A simple breathing circuit to maintain isocapnia during measurements of the hypoxic ventilatory response.

We report the development and testing of a simple breathing circuit that maintains isocapnia in human subjects during hypoxic hyperpnea. In addition, the circuit permits rapid switching between two gas mixtures with different partial pressures of oxygen. Eleven volunteers breathed repeated cycles of exposure to air (2 min of 21% O(2), balance N(2)) and hypoxia (2 min of 8.3+/-0.1% O(2), balance N(2)). Hypoxia induced significant increases in minute ventilation, breathing frequency and tidal volume (P < 0.05) that were consistent over repeated cycles of hypoxia (P > 0.1, one-way ANOVA). The system successfully maintained isocapnia in all subjects, with an average change in end-tidal CO(2) of only -0.2 mmHg during hyperventilation in hypoxia (range 0.4 to -0.8 mmHg). This system may be suitable for repeated tests of the hypoxic ventilatory response (HVR) and may prove useful for exploring intra- and inter-individual variability of HVR in humans.