Effect of carbon dioxide on the utilization of brain capillary reserve and flow

This study investigated effects of increased arterial carbon dioxide on the brain capillary perfusion pattern. Conscious rats were exposed to a 0%, 8% or 12% CO2 in air gas mixture. Arterial blood pressure, heart rate, arterial blood gases and pH were recorded, and either regional cerebral blood flow or the percent of capillary volume/mm3 or number/mm2 perfused were determined in cortical, hypothalamic, pontine or medullary regions of the brain. Arterial PCO2 increased from 37 +/- 1 in control to 74 +/- 1 torr in the high CO2 group. A position linear relationship was found between cerebral blood flow and arterial PCO2 in all examined regions. Approximately half of the capillaries in the examined regions were perfused under normocapnic conditions. Increasing arterial PCO2 had no effect on the percent of the capillary bed perfused in the cortex or hypothalamus. However, there was a significant linear relationship between carbon dioxide tension and the percent of the microvasculature perfused in the hindbrain. The percent of capillaries/mm2 perfused increased significantly in the medulla (to 60 +/- 5%) and pons (70 +/- 4%) with 12% CO2 in air. These data suggest that carbon dioxide may have differential effects on diffusion distances affecting the hindbrain to a greater extent than the forebrain.