The use of intensity modulated optical spectroscopy to measure cerebral saturation and haemoglobin concentration in the human fetus during labour
Background: Although intrapartum hypoxia-ischaemia is an important cause of death and permanent brain injury, current available methods for the detection of damaging fetal hypoxia are unsatisfactory and unreliable. Electronic fetal heart rate monitoring (EFM) is considered to be the "gold standard" for intrapartum fetal surveillance. However, monitoring of the fetal heart rate provides an indirect measure of fetal hypoxia and provides little indication of the adequacy of cerebral perfusion. Consequently, EFM has a false positive rate of 99.8% in the detection of fetuses that subsequently develop cerebral palsy. A direct consequence of the poor specificity of fetal heart rate monitoring is a high rate of unnecessary Caesarean sections with associated fetal and maternal morbidity. Most fetuses subjected to intrapartum hypoxia will be protected from brain injury by appropriate changes in cardiovascular distribution. It is the aim of fetal surveillance to detect the minority of fetuses in whom this response is absent and who are at risk of permanent brain injury. Intrapartum fetal pulse oximetry is a promising new method of fetal surveillance, enabling measurement of fetal arteriolar saturation but the technique provides no direct information on cerebral oxygen delivery. Intrapartum measurement of fetal cerebral saturation and changes in blood volume has been described using conventional near infrared spectroscopy (NIRS). However, the clinical relevance of these values may be limited. This is because these values are derived from changes in the concentrations of oxyhaemoglobin (HbCh) and deoxyhaemoglobin (Hb) from an arbitrary baseline. Furthermore, the contribution of artefact to NIRS measurements of Hb and HbC>2 changes, arising from possible changes in the geometry of the NIRS fetal probes during uterine contractions, has not been clearly defined. Using novel methods of collection and analysis of NIRS data, the technique of Intensity Modulated Optical Spectroscopy (IMOS) has the unique potential to provide direct information on fetal cerebral oxygenation and perfusion during labour from measured absolute values of fetal cerebral Hb02 and Hb. Furthermore, the technique of IMOS has the potential to provide more information on the contribution of probe movement during uterine contractions to conventional NIRS data.;Aims: The aims of this project were therefore to use this new technique of intensity modulated optical spectroscopy to (a) provide the first measurements of absolute cerebral blood volume and cerebral saturation in healthy normoxic human fetuses during labour and (b) to compare these values with those calculated from fetuses that develop hypoxia-ischaemia during labour and (c) to assess the contribution of probe movement during uterine contractions.;Methods: After assessing and optimising the technical performance of a specially designed and constructed intensity modulated optical spectrometer, a specially designed optical probe was placed against the scalp of 29 fetuses after rupture of amniotic membranes during labour and connected to the spectrometer.;Results: Of these 29 fetuses, data were collected from 18 fetuses during the first and second stage of labour through to delivery. Of these 18, data was suitable for analysis in 10 of these fetuses. In these 10 fetuses, a mean (+/-1 S.D.) value of cerebral saturation of 59 +/-12 % and a mean absolute cerebral blood volume of 2.8 +/-1.0mls/100g over 3 uterine contractions were derived from the mean concentrations of Hb and Hb02 of 30 +/-18 and 46 +/-21 umol/1, respectively. Concentration changes rather than artefact appeared to dominate the NIR signal in the calculation of these values.;Conclusion: This work has provided the first measurements of absolute values of fetal cerebral oxygenation and of cerebral perfusion, whilst the contribution of artefact to the data, certainly in the healthy fetus, appears to be negligible. However, despite these advances in near infrared technology and knowledge of intrapartum fetal cerebral haemodynamics, the number of fetuses studied with near infrared spectroscopy, in particular IMOS, remains small. In order for IMOS to be subjected to larger studies to assess it's usefulness as a realistic adjunct to fetal heart rate monitoring, advances in the technology are still required.