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

Abstract

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 CO2 levels, and a change in oxygen consumption with CO2 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.

Modelling Noninvasively Measured Cerebral Signals during a Hypoxemia Challenge: Steps towards Individualised Modelling

“…Further, the damped response of TOS is consistent with preliminary studies on the simultaneous response of TOS and vMCA to changes in inspired levels [39]. In this case, increased causes significant increases in vMCA, and hence presumably in CBF, which are not visible to the expected degree in the TOS signal.…”

Influence of probe pressure on the diffuse correlation spectroscopy blood flow signal: extra-cerebral contributions

“…The hyperventilation task is expected to decrease the arterial concentration of CO 2 , ultimately inducing a decrease in brain blood flow [40,41]. Importantly, both tasks have been previously suggested to induce a global response [42,43]. By global, we mean that the cerebral and the extra-cerebral (scalp/skull) tissues should both exhibit hemodynamic variations in response to the ventilation tasks, although the magnitude of the flow variation across tissue types might be different.…”

Cerebral Autoregulation

“…In particular, the model by Banaji et al (2005), which has been used extensively, assumes the model form above, but replaces the two feedback equations with a detailed model of the biochemical pathways, such that the activation factors are set by the MLC phosphorylation in each compartment. Banaji et al (2005) This model, with further adaptions, has been used in a number of studies, although these are mainly animal studies, apart from the work of Moroz et al (2012) in healthy adults. NO production is controlled by pressure and pH in each compartment, whereas intracellular calcium is controlled by a detailed model of flow and metabolism, Fig.…”