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Umbilical Cord Blood Gas Analysis at Delivery
S F Loh, A Woodworth, G S H Yeo

In Singapore, most obstetric units use Apgar score as the indicator of neonatal condition at birth. However, Apgar score is not a good indicator of perinatal asphyxia(1-3), and can be influenced by other factors eg. prematurity, maternal sedation with opioids, aspiration of mucus or meconium etc. In contrast, umbilical cord blood gas values are more objective in the assessment of fetal oxygenation at delivery(1, 4-7).

Measurement of acid base status of umbilical cord blood after delivery is recommended by the 1994 American College of Obstetrics and Gynaecology Committee Opinion on Obstetric Practice and the 1993 Royal College of Obstetrics and Gynaecology Study Group on Intrapartum Fetal Surveillance(8).

Before we start to use umbilical cord blood gas values routinely as an indicator of antepartum and intrapartum care, establishment of normal range of blood gas values in our labour ward is important, rather than relying on results from other centres which may reflect different labour ward practices. Moreover, the reported normal range of umbilical artery pH value varies greatly(1, 9-13). This study defines the normal range of blood gas values in our labour ward.

MATERIALS AND METHODS

During the periods from 21 March to 11 April 1994 and 21 June to 8 July 1994, 205 umbilical cord blood gas results were evaluated. Of these, 153 newborns were delivered vaginally and 52 newborns were delivered by Caesarean section.

AVL 995 analyser measures pH, pCO2, pO2 and calculates HCO3-, Base deficit (blood) and Base deficit in the extracellular fluid compartment (BD ecf) from the measured parameters.

The arterial and venous blood gas values were examined immediately after analysis. Those results with arterial pH and pO2 higher than venous pH and pO2 or arterial pCO2 lower than venous pCO2 were excluded from analysis.

Both umbilical artery and vein were sampled and the blood gas parameters recorded. All those cases with abnormal umbilical artery pH less than 7.05 (mean - 2SD in our series) were retrospectively analysed with regards to their antenatal events, intrapartum management and neonatal outcome.

RESULTS

Of the 153 vaginal deliveries, delivery was spontaneous for 141 patients but required assistance in 12 patients (forceps 9 cases, vacuum 3 cases). Of the 52 Caesarean sections, 25 were performed electively and 27 were done as emergency for indications other than fetal distress.

The period of gestation of the newborns ranged from 34 - 42 weeks with a mean of 37.2 weeks. Their birth-weights ranged from 1625 to 4215g with a mean of 3112g. One minute Apgar scores ranged from 5 to 9 with a mean of 8.89. Five-minute Apgar scores were all 9.

The means and standard deviations for umbilical arterial pH, pCO2, bicarbonate and base deficit in the extracellular fluid compartment (BDecf) for both vaginal deliveries and Caesarean sections are tabled in Table I.

The means and standard deviations for umbilical vein pH, pCO2, bicarbonate and BDecf for vaginal deliveries and Caesarean sections are given in Table II.

Case 1

A 35-year-old gravida 4 para 2 presented at 37 weeks amenorrhoea with abruptio placenta. Cardiotocogram showed bradycardia with poor baseline variability. Emergency LSCS was done. Retroplacental clot was formed covering 50% of placental surface. A baby girl weighing 2670g with Apgar scores of 9 at 1 and 5 minutes was delivered. The baby did well though afflicted with some transient tachypnoea of newborn and neonatal jaundice and was discharged on day 12. At 4-month follow-up, her developmental milestones were normal.

Case 2

A 33-year-old gravida 1 para 0 presented at 42 weeks with prolonged rupture of membrane for 3 days. Cardiotocogram showed tachycardia with poor baseline variability. Emergency LSCS was done. Liquor was scanty and foul smelling. A baby boy weighing 2740g with Apgar scores of 1 and 9 at 1 and 5 minutes respectively was delivered. The boy was resuscitated with bag and mask and had mild respiratory distress. He was discharged on day 11 and had normal milestones at 4-month follow-up.

Case 3

A 20-year-old primigravida was admitted in labour at 37 weeks. A cardiotocogram showed baseline tachycardia, poor baseline variability and late decelerations. An emergency LSCS was performed. Thick meconium stained liquor was seen. The baby boy weighed 2.9 kg and his Apgar score was 8 at 1 minute and 9 at 5 minutes. He did not develop meconium aspiration and was discharged on day 5 of life.

Case 4

A 30-year-old gravida 4 para 3 presented at 34 weeks with continuous abdominal pain and antepartum haemorrhage. Doptone was negative but bedside ultrasound examination showed feeble heart beat at about 40 beats/minute. An emergency LSCS was performed. Liquor was heavily blood stained. About 600 mLs of clots was found retroplacentally. A baby boy weighing 1500g was delivered. His Apgar scores were O at 1 minute and 2 at 5 minutes. The baby was intubated. Neonatal problems included hypotension requiring dopamine support, stage 2 hypoxic ischaemic encephalopathy, neonatal seizure needing phenobarbitone for 1 week and grade 4 Hyaline membrane disease. The baby was discharged on day 38 of birth. At 4 months, he had no seizures, was able to raise his head from prone position and was "babbling and cooing" at the time of consultation.

Case 5

A 26-year-old lady gravida 2 para 1 with 1 previous LSCS, was admitted in labour at 37 weeks. Epidural anaesthesia was administered and labour was augmented with syntocinon. At second stage of labour, CTG showed prolonged bradycardia. Vacuum assisted delivery was performed. Subsequently, a diagnosis of uterine rupture was made. The baby’s Apgar score was O at 1 minute and 2 at 5 minutes. The baby was intubated but developed hypoxic ischaemic encephalopathy with spastic diplegia and opisthotonic posturing, Swiss cheese periventricular leukomalacia, and was ventilator-dependent. The baby was extubated and died at 99 days of life.

DISCUSSION

Umbilical cord blood gas values reflect the last moment of fetal oxygenation and acid base balance prior to delivery. Severe fetal acidemia is associated with increased perinatal mortality and increased risk of subsequent impaired neurological development.

It is obvious from Table IV that there is no consensus in the literature on what constitutes acidosis. If we take mean - 2 standard deviation as abnormal, the reported lower limit of normal uAph ranges from 7.04 - 7.18.

In our study, statistically lower limit of uAph (mean - 2SD) is 7.05 for vaginal deliveries. This is in agreement with larger series of Sykes et al and Westgate et al(1, 7). In most studies trying to correlate uAph with neonatal outcome, the level of pH chosen to indicate acidosis was too high. This may explain the lack of strong correlation between uAph and outcome(15).

We found that the isolation of a segment of umbilical cord immediately after delivery is logistically easier for labour ward staff. Before our study, we performed consecutive measurements on clamped segments of umbilical cord at 0, 15, 30, 45 and 60 minutes of birth and found that clamped segments of umbilical cord are stable for blood gas analysis of up to 30 - 45 minutes of birth. These results concurred with Duerbeck’s(16) who found that a clamped segment of cord is stable for blood gas assessment for at least 60 minutes after delivery.

What are the parameters to look at in an umbilical cord blood gas results other than umbilical artery pH? We think umbilical artery BDecf and arterial venous difference in BDecf are two important parameters. Base deficit in the extracellular fluid compartment is less affected by elevated CO2 level in the newborn compared to the blood compartment. As the newborn is usually hypercapnoeic at birth, BDecf is a better parameter to quantitate metabolic acidosis than BD blood(17).

Umbilical arterial and venous difference in BDecf is markedly higher in cases where cord entanglement is the etiology of acidosis (eg. cases 2 and 3). Other studies also had the same observation(17, 18).

In conclusion, we are sure that umbilical cord blood gas analysis is useful to ascertain whether a particular case of fetal compromise is due to "perinatal asphyxia". Selectively paired umbilical cord blood gas analysis, when properly done and correctly interpreted offers insight into metabolic events occurring in the perinatal period and enables the obstetricians to learn from individual patient. It also provides the neonatologists with a baseline of the neonate’s metabolic condition. A good blood gas in a depressed newborn should alert the neonatologist to search more diligently for other causes of neonatal depression eg. sepsis, trauma or congenital abnormalities. It also provides an objective measure for the evaluation of antepartum and intrapartum care.

Correspondence to:
Dr S F Loh
Department of Reproductive Medicine
KK Women’s and Children’s Hospital