Effect of prolonged heavy exercise on pulmonary gas exchange in horses.

The study examines the effect of prolonged heavy exercise on the pulmonary gas exchange process in horses. The findings suggest that despite a noticeable increase in ventilation during exercise, horses maintain excellent pulmonary gas exchange with minimal impact from ventilation-perfusion inequality.

Introduction and Background

• The study is predicated on previous findings that have shown horses to have impaired pulmonary gas exchange during short-term maximal exercise. This impairment is displayed through diffusion limitation and arterial hypoxemia, without any significant ventilation-perfusion inequality.
• This study aims to understand whether this gas exchange worsens progressively during prolonged submaximal exercise.

Methodology

• Six thoroughbred horses were used for the study. They were made to perform treadmill exercise at approximately 60% of their maximum oxygen uptake until they reached exhaustion. The time of exercise ranged between 28 and 39 minutes.
• During this exercise, the team collected inert gas, blood-gas, hemodynamic, metabolic rate, and ventilatory data at 5-minute intervals and at rest.

Results

• The study found that the horses’ oxygen uptake, cardiac output, and alveolar-arterial PO2 gradient remained unchanged after the initial 5 minutes of exercise.
• Their alveolar ventilation increased progressively during exercise due to increased tidal volume and respiratory frequency. This led to an increase in arterial PO2 and a decrease in arterial PCO2.
• At rest, the horses showed minimal ventilation-perfusion inequality, represented as a log standard deviation of the perfusion distribution (log SDQ) value of 0.20.
• This value doubled to 0.40 by 5 minutes of exercise but did not continue to increase further during exercise.
• They found no evidence of alveolar-end-capillary diffusion limitation during exercise, but there was evidence for gas phase diffusion limitation at all time points and enflurane was preferentially overretained.

Conclusion

• The study concludes that horses can maintain excellent pulmonary gas exchange during exhaustive, submaximal exercise.
• Although the ventilation-perfusion inequality is greater than at rest, it does not have as significant an effect on gas exchange as it does in most other mammals, and the overall effect on gas exchange is minimal.