Determination of red blood cell oxygenation in vivo by dual video densitometric image analysis

We have developed a new video microspectrophotometric system for the in vivo determination of oxygen saturation in red blood cells in striated muscle capillaries. This method allows one to quantify changes in the oxygenation of small groups of red blood cells as they traverse the capillary. Simultaneous images of a single microscopic field are recorded using two silicon-intensified target cameras and high-resolution video recorders. One image is recorded at an oxygen-dependent wave-length (431 nm) and the other at an isosbestic wavelength (420 nm). Light intensities from 10 adjacent pixels aligned along the axis of the capillary from identical 10-s segments of the video-tapes are digitized once per frame. Both sets of data are redisplayed simultaneously as two-dimensional images (10 pixels high x 300 frames wide) using a graphics system. These images show alternating bright and dark bands corresponding to plasma gaps and red blood cells. Light intensities in the presence and absence of red blood cells are determined by positioning a window over the appropriate region of the graphics image. Optical densities of single red blood cells at the two wavelengths, OD431 and OD420, are computed as is their ratio (OD431/OD420), which is linearly related to oxygen saturation. In vivo calibration studies in capillaries of the hamster retractor muscle indicate that the error in measuring oxygen saturation with this technique is approximately 2.7% saturation for a group of 10 cells.