This study investigated the relation between local cerebrocortical oxygen tension (PO₂) and cerebrocortical microflow (CMF) during normoxia (F_io₂ = 0.3) and hypoxic hypoxemia (F_io₂ = 0.16 and 0.1). Measurements were performed on mechanically ventilated rats and rabbits anesthetized with 0.8% Ethrane and maintained within normocapnic limits. Polarographic techniques based on the use of multiwire surface electrodes were applied for measurements of local PO₂ and CMF. In both species the mean tissue PO₂ values were similar under normoxia (26.0 and 31.5 mm Hg for rats and rabbits, respectively). CMF histograms showed pronounced heterogeneity. The highest CMF values exceeded the lowest ones by a factor of 36 in the rat and by a factor of 26 in the rabbit. Mean CMF values were 6.67 ± 0.72 and 4.09 ± 0.14 relative units (for definition see text) in rats and rabbits, respectively. During hypoxemia, if the mean tissue Po₂ was >5 mm Hg, mean CMF did not change but a change in the pattern of microflow distribution was observed with increases in some CMF values (up to 670% of control) and decreases in others (down to 12% of control). When mean tissue Po₂ values of <5 mm Hg were observed during hypoxemia, mean CMF increased in both species by ∼50% on average. The increase in CMF could be seen in each individual CMF recording. We conclude that in the brain cortex local regulatory mechanisms are responsible for the change in the pattern of distribution of microcirculation during moderate tissue hypoxia. The same or similar mechanisms produce an increase in microflow if the local tissue Po₂ falls below a critical level. Our experiments show that this critical level lies within the range of 0–5 mm Hg.

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Cerebrocortical Microcirculation in Different Stages of Hypoxic Hypoxia