THE NONINVASIVE MEASUREMENT OF ABSOLUTE CEREBRAL DEOXYHEMOGLOBIN CONCENTRATION AND MEAN OPTICAL-PATH LENGTH IN THE NEONATAL BRAIN BY 2ND DERIVATIVE NEAR-INFRARED SPECTROSCOPY

We have used second differential near infrared spectroscopy of water t o determine the mean optical path length of the neonatal brain. By obt aining the ratio of the second differential features of deoxyhemoglobi n to those of water, the absolute cerebral concentration of deoxyhemog lobin can be monitored continuously and noninvasively. Nineteen neonat es were studied; the gestational age at birth varied from 23 to 38 wk, and the postconceptual age, when the spectra were recorded, ranged fr om 35 to 48 wk. The calculated mean deoxyhemoglobin concentration was 14.6 +/- 4.0 mu M; the differential path length factor (mean optical p ath length/optode separation) calculated from the water peak at 730 nm was 4.66 +/- 1.01, and that calculated at the 830-nm peak was 3.91 +/ - 0.75. These values are consistent with path length measurements usin g laser time-of-flight spectroscopy on postmortem neonates and phase-r esolved spectroscopy on live neonates. Induced arterial oxygen saturat ion decreases from 98 to 93% showed no significant change in the mean optical path length, despite significant cerebral desaturation. Change s in the deoxyhemoglobin concentration after this procedure were ident ical, whether measured by second differential analysis at 760 nm or by multilinear regression over the wavelength range 740-900 nm. When com bined with existing methods of measuring total cerebral hemoglobin con centration, second differential near infrared spectroscopy can be used to derive the mean cerebral oxygen saturation. A preliminary experime nt outlined the feasibility of this approach and yielded a saturation value of 63%, consistent with near infrared sampling of a predominantl y venous pool in the brain.