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Perinatal Hypoxia : the causes and consequences

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    In this fourth blog in our series relating to injuries suffered around the time of birth, we will be considering the importance of appropriate monitoring during pregnancy and labour to avoid brain injury caused by oxygen starvation.

    By Simon Elliman

The term perinatal hypoxia refers to a baby being deprived of oxygen in later pregnancy, around the time of their birth or in their early life.  This can happen at any time during pregnancy, and little can be done to prevent it prior to labour. However, during labour, the risk of a baby being deprived of oxygen rises because of the trauma of the process, so it is good practice to monitor the baby to see if there any signs of oxygen deprivation.

Causes of hypoxia

Probably the most common understood causes are:

  • Maternal smoking;
  • Prolapse of the umbilical cord;
  • Placental insufficiency.
  • Meconium aspiration.

Outcomes of hypoxia

The brain requires significant levels of oxygenated blood to reach it, or it starts to suffer damage. At first, if there is a lack of oxygen during labour, the foetus responds by redirecting blood to the brain, and increasing cerebral blood flow.

It is generally thought that the foetal brain remains undamaged by a partial deprivation of oxygen for up to one hour for this reason.  After that period, the brain begins to suffer damage. The area damaged is usually what is known as the “watershed” zones of the brain, within the borders between major cerebral arteries, where perfusion pressure is least.

Sometimes during labour there may be a period of acute profound hypoxia, where the brain is entirely, or almost entirely, deprived of oxygen for a short period. Again, the foetal brain can withstand a short period of total oxygen starvation, but damage is generally thought to be inevitable after ten minutes, with damage then occurring to the basal ganglia and the thalami areas of the brain.

Monitoring foetal distress during labour

There are two ways of monitoring the foetal heartbeat. In a “low risk” pregnancy, it is regarded as being satisfactory to monitor by “intermittent auscultation” – listening to the heartbeat with a sonicaid at regular intervals.  However, if there are recognised risk factors, such as raised blood pressure or temperature in the mother; premature rupture of the membranes (waters breaking early); significant meconium etc., it is necessary and mandatory to monitor by way of continuous CTG (electronic monitoring).

It is well established that CTG monitoring is much more reliable in picking up abnormalities in the foetal heart rate, which will indicate foetal distress due to oxygen deprivation, but in the majority of labours it is still regarded as acceptable to monitor the heart rate only intermittently.

Intervention

When foetal distress is detected by abnormalities in the foetal heart rate, it is necessary to intervene to deliver the baby, ideally before any brain damage occurs. Features of a CTG trace are categorised either as “normal/reassuring”, “non-reassuring” or “abnormal”; depending on how many features fall into these categories the trace itself is then graded as “normal”, “ suspicious”, or “pathological”.

Where a trace is suspicious or pathological the midwife should call an obstetrician urgently. Depending on the level of concern and stage of labour, the obstetrician may then try to get a foetal blood sample (FBS). This provides a pH reading, demonstrating whether there is acidosis, which is diagnostic of oxygen deprivation. If the FBS is not reassuring, an urgent caesarean section is likely to be indicated. If the foetus is too far advanced towards delivery for a caesarean section to be practical, an assisted delivery (forceps or ventouse) will be performed.

Categorising Caesarean Sections

Caesarean sections are categorised 1-4.  A Category 1 section means that there is immediate threat to the life of the woman or foetus. While the Royal College of Obstetricians and Gynaecologists (RCOG) guidelines encourage “an individualised approach to assessment of urgency of delivery”, there remains an underlying assumption that delivery within 30 minutes of the decision to perform a category 1 caesarean represents a minimum level of good practice, and many NHS Trusts will have such a target in their own guidelines. The reality is that in some obstetric emergencies (such as cord prolapse) delivery within 15 minutes may be feasible, and given that cord prolapse usually means a total or near-total deprivation of oxygen even should that be achieved the baby may still be damaged.

Measuring hypoxia after birth

After birth, bloods will usually be taken from the umbilical cord to measure the level of acidosis of the foetus, which is a strong indicator of the level of oxygen starvation suffered by the foetus during delivery and the timing of this.

A case where foetal heart monitoring was unsatisfactory

In the majority of cases the methods of foetal monitoring are a good way of ensuring the health of the baby or ensuring any concerns are identified early so that action can be taken to deliver the baby before they suffer any harm.  In some cases unfortunately the monitoring is not carried out as it should be, or not acted upon appropriately, and this can lead to babies suffering brain injury due to oxygen deprivation, and may lead to a claim for damages.

Interpreting all of the above factors, including foetal heart rate monitoring, scans of the brain to show the location of any areas of damages, and “blood gases” are often very important to lawyers when advising on pursuing a claim for damages for a brain injured baby, where the injury is thought to have occurred around the time of their birth.

I acted for a child recently, who has severe cerebral palsy as a result of oxygen starvation during labour. For the majority of the labour “intermittent auscultation” was applied, with the foetal heart being listened to periodically. However, this did not take place sufficiently regularly (every 5 minutes was the requirement), and insufficient attention was paid to a significant drop in the heart rate. When a CTG trace was belatedly started, it was misinterpreted as being normal when it was clearly abnormal, and also in part was picking up the mother’s heart rate as well as the foetal heart, further confusing matters. My client was eventually delivered vaginally, by which time he had suffered catastrophic brain damage, which would likely have been avoided had the monitoring been carried out and interpreted competently. My client was awarded agreed damages of £2.6million plus annual payments for the costs of his care, guaranteed every year for his lifetime.

Our next blog in this series will look at Group B Streptococcus infection, and how this infection can cause serious injury to newborn babies if passed on by their mother during labour and not treated appropriately.

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