Probing Brain Oxygenation with Near Infrared spectroscopy, the Role of Carbon Dioxide and Blood Pressure

TL;DR

This paper reviews near infrared spectroscopy (NIRS) for brain oxygenation, highlighting the influence of carbon dioxide and blood pressure on cerebral blood flow, supported by models and experiments on animals and humans.

Contribution

It introduces a simple four-parameter model relating cerebral blood flow to PaCO2, enabling data comparison across species and conditions.

Findings

PaCO2 significantly affects cerebral blood flow and oxygenation.

A physical model relates blood pressure to cerebral blood flow.

Experimental data on humans show PaCO2's impact on brain oxygenation.

Abstract

The fundamentals of near infrared spectroscopy (NIRS) are reviewed. Among the major factors controlling the cerebral blood flow (CBF), the effect of PaCO2 is peculiar in that it violates autoregulatory CBF mechanisms and allows to explore the full range of the CBF. A simple physical model, with a four parameter formula, relating the CBF to PaCO2 is presented. It can be used to transform the fits of one animal to the fits of another one. It enable the use of rats data as monkeys data simply by rescaling the PaCO2 values and the CBF data. Controlled breathing can change the PaCO2. Experiments on human subjects relating the PaCO2 to rSO2, measured with brain oximeters, are presented. A simple model relating the mean blood pressure to CBF is worked out.