Presented at the Neonatal Society 2004 Autumn Meeting.
Austin T1, Leung T2, Elwell C2, Henty J2, Meek J1, Wyatt J1
1 Department of Paediatrics and Child Health, University College London, UK
2 Department of Medical Physics and Bioengineering, University College London, UK
Background: Ensuring the adequacy of cerebral oxygen delivery to meet metabolic demand is important in the brain orientated care of newborn infants. Spatially resolved spectroscopy (SRS) is an optical method which provides quantitative measurement of the mean cerebral oxygen saturation (SmcO2) at the cotside (1). SmcO2 is dependent on both cerebral oxygen delivery and consumption.
Aim: The aim of this study was to use measurements of SmcO2 obtained by SRS and changes in oxyhaemoglobin (ΔHbO2) measured using differential spectroscopy to obtain absolute values of cerebral oxygen delivery (CDO2) and cerebral metabolic rate (CMRO2) in a cohort of newborn infants undergoing intensive care.
Methods: Ten infants born at a median (range) postmenstrual age of 25 (23-37) weeks were studied. Their median birthweight was 664 (512-2500)g and the median age at study was 1.5 (1-9) days post delivery. All infants required ventilatory support. SmcO2 data was collected using a NIRO 300 spectrophotometer (Hamamatsu Photonics, K.K., Japan). Cerebral blood flow (CBF) was measured using the oxygen bolus technique (2). Using measurements of SmcO2 and CBF it is possible to calculate CMRO2. using the following equation:
SaO2 is the arterial oxygen saturation (fractional value); Vven is the venous:arterial compartment volume ratio (assumed to be 0.75); K is the oxygen combining power of haemoglobin (Hb) (1.306 ml.g-1); tHb is the haemoglobin concentration in blood (g.100ml-1), measured on each baby from a venous sample; CBF is the cerebral blood flow (ml.100g-1.min-1).
Results: The median (range) SmcO2 was 58.2 (47.4-73.4%), CDO2 2.10 (1.33-4.78) ml O2 100g-1min-1, CMRO2 0.96 (0.8-1.40) ml O2 100g-1 min-1. CMRO2 increased with postmenstural age (r=0.73, p=0.02). There was a significant linear relationship between CMRO2 and CDO2 (r=0.75, p=0.01) (figure 1).
Discussion: This study represents a novel method for calculating CMRO2. The values are low and consistent with other studies measuring CMRO2 in the newborn. Although there is evidence of perfusion-metabolism coupling in the newborn brain, infants with a low SmcO2 as a result of a mismatch between CDO2 and CMRO2 may be vulnerable to hypoxic-ischaemic injury.
References
1. Matcher S.J. et al. Proc SPIE 1995;2389:486-495.
2. Edwards A.D. et al. The Lancet 1988;8614:770-771.