Interpretation of CTGs

Interpretation of CTG’s in birth claims 

The Head Injury UK team of experienced medical negligence lawyers includes members who specialise in claims involving birth injuries which have resulted in some kind of brain trauma.

We have many contacts in the medical professions who help us to unravel the truth of even the most complicated birth injury circumstances. This means we can make the strongest possible case for compensation and our rate of success is extremely high.

In investigating a possible claim, we will obtain all of your pre-natal and labour notes as well as the delivery notes of your baby. Within this will be important tracing information relating to your baby’s heart rate which tells the clinicians if your baby is distressed or not.

Cardiotocography (CTG)

Purpose

CTG’s are used during pregnancy to monitor both the foetal heart and the contractions of the uterus, usually only used in the 3rd trimester. Its purpose is to monitor foetal well-being and allow early detection of foetal distress. An abnormal CTG indicates the need for more invasive investigations and may lead to the need for emergency caesarian section.

Your Specialist Head Injury solicitor will obtain the CTG trace from the delivery of your baby and review it. The CTG will contain information about the condition of your baby by detailing the baseline rate, variability, accelerations, decelerations and the overall impression. The CTG will be read in the context of whether your pregnancy is defined as high or low risk.

If the pregnancy is high risk, the threshold for intervening may be lowered. High risk factors include maternal medical illness such as gestational diabetes, hypertension or asthma as well as obstetric complications such as multiple gestation, previous cesarean section, intrauterine growth restriction, premature rupture of membranes, congenital malformations, oxytocin induction/augmentation of labour and pre-eclampsia. Other risk factors include absence of prenatal care, smoking and drug abuse.

Reading the CTG

Commonly, the midwife will record the number of contractions present in a 10 minute period – e.g. 3 in 10.  Each big square on a CTG graph is equal to 1 minute, so the midwife will look at how many contractions occurred within 10 squares. Individual contractions are seen as peaks on the part of the CTG monitoring uterine activity. She will assess the contractions for duration and intensity. Most importantly, she will assess the baseline rate of the foetal heart. The baseline rate is the average heart rate of the baby within a 10 minute window. A “normal foetal heart rate” is between 110-150 bpm.

“Foetal tachycardia” is defined as a baseline heart rate greater than 160 bpm. It can be caused by foetal hypoxia, chorioamnionitis, hyperthyroidism, foetal or maternal anaemia or foetal tachyarrhythmia. “Foetal bradycardia” is defined as a baseline heart rate less than 120 bpm. Mild bradycardia of between 100-120 bpm is common in a postdate gestation or occiput posterior or transverse presentations. “Severe prolonged bradycardia” (< 80 bpm for >3 minutes) indicates severe hypoxia caused by prolonged cord compression, cord prolapse, epidural & spinal anaesthesia, maternal seizures and rapid foetal descent. If the cause cannot be identified and corrected, immediate delivery is recommended.

The midwife will also consider the “Baseline variability” which refers to the variation of foetal heart rate from one beat to the next. Variability occurs as a result of the interaction between the nervous system and cardiac responsiveness. Therefore it is a good indicator of how healthy the baby is at that particular moment in time. This is because a healthy baby will constantly be adapting its heart rate to respond to changes in its environment. Normal variability is between 10-25 bpm. To calculate variability the midwife will look at how much the peaks and troughs of the heart rate deviate from the baseline rate (in bpm).

Variability can be categorized as reassuring (>5 bpm), non-reassuring (<5bpm for between 40-90 minutes) and abnormal (<5bpm for >90 minutes). .Reduced variability can be caused by foetal sleeping (this should last no longer than 40 minutes – most common cause), foetal acidosis due to hypoxia (more likely if late decelerations are also present), foetal tachycardia, drugs (opiates / benzodiazepines / methyldopa / magnesium sulphate), prematurity (variability is reduced at earlier gestation (<28 weeks)) and congenital heart abnormalities.

Accelerations and Decelerations

Accelerations are an abrupt increase in baseline heart rate of >15 bpm for >15 seconds. The presence of accelerations is reassuring. Antenatally, there should be at least 2 accelerations every 15 minutes. Accelerations occurring alongside uterine contractions is a sign of a healthy baby. However the absence of accelerations with an otherwise normal CTG is of uncertain significance.

Decelerations are an abrupt decrease in baseline heart rate of >15 bpm for >15 seconds. There are a number of different types of decelerations, each with varying significance. “Early decelerations” start when uterine contraction begins and recover when uterine contraction stops. This is due to increased foetal intracranial pressure causing increased vagal tone. It therefore quickly resolves once the uterine contraction ends and intracranial pressure reduces. This type of deceleration is therefore considered to be physiological and not pathological. “Variable decelerations” are observed as a rapid fall in baseline rate with a variable recovery phase. They are variable in their duration and may not have any relationship to uterine contractions. They are most often seen during labour and in patients’ with reduced amniotic fluid volume. Variable decelerations are usually caused by umbilical cord compression. Their presence indicates the baby is not yet hypoxic and is adapting to the reduced blood flow. Variable decelerations can sometimes resolve if the mother changes position. The presence of persistent variable decelerations indicates the need for close monitoring. “Late decelerations” begin at the peak of uterine contraction and recover after the contraction ends. This type of deceleration indicates there is insufficient blood flow through the uterus and placenta. As a result blood flow to the baby is significantly reduced causing foetal hypoxia and acidosis. The presence of late decelerations is taken seriously and foetal blood sampling for pH is indicated. If foetal blood pH is acidotic it indicates significant foetal hypoxia and the need for emergency C-section.

A “reassuring” deceleration lasts more than 2 minutes. If it lasts between 2-3 minutes it is classed as “non-reassuring”. If it lasts longer than 3 minutes it is immediately classed as “abnormal” and action must be taken quickly – e.g. foetal blood sampling/emergency C-section.

Once the midwife has assessed all aspects of the CTG she will give her overall impression. The overall impression can be described as either reassuring, suspicious or pathological. The overall impression is determined by how many of the CTG features were either reassuring, non-reassuring or abnormal and the midwife will refer to the NICE guidelines which demonstrate how to decide which category a CTG falls into.

Metabolic Data

In addition, your specialist solicitor will obtain details of your baby’s blood cord gases. A baby depends on the mother for oxygen and carbon dioxide exchange during pregnancy. In the womb, a baby does not breathe in the same way humans do outside the womb (although “practice breathing” occurs from time to time). Rather, “breathing” (gas exchange) occurs in the intervillous space in the placenta. The placenta is an organ that connects the developing baby to the uterine wall to allow for nutrient uptake, waste elimination and gas exchange via the mother’s blood supply. The intervillous space is a space in the placenta where maternal blood travels. Oxygenated blood from the mother diffuses into capillaries in the placenta. The vein in the umbilical cord, called the umbilical vein, picks up this oxygenated blood from the capillaries, and carries it to the baby’s heart, which pumps the blood throughout the baby’s body. Once the baby uses this blood, it is carried away from the heart and back to the placenta by both umbilical arteries. In the intervillous space, carbon dioxide diffuses into the mother’s circulation so the mother can eliminate it by exhalation, and oxygen diffuses into the baby’s circulation.

To determine if a baby has suffered an oxygen-depriving event, called an anoxic or hypoxic-ishemic event, it is best to examine umbilical artery blood because this is the blood coming from the baby (as opposed blood going to the baby through the umbilical veins). If umbilical artery blood is acidemic (often called acidosis, which refers to acid in the tissues), it typically suggests that anaerobic metabolism occurred. Anaerobic metabolism occurs when oxygen is not available, and is therefore an indicator that an anoxic event occurred. Normal values in an umbilical arterial sample in a term newborn:

•PH: 7.18 – 7.38

•PCO2: 32 – 66 (mmHg)

•HCO3: 17 – 27 (mmol/L)

•PO2: 6 – 31 (mmHg)

•Base excess: -8 – 0 (mmol/L); (Base deficit: 0 – 8)

 

Normal umbilical arterial values in a preterm newborn are:

•PH: 7.14 – 7.4

•PCO2: 32 – 69 (mmHg)

•HCO3: 16 – 27 (mEq/L)

•Base excess: -7.6 – 1.3 (mEq/L)

 

Base excess is defined as the amount of acid required to titrate the blood gas sample to a normal pH (7.4) at a normal PCO2 (40) at normal body temperature. Most severely acidotic babies have a base deficit of 12 or greater.

The medical expert in your claim will, together with your specialist solicitor, examine the metabolic data available. If it is apparent that a hypoxic brain injury occurred, then an application for funding to the Legal Aid Authority will be made and, once granted, a formal medical report will be obtained dealing with breach of duty and causation.

If you experienced a traumatic birth leading to your baby suffering a brain injury at birth, contact our specialist team of solicitors at Head Injury UK and call 0345 111 5050.

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