Amnestic Mild Cognitive Impairment Does Not Alter Cerebrocortical Oxygenation Dynamics During Acute Hypoxia-Reoxygenation in Older Adults.

Background: This study examined the impact of amnestic mild cognitive impairment (aMCI) on dynamic changes in cerebrocortical oxygen saturation (ScO2) and O2 extraction during acute, moderately intense, normobaric hypoxia and reoxygenation in elderly adults (71 ± 6 years old). Methods: Thirty-two aMCI and thirty-five control subjects participated. Inspired and expired fractions of O2 and CO2 (mass spectrometry), arterial O2 saturation (SaO2) and prefrontal ScO2 (near-infrared spectroscopy), heart rate, tidal volume and breathing frequency were monitored while subjects breathed hypoxic air (fractional inspired O2 0.10) for 3-5 min (aMCI: 4.5 ± 0.7 min; control: 4.5 ± 0.6 min) and recovered on room air. Values at the pre-hypoxia baseline, the first and last min of hypoxia and the first min of recovery were compared within and between groups using two-factor ANOVA. Results: Despite a similar baseline SaO2 in aMCI (97.2 ± 1.6%) and control (97.3 ± 1.3%) subjects, prefrontal ScO2 was lower (p < 0.05) in the aMCI subjects in both the left (67.0 ± 1.7% vs. 69.6 ± 4.5%) and right (66.8 ± 4.6% vs. 69.4 ± 4.1%) hemispheres. Hypoxia similarly decreased SaO2 and ScO2 in both groups (last min hypoxia, aMCI vs. control subjects: SaO2 76.6 ± 5.3% vs. 77.4 ± 6.1%, left prefrontal ScO2 54.0 ± 4.9% vs. 55.2 ± 6.4%, right prefrontal ScO2 56.0 ± 4.3% vs. 58.2 ± 4.4%). Upon the resumption of room-air breathing, ScO2 recovered at similar rates in aMCI and control subjects. Conclusions: Although it produced a greater deoxygenation in the left vs. the right prefrontal cortex, acute, normobaric, moderate hypoxia was well tolerated by elderly adults, even those with aMCI. Dynamic changes in cerebral oxygenation during hypoxia and recovery were unaltered by aMCI. Brief, moderate hypoxia does not impose more intense cerebrocortical oxygen depletion in elderly adults with aMCI, despite pre-hypoxic cerebrocortical oxygenation below that of their non-MCI counterparts.