Effects of elevated oxygen and carbon dioxide partial pressures on respiratory function and cognitive performance

Area of Science:

• Diving physiology
• Hyperbaric medicine
• Respiratory physiology

Background:

• Hyperoxia is suspected to worsen hypercapnic narcosis and unconsciousness during diving.
• Understanding the physiological responses to hyperoxia and hypercapnia is crucial for diver safety.

Purpose of the Study:

• To test the hypothesis that hyperoxia exacerbates hypercapnic narcosis and promotes unconsciousness.
• To investigate the effects of hyperoxia on ventilation, carbon dioxide levels, cognitive function, and perceived discomfort under hyperbaric conditions.

Main Methods:

• Male volunteers (n=22) underwent hyperbaric chamber exposures breathing various gas mixtures.
• Inspired oxygen partial pressure (PiO2) was varied (0.21 vs 1.3 atm), with or without maximum tolerable inspired carbon dioxide (CO2).
• Measurements included end-tidal CO2 partial pressure (PetCO2), minute ventilation (V̇e), cognitive function (auditory n-back), and perceived discomfort (RPD).

Main Results:

• Minute ventilation significantly increased with hyperoxia (PiO2 = 1.3 atm) compared to normoxia (PiO2 = 0.21 atm) at rest and during exercise (P < 0.0001).
• End-tidal CO2 partial pressure (PetCO2) was consistently lower during hyperoxia compared to normoxia (P < 0.01).
• Cognitive performance (n-back scores) improved and perceived discomfort decreased during hyperoxic hypercapnia, with no serious symptoms reported, unlike normoxic hypercapnia trials.

Conclusions:

• Hyperoxia, contrary to hypothesis, did not exacerbate hypercapnic narcosis or cause unconsciousness in this study.
• Increased ventilation under hyperoxic conditions effectively reduced PetCO2, mitigating potential adverse effects of hypercapnia.
• Hyperoxic hypercapnia appears safe, with serious symptoms occurring only during normoxic hypercapnia, suggesting a protective role of hyperoxia via increased ventilation.