Ventilation and carbon dioxide exchange in exercising horses: effect of inspired oxygen fraction.

This research looked at the reaction of Thoroughbred horses to different levels of oxygen during exercise, particularly in relation to carbon dioxide production and ventilation. The study found that while horses do not respond to increased levels of CO2, they do react to low levels of inspired oxygen, indicating that it was not physical limitations of the horses causing lack of response, but rather the breathing control system.

Study Methodology

• The experiment involved five normal Thoroughbred horses(Lit: TB). These horses were put through three different tests involving varying levels of inspired oxygen – 0.16, 0.21, and 0.30 – while they were standing still and while galloping at speeds of 10 and 14 meters per second on a treadmill. They chose to study males and females separately because of the different physiological characteristics they possess.
• The researchers used a flow-through mask system to measure gas exchange – which included oxygen consumption and carbon dioxide production – and ventilation.
• In addition to the above, they tested and measured the partial pressures of oxygen (PO2) and carbon dioxide (PCO2), and oxygen content, in the arterial and venous blood of the horses. These measurements were crucial to ascertain if there were any significant variations in the quantities of these gases, due to the different oxygen fractions in the inhaled air.
• They also calculated cardiac output utilizing the Fick equation, which is a mathematical representation used in cardiovascular physiology for the calculation of the rate of oxygen consumption, or delivery.

Key Observations

• The researchers found that low fractions of inspired oxygen (0.16 as opposed to 0.21) affected horses significantly when galloping at 14 m/s. The arterial PO2 dropped to 38 Torr, compared to a standard 56 Torr for those horses breathing regular air.
• As a reaction to this low oxygen fraction, the horses increased their tidal volume and minute ventilation by 20% (over the calculated standard volumes of 12 liters and 1,475 l/min, respectively) while their respiratory frequency remained the same.
• Despite the hike in ventilation, the levels of oxygen consumption and CO2 production did not change. However, the alveolar ventilation(i.e., the amount of air per minute that reaches the alveoli and is participating in gas exchange) increased by 6% despite an increase in alveolar and physiological dead spaces (areas of lungs that don’t perform any gas exchange). This resulted in lower arterial PCO2 levels (45 versus 50 Torr on regular air).

Conclusion

• The study concluded that a decrease in inspired oxygen works as an effective stimulus for increased breathing in horses, and that minute ventilation was not mechanically limited in TBs breathing air at the speeds studied.
• It debunked the original hypothesis that Thoroughbred horses had no ventilatory response to increased CO2 levels during near-maximal exercise due to mechanical limitations. Rather, it demonstrated that the breathing control system in these horses is what controls this lack of response.