By: 1 June 2009

It has been said that the most dangerous journey we ever undertake is the one we take when we are born.
The latest figures for infant mortality show that, of 668,681 live births and 7,225 deaths, there were 1,193 late infant losses, 3,676 stillbirths and 2,356 neonatal deaths; taken from Birth and Death notifications to the Confidential Enquiry into Maternal and Child Health (CHEMACH), (Office for National Statistics (ONS) 2005 for England, Wales and Northern Ireland)4. These figures indicate that there are still improvements to be made. It is with this in mind, together with the early identification of the compromised fetus, as well as improving the care for women in labour, that fetal pulse oximetry is being put at the forefront of advanced monitoring in the delivery suite.

History of Oximetry
Pulse oximetry measures the percentage of haemoglobin saturated with oxygen in the blood. It is a non-invasive technique achieved by passing light of two specific wavelengths through the blood, and measuring the relative absorptions by the haemoglobin. From this the proportion of haemoglobin which is oxygenated can be calculated.

In 1875 Karl von Vierordt, a German physician, demonstrated that the oxygen in his hand was consumed when a tourniquet was applied, and that, by utilizing transmitted light waves, fully saturated blood could be distinguished from unsaturated blood. However, the development of the pulse oximeter was still a long way off.

It was Karl Matthes who developed the first ear saturation meter, which used two wavelengths of light. This was improved upon by Glenn Millikin who, in 1940, developed a lightweight oximeter to aid the military in solving aviation hypoxia.

The modern pulse oximeter was developed by Takuo Aoyagi in 1972 while working in Tokyo on non-invasive cardiac output measurement. He noticed a correlation in the difference between unabsorbed infrared and red light, and the oxygen saturation. This led to the development of the modern pulse oximeter.

It was in the 1980’s that Nellcor produced one of the first commercially reliable pulse oximeters. Since then its use has become standard procedure in many areas within the hospital, from pre and post op intensive care, to accident and emergency, as well as maternal pulse oximetry, but is yet to be widely adopted in the area of fetal pulse oximetry.

Nellcor produced a commercial fetal pulse oximetry unit, but unfortunately the probe could not be designed to stay in place long enough to provide continuous reliable information. The issues over loss of contact with the fetus were finally overcome by the Frauenhofer Institute in Munich, working in conjunction with Wolfartklinik Germany, and Nonin USA, by combining a pulse oximetry probe with a fetal scalp electrode5,7,8,9.

Current Practice
For over thirty years the main information that health professionals have had to monitor critical changes comes by fetal E.C.G.

In 1995 the 4th CESDI report highlighted the difficulties with electronic fetal monitoring. It stated that there were nearly 1,300 intrapartum deaths of normally formed fetuses in the United Kingdom4.

Almost 800 of these cases were attributed to failure to recognise the cardiotocographic (CTG) tracing to be abnormal , and failure to take timely action1,2,6. Even when the CTG was recognised as abnormal, there was a delay to intervene , and this could be due to the difficulty in identifying how fast hypoxia is progressing12, or due to the need to take a fetal scalp blood sampling (FBS) to confirm fetal acidosis6,15.

Delay in the delivery room after taking the decision to intervene almost doubles the admission rate of neonates to the intensive care unit6.

Traces on electronic fetal monitoring (EFM) have been abnormal in those who go on to develop hypoxia and acidosis in labour, and subsequent hypoxic ischaemic encephalopathy11,14. The EFM has high sensitivity, but the specificity is low , and since an abnormal fetal heart rate does not always mean fetal hypoxia or acidosis, the Royal College of Obstetrics and Gynaecology has recommended fetal scalp blood sampling13.

Fetal Scalp Blood Sampling

Until recently fetal scalp blood sampling (FBS) was not practised in nearly 50% of the delivery units in the U.K16, and even when the facilities exist, it is not used in all cases6. This could be due to the difficulties in obtaining a sample when the head is high, or the cervix is not sufficiently dilated16, or when clinically contra-indicated, however failure to take a sample in the presence of an abnormal cardiotocograph warrants a caesarean section10.

The Nonin “FetalSat” pulse oximeter consists of a computerised monitor and a probe that can be attached to the head of the fetus whilst in utero. The monitor displays digital percentage readout of a calculated estimate of the fetus’ haemoglobin that is saturated with oxygen (FSpO2) in “real time”.

The “FetalSat” pulse oximeter is a technically safe and practical system to use on the labour ward. Its application is as easy as applying most fetal scalp electrodes, and the readings are clear and accurate when connected to the existing electronic fetal monitor. The readings are printed on a single comprehensive strip chart.

In fetuses with good neonatal outcomes, the normal range of FSpO2 values throughout labour has been established. Studies from the USA, UK, Europe and Singapore have shown that FSpO2 values above 30% are not associated with fetal hypoxia and acidosis6,7,8,9.

When used with a suspicious or abnormal CTG, continuous monitoring of FSpO2 by pulse oximetry is a far better tool than repeated FBS.

In the long term, to be able to reduce intrapartum fetal and neonatal mortality and morbidity, unnecessary fetal scalp blood sampling, and operative intervention such as caesarean section or instrumental vaginal delivery for fetal distress, could translate into less neonatal and maternal morbidity and less cost to the NHS16.

The “FetalSat” pulse oximeter can assist the clinician and midwife by having an additional parameter on which to base their decision making, as well as giving more confidence when caring for women with a suspicious or an abnormal Fetal Heart Rate3.


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