Exercise induced ventilation/perfusion inequality in the horse

Abstract

Summary Exercise in normal human subjects causes deterioration of matching of ventilation to blood flow in the lungs, but only in about 50% of those examined. A previous study (Wagner et al. 1989) of 5 horses showed no significant worsening of ventilation/blood flow (V̇a/Q̇) relationships during heavy exercise as determined by multiple inert gas elimination technique (MIGET). Because of the small number of horses in that study and the 50% human incidence of exercise induced V̇a/Q̇ mismatch, we studied an additional 6 Thoroughbreds, comparing V̇a/Q̇ relationships at the walk (1.4 m/s, 0o incline) and during galloping (9.6 ± 0.3 m/s, 7% incline). Such data were collected under 4 different conditions wherein inspired gas was 1) air, 2) 21% O2 in helium, 3) 15% O2 in N2 and 4) 15% O2 in helium. Each horse exercised 4 times (morning and afternoon of 2 days, with inspired gas conditions randomised). There was a small but significant increase in V̇a/Q̇ mismatch (similar under all 4 conditions). The second moment of the V̇a/Q̇ distribution (determined by the MIGET) increased significantly (P < 0.01) from 0.31 ± 0.01 at the walk to 0.38 ± 0.02 during gallop. This increase however is small: 0.38 is well within the range of this parameter for normal human subjects (where the 95% upper confidence limit is 0.60). This study shows that a small amount of exercise induced V̇a/Q̇ mismatch can occur in the horse as in man, but the mechanism remains to be elucidated and its clinical significance remains to be established. It may reflect interstitial pulmonary oedema but effects on arterial oxygenation are minimal.

Gas exchange during intense exercise in Standardbreds with earlierRhodococcus equipneumonia

“…The increase in V A /Q inequality during intense exercise in both man and horse has been suggested to be the result of a low-grade interstitial oedema caused by increased fluid transudation secondary to pulmonary hypertension (West et al 1993;Birks et al 1997). In line with previous reports (Wagner et al 1989;Nyman et al 1995;Seaman et al 1995;Hopkins et al 1998), diffusion limitation of oxygen was the major contributor to exerciseinduced arterial hypoxaemia in racing horses. During the highest workload the diffusion limitation, calculated as the difference between measured PaO 2 and predicted PaO 2 , was responsible for 61% of the AaDO 2 .…”

Ventilation-perfusion inequality during normoxic and hypoxic exercise in the emu

“…There are clear-cut species differences in the relative contribution of these two factors. For example, the horse experiences very little increase in V /Q inequality, but the extent of pulmonary diffusion limitation for O 2 combined with mechanical constraint of ventilation is sufficient to cause marked arterial hypoxemia (24). Pigs, on the other hand, experience an increase in V /Q inequality with exercise but no appreciable diffusion limitation (12).…”

Exercise‐induced pulmonary perfusion redistribution in heaves

“…In healthy horses, arterial hypoxaemia is thought to be the result of (1) diffusion limitation (Wagner et al 1989), (2) mechanical limits to ventilation , and (3) a small but significant contribution of ventilation to perfusion inequalities (Seaman et al 1995). Because the locomotion-respiration coupling is not always present in Standardbreds who also experienced an exercise-induced hypoxaemia (Art and Lekeux 1995), the breathing frequency does not appear to be the major component of the mechanical limit to ventilation.…”