Selected Contribution: Chemoreflex responses to CO2 before and after an 8-h exposure to hypoxia in humans

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Abstract
The ventilatory sensitivity to CO2, in hyperoxia, is increased after an 8-h exposure to hypoxia. The purpose of the present study was to determine whether this increase arises through an increase in peripheral or central chemosensitivity. Ten healthy volunteers each underwent 8-h exposures to 1) isocapnic hypoxia, with end-tidal Po2 (Pet O2 ) = 55 Torr and end-tidal Pco2 (Pet CO2 ) = eucapnia; 2) poikilocapnic hypoxia, with Pet O2 = 55 Torr and Pet CO2 = uncontrolled; and 3) air-breathing control. The ventilatory response to CO2 was measured before and after each exposure with the use of a multifrequency binary sequence with two levels of Pet CO2 : 1.5 and 10 Torr above the normal resting value. Pet O2 was held at 250 Torr. The peripheral (Gp) and the central (Gc) sensitivities were calculated by fitting the ventilatory data to a two-compartment model. There were increases in combined Gp + Gc (26%, P < 0.05), Gp (33%, P < 0.01), and Gc (23%, P = not significant) after exposure to hypoxia. There were no significant differences between isocapnic and poikilocapnic hypoxia. We conclude that sustained hypoxia induces a significant increase in chemosensitivity to CO2 within the peripheral chemoreflex.