Relationship between brain tissue haemodynamics, oxygenation and metabolism in the healthy human adult brain during hyperoxia and hypercapnea.

This study investigates the relationship between changes in brain tissue haemodynamics, oxygenation and oxidised cytochrome-c-oxidase ([oxCCO]) in the adult brain during hyperoxia and hypercapnea. 10 healthy volunteers were studied. We measured the mean blood flow velocity of the right middle cerebral artery (Vmca) with transcranial Doppler (TCD) and changes in concentrations of total haemoglobin ([HbT]=[HbO2]+[HHb]), haemoglobin difference ([Hbdiff]=[HbO2]-[HHb]) and [oxCCO] with broadband near-infrared spectroscopy (NIRS). We also measured the absolute tissue oxygenation index (TOI) using NIR spatially resolved spectroscopy. During hyperoxia there was an increase in TOI (2.33 +/- 0.29%), [Hbdiff] (4.57 +/- 1.27 microM) and in the oxidation of [oxCCO] (0.09 +/- 0.12 microM); but a reduction in Vmca (5.85 +/- 4.85%) and HbT (1.29 +/- 0.91 microM). During hyperoxia there was a positive correlation between [oxCCO] and TOI and [Hbdiff] (r=0.83 and r=0.95) and a negative association between [oxCCO] and Vmca and [HbT] (r=-0.74 and r=-0.87). During hypercapnea there was an increase in TOI (2.76 +/- 2.16%), [Hbdiff] (7.36 +/- 2.64), [HbT] (2.61 +/- 2.7 microM), Vmca (14.92 +/- 17.5%) and in the oxidation of [oxCCO] (0.25 +/- 0.17 microM). Correlation analysis shows that there was association between [oxCCO] and TOI, [Hbdiff] and [HbT] (r=0.83, r=0.93 and r=0.82) but not with Vmca (r=0.33). We conclude that an increase in [oxCCO] was seen during both challenges and it was highly associated with brain tissue oxygenation.