Blood-brain tissue Pco2 relationships and ventilation during rebreathing.

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Abstract
Equations describing nonsteady CO2 exchange of medullary chemoreceptor tissue, for variable local blood flow, were solved using an analogue computer to predict the relations between: brain tissue PCO2 (PR) and arterial PCO2 (Pa); brain tissue-arterial PCO2 difference (PR - Pa) and time of rebreathing (t); ventilation (VE) and arterial PCO2. Theory predicted, after 20 sec of rebreathing 7% CO2 from a small bag: a markedly reduced and relatively constant PR - Pa; near linearity of PR/Pa and VE/Pa curves; insensitivity of PR/Pa and VE/Pa curves to alteration of receptor blood flow; VE/Pa curves which were displaced to the right of those for rebreathing 1% CO2 from a large bag, those for rebreathing without CO2 in the bag, and those for steady-state CO2 inhalation. Experimentally determined VE/Pa curves confirmed these predictions. These predictions and data are examined in relation to in vivo CO2 dissociation curves of arterial blood and brain tissue, transient fluxes of bicarbonate ions between brain interstitial fluid and cerebrospinal fluids, and peripheral chemoreceptor drive.