIONS AND CLINICAL

IMPORTANCE

Furosemide decreased EIPH severity and Ptm. However, Ptm remained above the pulmonary capillary stress failure threshold, and some EIPH was observed.