Development of a Model to Aid NIRS Data Interpretation: Results from a Hypercapnia Study in Healthy Adults

The use of a mathematical model of cerebral physiology and metabolism may aid the interpretation of experimentally measured data. In this study, model outputs of tissue oxygen saturation (TOS) and velocity of blood in the middle cerebral artery (Vmca) were compared with experimentally measured signals (TOS using near infrared spectroscopy and Vmca using transcranial Doppler) acquired during hypercapnia in healthy volunteers. Initially, some systematic discrepancies between predicted and measured values of these variables were identified. The model was optimised to best fit the measured data by adjusting model parameters. To improve the fit, three additional model mechanisms were considered. These were: an extracerebral contribution to TOS, a change in venous volume with CO(2) levels, and a change in oxygen consumption with CO(2) levels. Each mechanism, when used alone, improved the fit of the model to the data, although significant parameter changes were necessary. It is likely that a combination of these mechanisms will improve the success of modelling of TOS and Vmca changes during hypercapnia.