Small Changes in Relative CMRO2 During Stepped Hypercapnia?

Stefan Alexandru Carp¹, Woo Hyun Shim¹, Jeong Kon Kim¹, Young Ro Kim¹

Hypercapnic (CO2) calibration is necessary to be able to measure relative changes in the cerebral metabolic rate of oxygen (CMRO2) during evoked fMRI responses. The underlying assumption is that there are no metabolic changes associated with CO2 inhalation. In this work we administered several levels of CO2 (2.5, 5 and 7.5%) to anesthetized rats and compared relative CMRO2 values between these levels. We observe that when calibrating with the 2.5% to 5% transition, 7.5% CO2 appears to result in lower cerebral oxygen demand. This may be a result of BOLD signal contamination due to venous volume changes.