Hypoxia exaggerates inspiratory accessory muscle deoxygenation during hyperpnoea.

The purpose of this study was to elucidate inspiratory accessory muscle deoxygenation and myoelectric activity during isolated volitional hyperpnoea under hypoxic conditions. Subjects performed voluntary isocapnic hyperpnoea (tidal volume=30-40% of forced vital capacity, breathing frequency=60 breaths/min) in normoxia [inspired gas fraction (FIO₂)=0.21] and hypoxia (FIO₂). Muscle deoxyhemoglobin/myoglobin (Deoxy-Hb/Mb) and muscle oxygen saturation (SMO₂) using near-infrared spectroscopy and surface electromyography were measured from sternocleidomastoid (SCM) and intercostal (IC) muscles. During hyperpnoea, increases in Deoxy-Hb/Mb and reductions of SMO₂ in the SCM and IC muscles were larger (P<0.05) in hypoxia than those in normoxia. Under hypoxic conditions, the electromyogram increases from the SCM and IC muscles during hyperpnoea were greater than that under the normoxic conditions. These results suggest that inspiratory accessory muscle deoxygenation during isolated hyperpnoea is exaggerated in hypoxia compared with normoxia and that hypoxia has a significant impact on inspiratory accessory muscle activation.