Investigation of the changes in cerebral tissue oxygenation measured with near infrared spectroscopy in response to moderate hypercapnia

Introduction

The arterial partial pressure of carbon dioxide (PaCO₂) has a major influence on cerebral blood flow (CBF). Cranial near-infrared spectroscopy (NIRS) can monitor the resulting changes in cerebral haemodynamics and recent technical advances enable absolute measurements of tissue oxygenation using spatially resolved spectroscopy (SRS). This mixed (arterial, venous) oxygenation signal has been termed the tissue oxygenation index (TOI). TOI can be related to the arterial saturation (SaO₂), oxygen consumption (CMRO₂ in ml of O₂/min), CBF (in ml/min) and arterial (Va), venous (Vv) blood volume per unit mass of tissue (in l/g) by Eq. 1:

$\text{TOI}={\text{SaO}}_2-\left(\frac{\text{Vv}}{\text{Va}+\text{Vv}}\right)\left(\frac{{\text{CMRO}}_2}{\mathrm{k}\times \text{CBF}[\text{HB}\times {10}^{-2}]}\right)$

where [Hb] is the haematocrit (g/dl) and k is the oxygen combining power (∼1.306 ml O₂/g Hb). The aim of this study is to investigate the association between the cerebral TOI and systemic physiological changes during hypercapnia.

Methods

12 healthy volunteers (mean ± SD age 32 ±4 years) were investigated. A Hamamatsu NIRO 300 system recorded changes in oxyhaemoglobin ([HbO₂]) and deoxyhaemoglobin ([HHb]) concentrations and absolute TOI. End tidal CO₂ (EtCO₂) values were monitored continuously via a face mask and non-invasive beat-to-beat blood pressure with a Portapres® system. Data were recorded as the volunteers (i) breathed room air and (ii) during inspiration of 5% CO₂.

Results

Average values of each signal were calculated for 1 minute periods at the end of rest and hypercapnia. A statistically significant increase during hypercapnia was observed in EtCO₂ (7±4 mmHg), cerebral TOI (2.7±1.4%) and total haemoglobin [HbT]=[HbO₂]+[HHb] (2.4±1.7 μmol/l) (paired t-test, p<0.001). Correlation analysis showed that the changes in EtCO₂ were associated with the changes in [HbT] (r=0.54, p=0.03) but not the changes in [TOI] (r=0.46, p>0.05). However association analysis between the absolute values of TOI and EtCO₂ during hypercapnia show a substantial correlation (r=0.63, p=0.01). Multi-regression analysis between hypercapnic absolute values of EtCO₂, MBP and TOI revealed a relationship with r=0.74 and p<0.05.

Discussion

We have investigated cerebrovascular reactivity to induced hypercapnia observing a consistent increase in [HbT] and TOI confirming results of others^{1, 2}. Significant association was found between the absolute values of TOI and the absolute EtCO₂ and MBP values during hypercapnia suggesting that the absolute TOI value is dependent on the baseline state of these two variables. This might imply a relationship between CBF and TOI, which has already been shown in neonates ³ . Our data shows that the changes in TOI are not linearly related to the changes in [EtCO₂]. Clearly the relationship between TOI, CBF and the arterial:venous volume fraction requires further investigation particularly in studies where CBF is measured directly. These data have implications for the interpretation of changes in TOI during clinical monitoring.