Fetal scalp blood sampling during labor: an appraisal of the physiological basis and scientific evidence
- Edwin Chandraharan has no commercial interests or links with any non-NHS organizations. Nana Wiberg has no financial affiliation or involvement with any commercial organization with potential financial interest in the subject or materials discussed in this article.
Fetal cardiotocography is characterized by low specificity; therefore, in an attempt to ensure fetal well-being, fetal scalp blood sampling has been recommended by most obstetric societies in the case of a non-reassuring cardiotocography. The scientific agreement on the evidence for using fetal scalp blood sampling to decrease the rate of operative delivery for fetal distress is ambiguous. Based on the same studies, a Cochrane review states that fetal scalp blood sampling increases the rate of instrumental delivery while decreasing neonatal acidosis, whereas the National Institute of Health and Clinical Excellence guideline considers that fetal scalp blood sampling decreases instrumental delivery without differences in other outcome variables. The fetal scalp is supplied by vessels outside the skull below the level of the cranial vault, which is likely to be compressed during contractions. The self-regulated redistribution of oxygenated blood from peripheral to central organs causes peripheral ischemia, thus theoretically bringing into question the scalp capillary bed as representative of the central circulation.
fetal scalp blood sampling
The aim of intrapartum monitoring is to improve perinatal outcome while avoiding unnecessary operative interventions. Cardiotocography (CTG) was introduced into obstetric practice in the late 1960s to recognize and respond early to intrapartum fetal hypoxia in order to reduce the incidence of cerebral palsy and perinatal deaths. CTG is characterized by high inter-/intra-observer variation, high sensitivity and low specificity for fetal acidosis and perinatal outcome [1-4]. A normal CTG is therefore reassuring for a well-oxygenated fetus; however, few fetuses with non-reassuring CTG patterns are at risk of severe hypoxia . Fetal monitoring with only CTG results in variable but inappropriately high operative delivery rates and many methods have therefore been proposed as an accessory assessment for CTG , the best known and universally most accepted of which is fetal scalp blood sampling (FBS). Complications due to FBS are rare but potentially serious, such as drainage of cerebrospinal fluid, severe fetal bleeding from the puncture site, and scalp abscess [6, 7].
History and evidence
Fetal blood sampling was introduced in Germany in the 1960s as an additional test to the older intermittent auscultation of the fetal heart rate, prior to the availability of CTG machines [8, 9]. On the basis of 77 unselected cases, the pH cut-off values for non-acidosis, pre-acidosis and acidosis were determined, but to establish a reliable reference interval at least 120 selected individuals who met predefined criteria would have been required . The cut-off limits were re-tested in follow-up studies during the ensuing years, but with a remarkably constant low number of cases. The final conclusions and recommendations from the German study group were “if fetal scalp pH is pre-pathological or even pathological (<7.19), fetal blood sampling should be repeated again and if there is a further fall in pH, then delivery should be institute” [9, 11]. By the time electronic CTG was introduced, FBS naturally followed as an “additional test” with the aim to reduce the inappropriately high rates of operative delivery as a result of non-reassuring CTG patterns. To our knowledge only two small randomized controlled studies have been published on the subject, with conflicting results [12, 13]. At least two meta-analyses based on the same original studies have been published, also with conflicting results. The National Institute of Health and Clinical Excellence (NICE) in the UK states that the use of FBS with CTG will reduce the rate of instrumental vaginal deliveries without evidence for a difference in other outcomes , whereas in the Cochrane systematic review from 2013, the use of FBS as an adjunct to continuous CTG appeared to increase the rate of instrumental deliveries while decreasing the rate of neonatal acidosis . The German cut-offs and recommendations are still accepted and have been adopted by most of the obstetric societies worldwide, despite the lack of consistent agreement. It is noteworthy that the American College of Obstetricians and Gynecologists (ACOG) has recommended exclusion of the use of FBS, which is reflected by the low clinical use of under 1% compared with up to 15% in Sweden and the Netherlands [14-19]. In most STAN® protocols (ST analysis of fetal electrocardiogram, Neoventa, Gothenburg, Sweden) FBS is recommended as a supplementary tool, either before or during recording to ensure fetal wellbeing. There is a correlation between the lag-time between a significant ST-event and the fetal scalp blood pH, although only 10% of those cases fulfilling the STAN-criteria for FBS revealed acidosis in the scalp blood [18, 20]. Unfortunately, there are no long-term follow-up studies for fetuses where pH or lactate measurements were part of monitoring during labor.
In response to hypoxic stress, a fetus will release catecholamines (noradrenaline and adrenaline), which results in intense peripheral vasoconstriction with diversion of oxygen-saturated blood from peripheral tissues (fingertips, toes, scalp) to the most important central organs (heart, brain and adrenal glands) in order to preserve the function of these organs under hypoxic conditions. Moreover, the vascular supply to the scalp comes from superficial vessels running outside the skull bones below the level of the cranial vault from the temporal to the parietal area. Theoretically, due to the redistribution and the external compression of the vessels during uterine contractions, the scalp circulation can be disturbed and partly or even completely occluded, as has been seen from a low pO2 in the scalp blood in the presence of an otherwise well oxygenated fetus [21, 22]. In adults, a suspicion of hypoxia and acidosis warrants an arterial blood gas analysis to determine pH, base excess and lactate. Although a venous gas sample would be technically easier to obtain, this is avoided as it does not reflect oxygenation of the central organs. The only study that attempted to correlate the scalp pH with pH in the jugular vein and carotid artery was small and done in monkeys (11 animals). A perfect correlation was demonstrated but the study was not physiologically justifiable, as it was performed with anesthetized pregnant monkeys in the absence of uterine contractions or hypoxic stress . There is a correlation between scalp blood pH/lactate and umbilical cord blood pH/lactate, but the correlation is weak, supporting our opinion that the use of FBS in the management of labor requires knowledge of the anatomy and the physiological impact of uterine contractions [24, 26]. Other peripheral tests of fetal the scalp oxygenation, such as continuous fetal pulse oximetry and near-infrared spectroscopy, have not been shown to improve neonatal outcomes [22, 27].
pH, lactate or both?
Fetal scalp blood can be analyzed for pH or lactate, but both have low positive predictive values, low sensitivity and specificity (between 46 and 72%) for adverse neonatal outcome variables such as low Apgar scores, low cord artery pH and neonatal encephalopathy [28-31]. Measurement of lactate, the end product of anerobic metabolism, is suggested to be the best alternative, since estimation of scalp blood lactate is more likely to be successful (reduces the sample/analyzing failure rate from 11 to 20% to 1.2%) compared with pH, and lactate seems to be an earlier marker of severe hypoxia [30, 32, 33]. Compared with pH, lactate has higher sensitivity and specificity, and greater relevance in terms of the adverse neonatal outcome variables and umbilical cord blood acid-base values [28, 31, 34]. Analyzing pH values of paired scalp blood samples obtained from one fetus at the same time showed a difference greater than the acceptable analytical difference in 43% of cases . Combining the two tests often shows discrepancies, with the lactate values more often being abnormal. Only if both pH and lactate are abnormal in the same fetal blood sample, is there a significant relationship to acidemia in umbilical cord blood .
Fetal scalp blood sampling has been shown to be of questionable value in the presence of scalp edema (caput succedaneum). Saling stated that the presence of scalp edema would exert a negligible effect on the FBS . Contradicting this hypothesis, Odendaal suggested that scalp edema gives rise to more acidotic pH values when compared with a normal area of the fetal scalp , whereas N. Wiberg and K. Kallen (unpublished observations) showed that in the edematous area, falsely low lactate values will be measured. Contact of fetal scalp blood with amniotic fluid has been shown to alter the FBS values in different directions; in the case of non-meconium-stained amniotic fluid, which is alkaline, fetal acidemia can be masked ; conversely, during labor dystocia and in the presence of meconium, the amniotic fluid often contains high levels of lactic or bile acid that can be erroneously interpreted as fetal acidemia [40, 41]. In addition, if a fetal scalp sample is taken during a uterine contraction, more acidotic values may be observed as opposed to during uterine relaxation . Caution must also be exercised when interpreting lactate values from different point-of-care devices, since the values differ significantly .
Fetal scalp blood sampling is a test of historical importance as it was developed as a supplementary tool to intermittent auscultation with the Pinard stethoscope. It was subsequently used as an additional test of fetal wellbeing to reduce the false positive rate of CTG. However, to justify FBS in modern obstetric practice the obstetrician must have the anatomical and the pathophysiological aspects, current evidence and the described pitfalls in mind. The NICE Guideline Development Group and the latest Cochrane systematic review [5, 14] claim that there little evidence of improved outcomes and decreased rates of instrumental delivery using FBS in the case of non-reassuring CTGs and, in the USA the test has been excluded from clinical practice . The recommendation to use FBS, despite the lack of good scientific evidence, highlights one of the main difficulties in fetal monitoring. The lack of an optimal test, which so far has not been found, leaves few other opportunities for obstetricians when they endeavor to explain why we continue to advocate FBS.