Mechanism of reduction in alveolar-arterial PO2 difference by helium breathing in the exercising horse.

The research explored the mechanisms behind the reduction in exercise-induced alveolar-arterial PO2 difference in horses when nitrogen in air is replaced by helium, using a technique called multiple inert gas elimination.

Overview of the research

• The study set out to investigate the mechanisms that lead to the reduction in the exercise-induced alveolar-arterial PO2 difference when horses breathe helium-oxygen instead of air. This was motivated by prior studies that showed such a reduction but did not explain the underlying process.
• Using the multiple inert gas elimination technique, six horses were tested on a high-speed treadmill while breathing either normal air or a helium-oxygen mix.

Findings of the study

• The study found that the volume of oxygen uptake remained consistent at 138.0 ml.min-1.kg-1, regardless of the gas breathed.
• The recorded arterial PO2 levels, adjusted for temperature, were 76.7 Torr and 86.9 Torr for air and helium-oxygen respectively. The corresponding AaPO2 values were 22.3 and 15.9 Torr. Thus, the helium-oxygen mix resulted in higher arterial PO2 and lower AaPO2 – indicating more efficient oxygen uptake.
• The results showed significant differences in mean arterial PCO2 levels when horses breathe air and helium-oxygen – 50.1 Torr for air and 44.1 Torr for He-O2. Ventilation and tidal volume were correspondingly higher when horses breathed in helium-oxygen.
• Interestingly, the study found no changes in pulmonary O2 diffusing capacity, cardiac output, and all ventilation-perfusion dispersion indexes when horses breathed helium-oxygen. Intrapulmonary shunt was also found to be insignificant.
• The study concluded that higher ventilation with He-O2 only accounted for about 4 Torr of the 10-Torr increase observed in arterial PO2. The remaining increase (and the corresponding decrease in AaPO2) was due to more efficient diffusion equilibration, following higher minute ventilation and alveolar PO2 caused by helium breathing. This also resulted in reducing the average slope of the O2 dissociation curve, hence increasing the ratio of diffusive to perfusive conductance, leading to improved oxygen uptake.