Investigation of the changes in cerebral tissue oxygenation measured with near infrared
spectroscopy in response to moderate hypercapnia.
Tachtsidis I, Jones H, Oliver C, Delpy DT, Smith M, Elwell CE
Medical Physics and Bioengineering, University College London,
Department of Neuroanaesthesia & Neurocritical Care, The National Hospital for Neurology & Neurosurgery,
London
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:
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 1minute periods at the end of rest and hypercapnia. A
statistically significant increase during hypercapnia was observed in EtCO2 (7±4mmHg), cerebral TOI
(2.7±1.4%) and total haemoglobin [HbT]=[HbO2]+[HHb] (2.4±1.7µmol/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.
References:
[1] Smielewski P, Kirkpatrick P, Minhas P, Pickard JD, and Czosnyka M; Stroke 26, 2285-2292 (1995)
[2] Yoshitani K, Kawaguchi M, Tatsumi K, Kitaguchi K, and Furuya H; Anesth Analg 94, 586-590 (2002)
[3] Elwell CE, Henty JR, Leung TS, Austin T, Meek JH, Delpy DT, and Wyatt JS; Adv.Exp.Med.Biol. In press
2005