Mitochondrial NADH as the bellwether of tissue O2 delivery.

It is proposed that the redox state of mitochondrial NADH will complement blood gas analysis for measuring the health and welfare of human tissues. Use of arterial oxygen saturation levels (SaO2), especially as assayed by the Nellcor instrument, has spread almost everywhere in medicine despite the fact that hypoxia of internal organs, liver, kidney, brain, pancreas, etc. is not well indicated by peripheral digital oxygenation. Indeed, there is an implied liability in the failure to infer central oxygenation from peripheral values. Near infrared (NIR) sensing of deep tissue saturation of hemoglobin (StO2) requires multi-wavelength, multi-site measurement of both absorption and scattering properties by time or frequency domain NIR methods. Corrections for underlying water and lipid absorptions can be made so that the correct value for, and saturation oh hemoglobin are obtained. Nevertheless, the significance of blood oxygen saturation, even localized to particular organs, can be questioned from the standpoint of what is the critical value of the desaturation from which the tissue can recover; for example, in the case of cortical neurons where stroke, compression ischemia, etc. cause O2 lack, this value becomes of significant clinical importance in both the brain and the spinal chord. These approaches are actively pursued and the possibility of subsurface redox state measurement in human tissues may eventually emerge as the quantitative metric of tissue metabolic state and of hypoxic stress. The great flexibility and versatility of the fast, economical and "tetherless" nature of opto-electronic technology is appropriate to the manifold challenges of neuronal function as currently measured by intrinsic signals and soon to be studiable by extrinsic signals of metabolism and electrophysiological functions.