Dynamics of oxygen delivery and consumption during evoked neural stimulation using a compartment model and CBF and tissue P(O2) measurements.

The dynamics of blood oxygen delivery and tissue consumption produced by evoked stimulation of the rat somato-sensory cortex were investigated. Tissue oxygen tension (P(O2)) and laser Doppler flowmetry (LDF) measurements were recorded under two experimental conditions: normal, which represented both oxygen delivery and consumption, and suppressed CBF (achieved using a vasodilator), which only represented tissue oxygen consumption. Forepaw stimulation for 10 s produced increases of 27.7% and 48.8% in tissue P(O2) and LDF signal under normal conditions, respectively. The tissue P(O2) response peaked 9.8 s after stimulation onset and did not show any early transient decreases indicating that measurable oxygen deficits are not required to increase the delivery of oxygen by blood flow. Under suppressed CBF conditions, the LDF signal was mostly suppressed while the tissue P(O2) decreased by 11.7% and reached a minimum 10.8 s after stimulation onset. These data were analyzed using a dynamic model that described the transport of oxygen from blood to tissue. In order to explain the differences between the model prediction of the tissue P(O2) changes and the experimental data, several hypothetical scenarios were considered, such as changes in the vascular volume, permeability-surface area or arterial oxygenation. The increase in tissue P(O2) was found to probably require the recruitment of upstream oxygen from larger arteries as well as increases in the vascular volume at the oxygen exchange sites. The amplitude of the estimated tissue tension of oxygen delivered was about 2.7 x larger than the estimated consumption under normal conditions (45.7% vs. 17.1%, respectively).