Measuring the Influence of Hypercapnia on Absolute CMRO2 in Humans

Kevin Murphy¹, Ashley D. Harris¹, Richard G. Wise¹

Hypercapnia is often used in calibrated BOLD techniques to determine the maximum possible BOLD response. The assumption is that an increase in arterial CO₂ tension will not affect CMRO₂, a measure of brain metabolism. This assumption has been called into question with studies showing conflicting results. A framework is presented that models changes in CMRO₂ caused by hypercapnia at two different CO₂ levels: +4mmHg and +8mmHg. Decreases in CMRO₂ during hypercapnia were observed but only for the +8mmHg condition. This suggests that a low level of hypercapnia (~4mmHg) must be used for the assumption of calibrated BOLD to hold.