Brain stem Po-2 and pH of the working heart-brain stem preparation during vascular perfusion with aqueous medium

The rat working heart-brain stem preparation (WHBP) is an in situ preparati on having many of the advantages associated with in vitro preparations whil e retaining cardiovascular response functionality and an eupnoeic respirato ry motor pattern. The preparation is perfused arterially with an aqueous me dium having a much lower oxygen-carrying capacity than blood. To evaluate t he efficacy of the artificial perfusion in providing adequate gas exchange within the brain stem, we used polarographic PO2 and pH microelectrodes to determine the tissue PO2 and pH of the medulla oblongata at various depths. When the perfusate was equilibrated with 5% CO2 and 95% O-2, average tissu e PO2 was 294 Torr and no hypoxic areas were encountered. Tissue pH was rem arkably uniform throughout the tissue, and on average was only 0.04 +/- 0.0 2 pH units more acidic than that of the perfusate. Increasing the PCO2 of t he perfusate increased tissue PO2 and decreased arterial resistance. Decrea sing perfusate PCO2 (while keeping pH constant) decreased tissue PO2 and re duced the respiratory activity. These results suggest that arterial PCO2, i ndependent of arterial pH, is an essential variable in determining both res piratory drive and cerebrovascular perfusion. We conclude that the medulla of the WHBP is oxygenated and within a physiological pH, which accounts for the eupneic pattern of respiratory motor activity it generates. Furthermor e, this preparation may be a useful model for exploring mechanisms of centr al chemoreception as well as the dynamics of the cerebral vasculature respo nses following changes in blood gases.