The severity of acute hypoxaemia determines distinct changes in intracortical and spinal neural circuits

The purpose of this study was to examine how two common methods of continuous hypoxaemia impact the activity of intracortical circuits responsible for inhibition and facilitation of motor output, and spinal excitability. Ten participants were exposed to 2 h of hypoxaemia at 0.13 fraction of inspired oxygen ( FIO2${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol) and 80% of peripheral capillary oxygen saturation ( SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol) using a simulating altitude device on two visits separated by a week. Using transcranial magnetic and peripheral nerve stimulation, unconditioned motor evoked potential (MEP) area, short‐interval intracortical inhibition (SICI) and intracortical facilitation (ICF), and F‐wave persistence and area were assessed in the first dorsal interosseous (FDI) muscle before titration, after 1 and 2 h of hypoxic exposure, and at reoxygenation. The clamping protocols resulted in differing reductions in SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ by 2 h ( SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol: 81.9 ± 1.3%, FIO2${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol: 90.6 ± 2.5%). Although unconditioned MEP peak to peak amplitude and area did not differ between the protocols, SICI during FIO2${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping was significantly lower at 2 h compared to SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping (P = 0.011) and baseline (P < 0.001), whereas ICF was higher throughout the FIO2${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping compared to SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping (P = 0.005). Furthermore, a negative correlation between SICI and SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (rrm = −0.56, P = 0.002) and a positive correlation between ICF and SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (rrm = 0.69, P = 0.001) were determined, where greater reductions in SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ correlated with less inhibition and less facilitation of MEP responses. Although F‐wave area progressively increased similarly throughout the protocols (P = 0.037), persistence of responses was reduced at 2 h and reoxygenation (P < 0.01) during the SpO2${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol compared to the FIO2${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol. After 2 h of hypoxic exposure, there is a reduction in the activity of intracortical circuits responsible for inhibiting motor output, as well as excitability of spinal motoneurones. However, these effects can be influenced by other physiological responses to hypoxia (i.e., hyperventilation and hypocapnia).