Multi-wavelength optical imaging of experimentally induced hypercapnia

Guiou, M.; Pouratian, N.; Nemoto, M.; Sheth, S.; Toga, A. W.

Changes in PaCO2 have a marked effect on cerebral blood flow. Previous studies suggest hypercapnia (HC) has a long-lasting effect on functional activation. We sought to characterize the change in intrinsic optical signals in response to HC. Optical imaging was performed using a thinned skull preparation. The cortex was epi-illuminated by light serially filtered at 850, 610, and 570 nm via a high-speed filter wheel allowing for interleaved image acquisition with a CCD camera. HC was induced by adding 5% CO2 to the inspired air for 30 min. Reflectance values were calculated for each wavelength in an ROI comprising the right parietal cortex. Following HC induction a large increase (25+-6.5%) in reflectance was observed at 610 nm. Within 30s these values leveled off and remained stable for the HC period. After cessation of the 5% CO2 mixture, a large decrease in reflectance was noted. Within 90s, reflectance values returned to baseline levels. Interestingly, an ROI analysis of tissue vs. vasculature showed during the HC period values in the large veins were 4.26+-1.00% higher than in the surrounding tissue. At 850 nm, following HC induction reflectance values decreased (.74+-.37%). Within 30s these values equilibrated and remained stable for the remainder of the HC period. Upon termination of HC, reflectance values approached baseline levels. Results at 570 nm were difficult to characterize, however a decrease in reflectance with HC was seen in several animals. HC results in a rapid increase in blood volume and hemoglobin concentration. The large values recorded at 610 nm suggest HC has little effect on the rate of O2 extraction resulting in elevated oxy-hemoglobin concentrations and venous hyper-oxygenation.