Auditory nerve-brain stem potentials in man and cat under hypoxic and hypercapnic conditions

Area of Science:

• Neuroscience
• Physiology

Background:

• Hypoxia and hypercapnia can significantly impact brain function.
• Auditory nerve brain stem evoked potentials (ABP) are a measure of auditory pathway integrity.

Purpose of the Study:

• To investigate the effects of hypoxic and hypercapnic respiratory gas mixtures on brain activity, specifically ABP.
• To determine the brain stem's resilience to altered blood gas levels.

Main Methods:

• Recorded ABP in human volunteers and cats breathing various gas mixtures.
• Monitored physiological parameters including blood gases, pH, blood pressure, body temperature, and EEG in cats.
• Exposed cats to extreme levels of CO2 and O2, and combined hypoxic-hypercapnic conditions.

Main Results:

• Human ABP showed no significant changes with moderate hypoxia (9-13% O2) or hypercapnia (7.5-10% CO2).
• Cat ABP wave form, amplitude, and latencies were generally unaffected by severe hypercapnia (up to 25% CO2) and moderate hypoxia (down to 5.5% O2).
• Loss of ABP in cats occurred only at approximately 5% O2, linked to cardiac failure and brain ischemia, not directly to gas levels.

Conclusions:

• The brain stem auditory pathway demonstrates remarkable resistance to significant alterations in blood oxygen and carbon dioxide levels.
• Cortical activity (EEG) is more susceptible to hypercapnia than brain stem auditory function.
• Brain stem auditory structures appear resilient to hypoxic and hypercapnic challenges, except under conditions leading to critical ischemia.