The subject was fitted with a nose clip and a mouthpiece to allow mouth breathing. The subject inspired room air and the hypercapnic gas in an interleaved fashion (60 sec CO2 followed by 60 sec room air, repeated three times with additional 60 sec room air inhalation at the end). The subject with sealed nose breathed through a mouth piece that connected to a 5% CO2 gas bag or room air. The composition of the hypercapnic gas was 5% CO2, 74% N2 and 21% O2. The type of air inhaled was switched via a valve in a 1min interleaved manner while BOLD images and physiologic parameters (end-tidal CO2, breathing rate, heart rate, and arterial oxygenation) were continuously collected for 7min. A research assistant was always present inside the scanner room during the experiment to monitor the subject and switch the valve that controls delivery of hypercapnic gas and room air. End-tidal CO2 (Et-CO2), which measures the concentration of CO2 in the lungs and represents a surrogate for concentration of CO2 in the arterial blood, was recorded for the entire duration of the scan using a capnograph device (Capnogard, Model 1265, Novametrix Medical Systems, CT). Imaging parameters used in this study were FOV = 220 ? 220 mm2, matrix size = 64 ? 64, 43 axial slices, thickness = 3.5 mm, no gap, TR/TE/flip angle = 2000 ms/25ms/80?, single-shot EPI. …show more content…
The error bar represents standard error. Note that the c was significantly greater than zero.
In this study the nonlinearity in BOLD response to EtCO2 change was investigated for moderate hypercapnia. Contrary to sigmoidal response hypothesized previously for ramped stimulus, we observed opposite pattern i.e. inverse sigmoid pattern in BOLD response. A new biophysical model was proposed to account for this non-linearity. We found that the non-linear model showed improved r2 which is an indicator of good ness of fit for a model. This would suggest that the new proposed model better describes the BOLD response. We then investigated the age long variation in the fitted parameters in BOLD response We found that there is non-linearity the BOLD response present irrespective of age and this non-linearity is significantly greater than zero. Suggesting the nonlinearity may be governed by other factors independent of aging. We also found significant age related changes to EtCO2 and these changes was mostly driven by the changes in male