Hyperpnea of exercise at various PIO2 in normal and carotid body-denervated ponies.

The research investigates the effect of varying oxygen levels (inspired [O2]) on the partial pressure of CO2 in the blood during exercise in normal and carotid body-denervated (CBD) ponies, both acutely and chronically. The main findings suggest that carotid chemoreceptors typically manage ventilation at the onset of exercise, and that mechanisms working to minimize the drop in blood CO2 levels during exercise adjust ventilation by modulating tidal volume.

Study Methodology

• Researchers conducted the study on normal and carotid body-denervated (CBD) ponies, which had been in a CBD state for either 2-4 weeks (acute) or 1-2 years (chronic).
• The ponies were subjected to treadmill exercise at a speed of 3 mph on a 3% gradient.
• They examined the impact of changes in oxygen levels inspired on the partial pressure of CO2 (PaCO2) in the pony’s arterial blood during the exercise.

Key Findings

• In all cases, PaCO2 decreased from rest during exercise, typically reaching the lowest point between 15-30 seconds into the exercise.
• The PaCO2 lowest level was 2.3 +/- 0.6 Torr below the resting level in normal ponies. However, this lowest point was more significant in acute (delta = 6.4 +/- 0.8 Torr) and chronic (delta = -4.7 +/- 1.1 Torr)
• Hyperoxia (increased levels of oxygen in the blood) exaggerated the low CO2 level in the blood (hypocapnia) only in the normal ponies. In contrast, hypoxia (below normal level of oxygen) lessened the exercise-induced hypocapnia by 3-5 Torr in all ponies.
• The PaCO2 usually slowly increased after 30 seconds of exercise, attaining a stable level 1-3 Torr below rest by about 2 minutes (P less than 0.05).
• The tidal volume (VT – the depth of breathing) increased from rest during the first 15 seconds of exercise only when there was a large decrease in PaCO2.

Conclusions

• Carotid chemoreceptors normally manage alveolar (connected to the lungs) ventilation (VA) at the onset of exercise. This was suggested by the exaggerated hypocapnia caused by eliminating (in CBD ponies) or reducing (under hyperoxia) carotid chemoreceptor activity.
• The reduction in hypocapnia at the onset of exercise by hypoxia in CBD ponies suggests that a direct effect of hypoxia on the central nervous system (CNS) manages VA.
• Mechanisms working to minimize the hypocapnia during exercise appear to adjust VA by influencing the tidal volume (VT).