Ventilatory response to 2-h sustained hypoxia in humans

Area of Science:

• Physiology
• Respiratory Control
• Hypoxia

Background:

• Hypoxic ventilatory decline (HVD) describes the decrease in ventilation during sustained hypoxia.
• The precise mechanisms of HVD, whether involving changes in the slope or intercept of the hypoxic ventilatory response (HVR), remain incompletely understood.

Purpose of the Study:

• To investigate whether HVD involves alterations in the slope and/or intercept of the isocapnic hypoxic ventilatory response (HVR) in humans.
• To differentiate the effects of hypoxic exposure duration versus depth on HVR parameters.

Main Methods:

• Two protocols were employed to measure HVR under isocapnic conditions (PaCO2 maintained at 1.5 mmHg above hyperoxic levels).
• Protocol I involved sequential hypoxic exposures (85% and 75% SaO2) for 25 minutes each (n=7).
• Protocol II involved a 14-minute exposure to 90% SaO2, extended to 2 hours in a subset of subjects (n=15), with HVR measured at various time points.

Main Results:

• The HVR intercept (predicted ventilation at 100% SaO2) significantly decreased after 14 and 25 minutes of hypoxia in both protocols.
• Changes in the HVR slope were observed only at deeper hypoxia (75% SaO2) in Protocol I, suggesting depth sensitivity.
• After 2 hours of hypoxia, the HVR intercept showed a partial return towards baseline, while the slope remained largely unchanged.

Conclusions:

• HVD in humans is characterized by a decrease in the hyperoxic ventilatory drive (HVR intercept) that can occur independently of significant changes in the HVR slope.
• The findings suggest that the HVR slope is more sensitive to the depth of hypoxia than its duration.
• Partial reversal of HVD after prolonged hypoxia may indicate early stages of ventilatory acclimatization to low oxygen levels.