Chapter 12 The role of scalp pH

The availability of fetal scalp blood sampling for the assessment of fetal scalp capillary blood pH and its application in practice vary enormously. The NICE guidelines have suggested the use of scalp pH in situations with suspicious and/or pathological cardiotocography (CTG) after due consideration to the clinical situation.¹³ At times the clinical situation may demand early delivery rather than a scalp pH. In reality, junior doctors use scalp pH more when they have less experience of labour ward responsibilities. This is understandable as they have a greater anxiety with a less-developed degree of understanding. As they gain experience and use pH as a guide, they then understand better the associations of an abnormal and a normal pH and will need this reassurance less often. The process of scalp blood sampling is undignified and uncomfortable for the woman. This is not to say it should not be done if properly indicated.

When the fetal heart rate (FHR) is reactive and normal, the chance of fetal acidosis is extremely low.^112–114 On the other hand, suspicious and abnormal FHR changes are not always associated with acidosis.^113–116 Such observations form the basis of the perceived need to measure fetal scalp pH for further investigation.

Changes in the CTG cause anxiety to the person not familiar with CTG interpretation. An inexperienced person in a centre with fetal blood sampling (FBS) facilities might perform FBS more frequently. When properly interpreted, assessment of FHR changes in most cases proves of equal value to pH in predicting fetal outcome.¹⁴ FBS is a useful adjunct because, even with the worst pattern of tachycardia, reduced baseline variability and decelerations, only 50–60% of the fetuses are acidotic.¹¹³ A wall chart correlating different FHR patterns to the percentage who are likely to be acidotic is available in most labour wards. It is clear from that chart and other studies that when the FHR pattern exhibited accelerations the chance of fetal acidosis was zero, emphasizing accelerations as the hallmark of fetal health.¹¹³ One problem of these charts is that all fetuses do not conveniently provide a fetal heart tracing that easily falls into one category. There is the added perspective of the need for a time continuum, which is so important in trace analysis.

Baseline variability is another good indicator of fetal health. When normal baseline variability is observed in the last 20 min prior to delivery, the babies are in good condition at birth regardless of other features of the trace.¹¹⁷ Fetal acidosis is more common when there is a loss of baseline variability with tachycardia or late decelerations.^113, ¹¹⁸ The preservation of normal baseline variability indicates that the autonomic nervous system is responsive and the fetus is trying to compensate despite other abnormal features in the trace. The reason that, with a given FHR pattern, there are different percentages of fetuses showing acidosis depends on the duration for which the suspicious or abnormal FHR pattern was present before the time of FBS.¹⁵ The approximate duration after which acidosis develops in an appropriately grown term fetus with a given FHR pattern has been discussed previously. It is also known that, in fetuses with less ‘placental reserve’ such as those with intrauterine growth restriction (IUGR), thick scanty meconium-stained fluid,¹¹⁹ in the presence of bleeding and in post-term infants, the rate of decline of pH is steep compared with term infants appropriately grown with abundant, clear amniotic fluid.