Ventilation‐perfusion relationships during graded exercise in the Standardbred trotter

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Abstract
Summary: The aim of this study was to evaluate pulmonary function in Standardbred trotters during graded exercise. The exercise test consisted of 4 work loads corresponding to 10%, 42%, 70% and 96% of the individual V̇O2max. Oxygen uptake was detected from an open bias flow system without valves. Cardiovascular and haemodynamic data were recorded at walk and during steady state exercise in 7 horses. Pulmonary gas exchange was assessed by conventional blood gas variables (arterial and mixed venous blood gas tensions) and the ventilation‐perfusion distribution V̇A|Q̇ as estimated by the multiple inert gas elimination technique. The dispersion of perfusion and ventilation distribution respectively (SDQ) and the difference between measured PaO2 and that predicted on the basis of amount of ventilation‐perfusion mismatching and shunt that was observed (predicted‐measured), were determined. The latter reflects mostly diffusion limitation. At the highest work load the PaCO2 increased to 50.3 torr. V̇A|Q̇ inequality increased significantly with exercise [mean log = 0.32 ± 0.03 (walk) and 0.46 ± 0.06 (heavy exercise), P < 0.01]. Alveolar‐capillary diffusion limitation of oxygen was evident at and above exercise at 70% of V̇O2max. The arterial hypoxaemia seen during the highest work load was a result of a) hypoventilation, accounting for 4%, b) increase in V̇A|Q̇ mismatch, accounting for 41% and c) considerable diffusion limitation of oxygen, accounting for 55%.