Photoacoustic Imaging for the Detection of Hypoxia in the Rat Femoral Artery and Skeletal Muscle Microcirculation

Photoacoustic (PA) imaging is an emerging technology that combines structural and functional imaging of tissues using laser and ultrasound energy. We evaluated the ability of PA imaging system to measure real-time systemic and microvascular mean oxygen saturation (mSAO(2)) in a rat model of hypoxic shock. Male Sprague Dawley rats (n=6) underwent femoral artery catherization and were subjected to acute hypoxia by lowering the fraction of inspired oxygen (FiO(2)) from 1.0 to 0.21, and then to 0.08. PA measurements of mSaO(2) were taken in the femoral artery near the catheter tip using the Vevo 2100 LAZR at each FiO(2) and compared to co-oximetry on blood removed from the femoral catheter. Both co-oximetry and PA imaging measured a similar stepwise decline in femoral artery mSaO(2) as FiO(2) was lowered. We also measured mSaO(2) in the feed arteriole of the rat spinotrapezius muscle and adjacent microvessels (n=6) using PA imaging. A significant decrease in mSaO(2) in both the feed arteriole and adjacent microvessels was recorded as FiO(2) was decreased from 1.0 to 0.08. Moreover, we detected a rapid return toward baseline mSaO(2) in the feed arteriole and microvessels when FiO(2) was increased from 0.08 to 1.0. Thus, PA imaging is noninvasive imaging modality that can accurately measure real-time oxygen saturation in the macro and microcirculation during acute hypoxia. This proof-of-concept study is a first step in establishing PA imaging as an investigational tool in critical illness.