Hypoxic Ventilatory Response and Breathlessness Following Hypocapnic

Study objectives: To investigate the etiology of posthyperventilation (post-HV) hypoxemia following voluntary hyperventilation (VHV), we examined the effects of hypocapnic (hypo-C02) and isocapnic (iso-C02) VHV on the hypoxic ventilatory response (02-response) and on the sensation of breathlessness during the 02-response. Methods: 02-responses and visual analog scale (VAS) scores for estimating breathlessness in 10 normal subjects during the 02-response under iso-C02 conditions and under hypo-C02 condi¬ tions 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-C02 vs iso-C02 VHV, the VAS scores at the start of the 02-response following hypo-C02 VHV (30.2±24.2 mm) were significantly higher (p<0.05) than the VAS scores at the start of the 02-response following iso-C02 VHV (13.7±8.4 mm). However, VHV did not have a significant effect on the 02-response at 2 min after theVHV when the arterial 02 saturation (Sa02) was below 90%. The nonsteady-state hypo-C02 induced by VHV greatly attenuated the 02-response below 90% Sa02 and VAS scores at 70% Sa02. Conclusions: Elevated VAS scores immediately following the hypo-C02 VHV, which might be independent of actual breathing levels, and the attenuation of the 02-response following the hypo-C02 VHV were not due to input from lung and chest wall mechanoreceptors induced by the hyperpnea itself, but rather to the hypo-C02 induced by hyperpnea. (CHEST 1997; 112:154-63)