title: Searching for a truly "iso-metabolic" gas challenge in physiological MRI.
abstract: Hypercapnia challenge (e.g. inhalation of CO2) has been used in calibrated fMRI as well as in the mapping of vascular reactivity in cerebrovascular diseases. An important assumption underlying these measurements is that CO2is a pure vascular challenge but does not alter neural activity. However, recent reports have suggested that CO2inhalation may suppress neural activity and brain metabolic rate. Therefore, the goal of this study is to propose and test a gas challenge that is truly "iso-metabolic," by adding a hypoxic component to the hypercapnic challenge, since hypoxia has been shown to enhance cerebral metabolic rate of oxygen (CMRO2). Measurement of global CMRO2under various gas challenge conditions revealed that, while hypercapnia (P = 0.002) and hypoxia (P = 0.002) individually altered CMRO2(by -7.6 ± 1.7% and 16.7 ± 4.1%, respectively), inhalation of hypercapnic-hypoxia gas (5% CO2/13% O2) did not change brain metabolism (CMRO2change: 1.5 ± 3.9%, P = 0.92). Moreover, cerebral blood flow response to the hypercapnic-hypoxia challenge (in terms of % change per mmHg CO2change) was even greater than that to hypercapnia alone (P = 0.007). Findings in this study suggest that hypercapnic-hypoxia gas challenge may be a useful maneuver in physiological MRI as it preserves vasodilatory response yet does not alter brain metabolism.
authors.affiliation: 1 Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, USA.|2 Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, USA.|3 Department of Biomedical Imaging and Radiological Science, China Medical University, Taichung, Taiwan.
authors.forename: Shin-Lei
authors.initials: SL
authors.lastname: Peng
authors.affiliation: 1 Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, USA.|2 Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, USA.|4 Department of Bioengineering, UT Arlington, Arlington, USA.
authors.forename: Harshan
authors.initials: H
authors.lastname: Ravi
authors.affiliation: 2 Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, USA.
authors.forename: Min
authors.initials: M
authors.lastname: Sheng
authors.affiliation: 2 Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, USA.
authors.forename: Binu P
authors.initials: BP
authors.lastname: Thomas
authors.affiliation: 1 Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, USA.|2 Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, USA.
authors.forename: Hanzhang
authors.initials: H
authors.lastname: Lu
mesh_terms: D000328:Adult; D001921:Brain / Q000098:blood supply* / Q000378:metabolism; D002245:Carbon Dioxide / Q000097:blood* / Q000378:metabolism; D002560:Cerebrovascular Circulation; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000097:blood* / Q000378:metabolism; D000860:Hypoxia / Q000097:blood* / Q000378:metabolism; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010100:Oxygen / Q000097:blood* / Q000378:metabolism; D010101:Oxygen Consumption; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen
keywords: Carbon dioxide; cerebral blood flow; cerebral metabolic rate of oxygen; hypercapnia; hypoxia