EFFECT OF PROLONGED HEAVY EXERCISE ON PULMONARY GAS-EXCHANGE IN HORSES

During short-term maximal exercise, horses have impaired pulmonary gas exchange, manifested by diffusion limitation and arterial hypoxemia, without marked ventilation-perfusion ((V) over dot A/(Q) over dot) ine quality. Whether gas exchange deteriorates progressively during prolon ged submaximal exercise has not been investigated. Six thoroughbred ho rses performed treadmill exercise at similar to 60% of maximal oxygen uptake until exhaustion (28-39 min). Multiple inert gas, blood-gas, he modynamic, metabolic rate, and ventilatory data were obtained at rest and 5-min intervals during exercise. Oxygen uptake, cardiac output, an d alveolar-arterial PO2 gradient were unchanged after the first 5 min of exercise. Alveolar ventilation increased progressively during exerc ise, from increased tidal volume and respiratory frequency, resulting in an increase in arterial PO2 and decrease in arterial PCO2. At rest there was minimal (V) over dot A/(Q) over dot inequality, log SD of th e perfusion distribution (log SD(Q over dot)) = 0.20. This doubled by 5 min of exercise (log SD(Q over dot) = 0.40) but did not increase fur ther. There was no evidence of alveolar-end-capillary diffusion limita tion during exercise. However, there was evidence for gas-phase diffus ion limitation at all time points, and enflurane was preferentially ov erretained. Horses maintain excellent pulmonary gas exchange during ex haustive, submaximal exercise. Although (V) over dot A/(Q) over dot in equality is greater than at rest, it is less than observed in most mam mals and the effect on gas exchange is minimal.