Cord Blood Gas

<Investigations>

<Index>

 

Definition

-  Sampling of placental cord blood post delivery to ascertain foetal metabolic condition at time of birth

 

Features

-  Assists in distinguishing between intrapartum & delivery causes of neonatal outcomes

-  Helps exclude birth asphyxia as a cause of neonatal depression

-  Medicolegal significance

-  Normal values exclude perinatal asphyxia or hypoxia from birth process

-  Arterial pH most important value

-  Arterial acidaemia may occur w normal venous pH

-  Paired samples prevent confusion as to true sample

 

Indication

-  Consider in all deliveries

-  Intrapartum event which might impact neonatal outcome

-  APH

-  Fever

-  +/- Abnormal CTG

-  Expediated birth: instrumental delivery, emergency LUSCS

-  APGARs <7 at 5 minutes

-  IUGR

-  Mutiple gestations: twins..

-  Shoulder dystocia

-  Vaginal breech

-  Premature delivery

-  Maternal thyroid disease

 

Equipment

-  21G needle x2

-  Arterial blood gas syringe (Heparinised) x2

-  Allows specimen to remain stable for up to 60min

 

Procedure

-  Double cord clamp 10-20cm

-  Should be done immediately or <30 sec

-  Delayed clamping significantly effects measured values

-  Take as soon as possible after delivery

-  Can be delayed up to 1hr wo significant changes in pH, PCO2 or PO2

-  Can be left at room temperature

-  Identify artery & vein

-  x2 arteries

-  Typically arteries wind around central larger vein

-  If difficulty in taking sample use artery on fetal surface of placenta (arteries cross over veins)

-  Similar but not identical results

-  Clean segment of cord

-  Insert needle & withdraw >0.2ml blood from artery then label sample

-  Take care as easy to do needle stick injury

-  Insert needle & withdraw >0.2ml of blood from vein then label sample

-  Note often x2 venous samples OR x2 arterial samples

-  Remove needle from syringes & expel air

-  Cap each syringe

-  Invert & mix specimens

 

Results

-  Must DDx same vessel samples

-  Median arteriovenous pH difference 0.09

-  Need at least 0.03 difference in pH to reassure different samples

-  pH & BE most useful values

-  Arterial pH

-  Preterm: 7.25 – 7.29

-  Term: 7.27 – 7.28

-  Normal value excludes birth asphyxia

-  Pathological fetal acidaemia <7.00

-   <6.90 highest risk of neonatal morbidity

-   Hypoxic-ischaemic encephalopathy

-   CPR

-   Seizure

-   Intubation

-  Arterial BE

-  Preterm: -2.5 to -3.3 

-  Term: -2.7 to -3.6

-  Metabolic acidosis >-12 mmol/L

-  Neonatal complications related to severity & duration of metabolic acidosis

 

 

 

 

 

 

References

 

NSW Health – Eastern Sydney:  Cord Blood Gas Collection, 01/06/2009

Pic (vessles): library.med.utah.edu/nmw/mod2/Tutorial2/vessels.html

Pic (diagram): McKinney, 2000
Upto Date: Umbilical Cord Blood Acid-Base Analysis, Yeomans et al, 24/03/2011