Intermittent hypoxia and the chemoreflex control of breathing

Rationale: Intermittent Hypoxia (IH) consists of bouts of hypoxic exposure interspersed with normoxic intervals. In animals, there is some evidence that multiple brief short duration exposures to intermittent hypoxia (SDIH) provoke more profound changes in chemosensitivity than longer duration bouts of intermittent hypoxia (LDIH). The purpose of this study was to test the hypothesis that SDIH would have differential effects from LDIH on chemosensitivity during rest and exercise in humans. Methods: Ten males underwent two intermittent hypoxic protocols of 7 days duration/each. The LDIH protocol consisted of daily 60-minute exposures to normobaric 12% O₂. The SDIH protocol consisted of twelve 5-minute bouts of normobaric 12% O₂, separated by 5-minute bouts of normoxia. Measured resting variables included the hypoxic ventilatory response (HVR), hypercapnic ventilatory response (HCVR), CO₂ threshold and CO₂ sensitivity. Submaximal exercise variables included minute ventilation, oxygen saturation, hyperoxic and hypercapnic ventilatory response in both hypoxia and normoxia. Peak exercise variables included power and oxygen consumption in hypoxia. Measurements were made immediately prior to intermittent hypoxic training and on the first day following IH. Resting measures were repeated 7 days following IH. Results: For both protocols, the HVR was significantly (p< 0.05) increased after IH. One week post IH, the HVR was not different from pre-IH. The HCVR was increased and remained elevated at 7 days post-IH (p<0.01). The CO₂ sensitivity was unchanged by either intervention. In hypoxia and hyperoxia, the CO₂ threshold was significantly reduced following IH (p<0.05). The submaximal minute ventilation, hyperoxic and hypercapnic responses in normoxia and hypoxia were unchanged by IH. Submaximal oxygen saturation and peak power were both increased (p