Tissue oxygen saturation measured by near infrared spectrophotometry correlates with arterial oxygen saturation during induced oxygenation changes inneonates

The aim of this study was to compare quantitatively the changes in tissue o xygen saturation (TOS), determined by two algorithms (TOSc and TOSa) based on near-infrared spectrophotometry, to the changes in arterial oxygen satur ation (SaO(2)) measured by pulse oximetry. TOSe is an algorithm derived by the manufacturer (Critikon) based on a modified Beer-Lambert law; TOSa, our own algorithm, uses the diffusion approximation of light transport for the semiinfinite boundary condition. Slow changes of more than 3% in SaO(2) were carried out in 20 mechanically ventilated neonates by altering the inspired;ed oxygen fraction. For each c hange the regression lines of TOSc versus SaO(2), TOSa versus SaO(2) and TO Se versus TOSa were calculated. For each infant the mean slope, intercept a nd r(2) of these lines were determined. In 18 preterm infants we obtained median 9.5 (range one to 13) measurements corresponding to a total of 166 measurements. The mean SaO(2) was 91.6 (SD 2.3)%, TOSc was 64.7 (SD 7.2)% and TOSa was 71 .4 (SD 11.0)%. Changes in TOSe and TOSa were strongly correlated to changes in SaO(2) (r(2) = 0.86 and r(2) = 0.87). TOSe considerably but systematica lly underestimated the size of the change: Delta TOSc - 0.49 Delta SaO(2). TOSa quantified changes reasonably correctly: Delta TOSa = 0.90 Delta SaO(2 ). Changes in TOSe and TOSa were highly correlated (r(2) = 0.98). These res ults are promising, but the large inter-individual variation requires furth er work.