Presented at the Neonatal Society 2005 Autumn Meeting.
Wong FY1,3, Austin T1, Henty J2, Elwell CE2, Walker AM3, Meek JH1, Wyatt JS1
1 Department of Paediatrics and Child Health, University College London, UK
2 Department of Medical Physics and Bioengineering, University College London, UK
3 Ritchie Centre for Baby Health Research, Monash University, Melbourne, Victoria, Australia
Background: Preterm infants are often prescribed inotropes to maintain mean arterial blood pressure (MAP). The effect of inotropes on cerebral oxygen metabolism remains unknown. Spatially resolved spectroscopy (SRS) measures the mean cerebral oxygen saturation (SmcO2) at the cotside (1). SmcO2 is dependent on both cerebral oxygen delivery and consumption.
Aim: Using SRS, we studied normotensive infants (NT) and infants who were on inotropes (INO, dopamine) for hypotension. We aim to compare the relationship between SmcO2 and cerebral oxygen delivery (CDO2) in these infants.
Method: The study was approved by the Joint UCL/UCLH Committees on the Ethics of Human Research, and parental consent was obtained prior to study. Fourteen infants in the NT group and 10 infants in the INO group, born at median (range) gestational age of 25 (23-32) weeks, were studied at median (range) postnatal age of 18 (4-103) hours. Four infants in the NT group and 3 infants in the INO group developed intraventricular haemorrhage. Using a NIRO-300 spectrometer (Hamamatsu Photonics, K.K., Japan), stable baseline SmcO2 was averaged over the 5 minutes immediately prior to cerebral blood flow (CBF) measurements. CBF was measured using the oxygen bolus method (2). The averaged value of 5 CBF readings was obtained to compute CDO2(CDO2 = CBF x cerebral arterial oxygen content). Between 1 and 3 sets of replicate measurements of SmcO2 and CDO2 were obtained over 1 to 3 hours to derive the mean value for each infant.
Results: Median (IQR) SmcO2 was 58.4 (53.7-65.1)% and CDO2 was 2.3 (1.8-2.6)ml O2/100g/min. There was no significant difference in SmcO2 and CDO2 between the NT and INO groups. In NT infants, CDO2 positively correlated with SmcO2 (r=0.78, p<0.001) (fig.1), suggesting coupling of cerebral perfusion and tissue oxygenation. In contrast, no correlation existed between CDO2 and SmcO2 in the INO infants.
Conclusion: Infants on inotropes have uncoupling of cerebral perfusion and tissue oxygenation. The underlying aetiology of our findings remains to be clarified, but may relate to the initial clinical condition leading to hypotension or to a direct effect of inotropes on the cerebral circulation. Whilst effective in raising systemic blood pressure, inotropes might not improve cerebral oxygenation.
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.