Ventilatory response to low levels of CO2

Area of Science:

• Respiratory Physiology
• Environmental Physiology

Background:

• Carbon dioxide (CO2) is a critical regulator of ventilation.
• Isocapnic hyperpnea, an increase in ventilation without a rise in arterial CO2 (PaCO2), is a known physiological response.
• The precise stimulus for isocapnic hyperpnea at low inspired CO2 concentrations is not fully understood.

Purpose of the Study:

• To investigate the ventilatory response to low concentrations of inspired CO2 (1-2%) under hyperoxic and hypoxic conditions.
• To determine if breathing through dead space elicits similar responses to direct CO2 enrichment.
• To explore the relationship between the CO2 response curve and the increase in PaCO2.

Main Methods:

• Six subjects breathed 1-2% CO2 during hyperoxia and hypoxia.
• Inspired CO2 was manipulated by direct enrichment or by breathing through added dead space.
• Ventilation and PaCO2 were measured, and the CO2 response curve was assessed via rebreathing.

Main Results:

• During hyperoxia, subjects exhibited isocapnic hyperpnea at 1% inspired CO2, primarily due to increased tidal volume.
• Hypoxia attenuated the isocapnic hyperpneic response to 1% CO2.
• The increase in PaCO2 at 2% CO2 correlated negatively with the slope of the CO2 response curve.

Conclusions:

• Isocapnic hyperpnea in response to low inspired CO2 levels is common in normal individuals.
• Hypoxia depresses this isocapnic hyperpneic response.
• The specific stimulus driving isocapnic hyperpnea at low CO2 levels requires further investigation.