Hypoxic Ischemic Encephalopathy (HIE)
Oxygen Deprivation During Labour and Delivery
The medical term is hypoxic ischemic encephalopathy (HIE). This simply means brain sickness from inadequate oxygenation to the brain. The fetus receives his or her necessary oxygen and nutrients from the mother through the umbilical cord. If this mechanism of delivery of oxygen and nutrients is compromised the fetus may suffer an irreversible neurological injury.
Causes of HIE
Contractions during labour result in a reduction of blood flow through the umbilical cord and thus a reduction in the delivery of oxygen and nutrients to the fetus. When the contraction ends, the blood supply through the umbilical cord usually returns to normal. In the vast majority of situations, the fetus can tolerate the momentary reduction in blood supply during a contraction and recovers during the resting phase in between contractions. In some cases, however, the repeated reduction in the supply of oxygen and nutrients during contractions will exceed the reserves of the fetus and, if this pattern continues, can result in neurological injury. Fetuses that are growth restricted, or are preterm, have lower reserves and are therefore less able to tolerate the reduction in blood supply with contractions. If labour is induced or augmented with oxytocin this can cause the contractions to be more frequent and longer, reducing the time between contractions and limiting the fetus’s ability to recover. There can be a transfer of blood between twins (referred to as twin-to- twin transfusion) that reduces the ability of the twins, or one of them, to tolerate the stress of labour.
Can HIE be Avoided?
One of the responses of the fetus to the decrease of blood flow with contractions is a change in the fetal heart rate and its pattern. Thus, the theory behind electronic fetal monitoring is to detect any changes in the fetal heart rate or pattern that suggests that the fetus is not tolerating labour and to allow for intervention prior to neurological injury occurring. It is the responsibility of both nurses and doctors to respond to any evidence of impaired fetal oxygenation suggested on the fetal heart tracing. By doing so, injury to the baby can, in some circumstances, be avoided or mitigated.
There are also situations where the fetal blood supply is suddenly and significantly decreased. This can occur with uterine rupture during an attempt at a vaginal birth after cesarean, cord prolapse, placental abruption and shoulder dystocia. This profound decrease in the blood flow will result in a profound decrease in the fetal heart rate which is a sign that the baby will need to be delivered immediately.