Comparison of ventilatory responses to sustained reduction in arterial oxygen tension vs. content in awake ponies.

This study examines how decreasing oxygen in the blood (either through lowering inhaled oxygen or raising levels of carboxy-hemoglobin) affects the breathing of ponies. The researchers propose that central and peripheral factors affect how breathing changes during low oxygen conditions.

Research Methods

• The researchers carried out two experiments on twelve awake ponies. In the first experiment, they introduced carboxy-hemoglobin (COHb) into the ponies’ bloodstreams, which decreases the blood’s oxygen-carrying capacity (referred as CO hypoxia). In the second experiment, they caused hypoxic hypoxia by reducing the ponies’ inhaled oxygen levels.
• They then measured the animals’ respiratory responses in terms of the levels of certain gases in their blood (mainly CO2 and O2), and the volume of their lung ventilation.
• The study lasted 1 hour when the ponies breathed normal air, followed by 6 hours when carboxy-hemoglobin was raised to 25%, then oxygen content in the blood was decreased by 17%.

Results

• During the first 30 minutes of the increased carboxy-hemoglobin, the ponies’ arterial carbon dioxide levels (PaCO2) and pulmonary ventilation both increased. Notably, however, this initial hypoventilation reduced after approximately 45 minutes, with the PaCO2 gradually decreasing again.
• After 45 minutes of elevated COHb, the arterial CO2 started to drop and further decreased to 1.5 Torr below the control level at the end of 4.5 hours of CO hypoxia. The ventilation rate did not change significantly after 30 minutes of COHb increase.
• In the second experiment when the ponies were subjected to hypoxic hypoxia, similar but more intense results were observed. The ponies’ PaCO2 fell rapidly and their pulmonary ventilation increased, but after a while, the ventilation rate also fell, consistent with the concept of hypoxic ventilatory acclimatization.

Conclusions

• The study concluded that the initial reduction in ventilation observed both in hypoxic and CO hypoxia is consistent with a theory called hypoxic ventilatory depression within the brain. This suggests that when the brain notices low oxygen levels, it reacts by temporarily slowing breathing.
• The research also hypothesized that the hyperventilation observed with prolonged CO hypoxia might be due to a central nervous system mechanism contributing to hypoxic ventilatory acclimatization.