836 A breathing apparatus providing improved control of FiO 2 and FiCO 2 for calibrated fMRI

Purpose: Respiratory manipulations modulating the fractional concentration of inspired O2 and CO2 (FiO2 and FiCO2) to induce hyperoxia and/or hypercapnia as a vascular stimulus have become an important component of fMRI studies measuring the vascular as well as the metabolic function in the brain. While individualized mixing of gas concentrations to achieve prospective control of end-tidal levels is useful in certain situations, the requirements for many applications are amply met by delivering predetermined concentrations of inspired gases, so long as end-tidal values are recorded for retrospective normalization of the fMRI signal. The most common practice has been to administer fixed fractional concentrations for inhalation, delivering O2/CO2 enriched mixtures through low-cost nonrebreathing masks commonly used in clinical oxygen therapy. However, either due to variations in the shape of the subject’s face or to incomplete sealing of one-way valves used to release excess gas flow, such oxygen masks are often very leaky, hampering precise adjustments in fractional inspired concentrations and thus limiting the reproducibility of the hyperoxic/hypercapnic stimuli. One way around this limitation would be to ensure a tight seal of the face mask using surgical tape or other means. However, this approach should be avoided in a closed circuit where the gas supply directly connected to the mask because it creates a risk of asphyxia in the event that the medical gas supply is interrupted. We aimed at developing an apparatus for improved control over the fractional concentration of inspired gases which would be safe and suitable for functional MRI applications.