The Effect of Two Different Intermittent Hypoxia Protocols on Ventilatory Responses to Hypoxia and Carbon Dioxide at Rest

Intermittent hypoxia (IH) consists of bouts of hypoxic exposure interspersed with normoxic intervals. The optimal IH regime for increasing the ventilatory response in humans is unknown, although in animals there is evidence that multiple short duration bouts of intermittent hypoxia (SDIH) provoke larger changes in chemosensitivity than longer duration bouts of intermittent hypoxia (LDIH). The purpose of this study was to compare responses to both hypercapnia and hypoxia between the two protocols. Methods: In a randomised crossover design, 10 healthy males underwent two 7-day poikilocapnic IH protocols. The LDIH protocol consisted of daily 60-minute exposures to normobaric 12% O₂ (balance N2). The SDIH protocol comprised twelve 5-minute bouts of normobaric 12% O₂, separated by 5-minute bouts of room air daily. We measured the isocapnic acute hypoxic ventilatory response (HVR), hypercapnic ventilatory response (HCVR) and CO₂ threshold and sensitivity by the modified Read rebreathing technique. Pre-testing was performed immediately prior to intermittent hypoxic training. Follow-up testing occurred on the first day following IH and at 7 days after completion of IH. HVR testing was also performed on every day of the IH protocol prior to the hypoxic exposure.