Fetal scalp blood sampling

Sampling capillary blood from the fetal scalp during labour of pregnancy, in order to determine its pH, was introduced to obstetric care in the late 1960s. Despite current scepticism surrounding its utility, most notably in the US, fetal-scalp blood sampling (FSBS) is still considered a useful fetal monitoring test by expert bodies, and continues to be used in obstetric units around the world. The intended purpose of this review article is to detail the clinical value of FSBS and explain just why, and under what circumstances, knowledge of the unborn baby’s blood pH is useful for management of labour of pregnancy. The technique and associated risks of sampling blood from the fetal scalp will be discussed. The article will also include consideration of the controversy surrounding the utility of the test. Finally, there will be brief discussion of the notion that it might be preferable to measure lactate concentration of the fetal-scalp blood sample rather than its pH. The article begins with consideration of the rationale for monitoring fetal heart rate during labour. This is a good starting point for discussion of FSBS, because FSBS is only performed when and if fetal heart rate monitoring gives cause for concern about the condition of the unborn baby. Page 1 Article downloaded from acutecaretesting.org Chris Higgins: Fetal scalp blood sampling Fetal heart monitoring detects possible fetal hypoxia Fetal well-being depends on an adequate supply of oxygen derived from maternal circulation and delivered to the fetus via the placenta and umbilical-cord vein [1]. Uterine contractions decrease placental blood flow and thereby oxygen delivery, so during labour the fetus is physiologically predisposed to reduced oxygen (hypoxia). Whilst healthy fetuses normally have the metabolic reserve to tolerate the potential transient hypoxia normally associated with labour, there are a number of conditions related to either the mother, placenta or fetus that can have the effect of further reducing oxygen delivery to fetal organs, causing prolonged or severe intrapartum hypoxia; these conditions are listed in Table I. If sufficiently severe and prolonged, intrapartum hypoxia can result in asphyxia (i.e. hypoxia in association with metabolic acidosis) and consequent possibility of the hypoxia-mediated brain damage known technically as hypoxic-ischemic encephalopathy (HIE) [2]. The damage may be permanent; cerebral palsy is the outcome for some. If brain damage is particularly severe and widespread, HIE can be fatal; it is one of the more common causes of perinatal death. The principal objective of fetal heart rate monitoring during labour is to detect hypoxia and impending intrapartum asphyxia so that treatment to improve fetal oxygenation can be implemented, or if necessary, urgent operative birth (cesarean section or instrumental vaginal birth) performed, before hypoxia-mediated damage occurs. Page 2 Article downloaded from acutecaretesting.org Chris Higgins: Fetal scalp blood sampling Maternal factors Utero-placental factors Fetal factors Maternal hypoxemia due to: • respiratory disease • hypoventilation • seizure, trauma • smoking Excessive uterine activity: • hyperstimulation by drugs • prolonged spontaneous labor • placental abruption Umbilical cord compression: • oligohydramnios • cord prolapse or entanglement Maternal reduced oxygen-carrying capability due to: • anemia • carboxy-hemoglobinemia Utero-placental dysfunction: • placental abruption • placental infarction/dysfunction marked by intrauterine growth restriction, oligohydramnios or abnormal Doppler studies • chorioamnionitis (infection) Decreased fetal oxygen-carrying capability: • significant anemia due to isoimmunization, maternal fetal bleed or vasa previa • carboxyhemoglobinemia (if mother is a smoker) Decreased uterine blood flow due to: • hypotension (e.g. shock, sepsis) • regional anesthesia • maternal positioning Chronic maternal conditions: • diabetes • chronic hypertension • SLE • antiphospholipid syndrome TABLE I: Conditions that can affect fetal oxygenation [2]