Hypoxic Ventilatory Response and Breathlessness Following Hypocapnic and Isocapnic Hyperventilation

Study objectives To investigate the etiology of posthyperventilation (post-HV) hypoxemia following voluntary hyperventilation (VHV), we examined the effects of hypocapnic (hypo-CO 2 ) and isocapnic (iso-CO 2 ) VHV on the hypoxic ventilatory response (O 2 -response) and on the sensation of breathlessness during the O 2 -response. Methods O 2 -responses and visual analog scale (VAS) scores for estimating breathlessness in 10 normal subjects during the O 2 -response under iso-CO 2 conditions and under hypo-CO 2 conditions immediately following voluntary maximal HV of 3 min duration were examined. Results Although there was no significant difference in the post-HV ventilation levels following hypo-CO 2 vs iso-CO 2 VHV, the VAS scores at the start of the O 2 -response following hypo-CO 2 VHV (30.2±24.2 mm) were significantly higher (p 2 -response following iso-CO 2 VHV (13.7±8.4 mm). However, VHV did not have a significant effect on the O 2 -response at 2 min after the VHV when the arterial O 2 saturation (SaO 2 ) was below 90%. The nonsteady-state hypo-CO 2 induced by VHV greatly attenuated the O 2 -response below 90% SaO 2 and VAS scores at 70% SaO 2 . Conclusions Elevated VAS scores immediately following the hypo-CO 2 VHV, which might be independent of actual breathing levels, and the attenuation of the O 2 -response following the hypo-CO 2 VHV were not due to input from lung and chest wall mechanoreceptors induced by the hyperpnea itself, but rather to the hypo-CO 2 induced by hyperpnea.