O2-Hb reaction kinetics and the Fåhraeus effect during stagnant, hypoxic, and anemic supply deficit.

We modified our previous computer model of O2 and CO2 transport in the cerebral microcirculation to include nonequilibrium O2-Hb kinetics and the Fåhraeus effect (reduced tube hematocrit in small microvessels). The model is a steady-state multicompartmental simulation which includes three arteriolar compartments, three venular compartments, and one capillary compartment. Three different types of oxygen deficits (stagnant, hypoxic, and anemic conditions) were simulated by respectively reducing blood flow, arterial O2 saturation, and systemic hematocrit to one half of normal. Microcirculatory distributions for PO2, PCO2, O2 saturation and deviations from equilibrium, and the O2 and CO2 fluxes for each compartment were predicted for the three O2 supply deficits. Differences were found for O2 extraction ratios and relative contributions of arteriolar, venular, and capillary gas fluxes for each type of deficit. The Fåhraeus effect and O2-Hb kinetics reduced O2 extraction in all cases and altered microcirculatory gas distributions depending on the specific type of O2 supply deficits. The modified model continues to predict that capillaries are the major site where gas exchange takes place, and demonstrates that the Fåhraeus effect and nonequilibrium O2-Hb kinetics are important mechanisms that should not be neglected in O2 and CO2 transport modeling. While this model provides useful insight regarding the influence of the Fahraeus effect and O2-Hb kinetics under steady state, the addition of a distributed and dynamic simulation should further elucidate the effects of the brain's heterogeneous properties and transient behavior.