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HIGHLIGHTED TOPICS
Oxygen Sensing in Health and Disease

Peripheral chemoreflex responsiveness is increased at elevated levels of carbon dioxide after episodic hypoxia in awake humans

Departments of ¹Internal Medicine, ³Biomedical Engineering, and ⁴Physiology, Wayne State University and ²John D. Dingell Veterans Affairs Medical Center, Detroit, Michigan 48201

Submitted 2 June 2003 ; accepted in final form 10 November 2003

We hypothesized that the acute ventilatory response to hypoxia is enhanced after exposure to episodic hypoxia in awake humans. Eleven subjects completed a series of rebreathing trials before and after exposure to eight 4-min episodes of hypoxia. During the rebreathing trials, subjects initially hyperventilated to reduce the partial pressure of carbon dioxide (PET_CO2) below 25 Torr. Subjects then breathed from a bag containing normocapnic (42 Torr), low (50 Torr), or high oxygen (140 Torr) gas mixtures. During the trials, PET_CO2 increased while a constant oxygen level was maintained. The point at which ventilation began to rise in a linear fashion as PET_CO2 increased was considered to be the ventilatory recruitment threshold. The ventilatory response below and above the recruitment threshold was determined. Ventilation did not persist above baseline values immediately after exposure to episodic hypoxia; however, PET_CO2 levels were reduced compared with baseline. In contrast, compared with baseline, the ventilatory response to progressive increases in carbon dioxide during rebreathing trials in the presence of low but not high oxygen levels was increased after exposure to episodic hypoxia. This increase occurred when carbon dioxide levels were above but not below the ventilatory recruitment threshold. We conclude that long-term facilitation of ventilation (i.e., increases in ventilation that persist when normoxia is restored after episodic hypoxia) is not expressed in awake humans in the presence of hypocapnia. Nevertheless, despite this lack of expression, the acute ventilatory response to hypoxia in the presence of hypercapnia is increased after exposure to episodic hypoxia.

hypocapnia; long-term facilitation; ventilatory recruitment threshold; modified rebreathing protocol

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