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Cord Blood Gases

The umbilical cord contains three blood vessels: one large vein carrying oxygenated blood from the mother to the fetus and two much smaller arteries carrying deoxygenated blood that is relatively rich in carbon dioxide and other metabolic waste products from the fetus to the mother. A fetus depends on the mother for gas exchange, nutrients and waste product removal. Oxygen and nutrients diffuse across the placental membrane from maternal arterial blood and is transported to the fetus by an umbilical vein. Following extraction of oxygen and nutrients by fetal tissues, fetal blood returns to the placenta in two small umbilical arteries. Fetal waste products, such as carbon dioxide, are eliminated by maternal lungs and kidneys.

The normal physiological difference between venous and arterial cord blood gas and acid-base values is summarized in the following table.

BG Parameter Umbilical Artery Umbilical Vein
pH 7.12 - 7.35 7.23 - 7.44
pO2 6.2 - 27.6 16.4 - 40.0
pCO2 41.9 - 73.5 28.8 - 53.3
Bicarbonate 18.8 - 28.2 17.2 - 25.6
Base Deficit +9.3 to -1.5 +8.3 to -2.6

 

Umbilical venous blood gas values more closely resemble those of adult arterial blood than do those of umbilical arterial blood because it carries oxygenated blood. Venous cord blood reflects the combined effect of maternal acid-base status and placental function, while arterial cord blood reflects neonatal acid-base status. Acid base parameters, including pH and base deficit (BD) should be measured in umbilical cord arterial blood to assess neonatal hypoxia.

PH and BD of umbilical cord arterial blood are measured immediately after birth to detect hypoxia and metabolic acidosis. Cord blood gas analysis has been shown to be more reliable than the Apgar scoring system. Metabolic acidosis occurs in hypoxic individuals because of the accumulation of lactic acid that occurs during anaerobic metabolism of glucose. High levels of lactic acid overwhelm the normal buffering capacity of blood resulting in decreased blood pH and increased BD.

The presence of cord blood metabolic acidosis indicates a higher risk of hypoxic brain injury and hypoxic ischemic encephalopathy (HIE). Symptoms of HIE include hypotonia, poor feeding, respiratory difficulties, seizures and reduced level of consciousness. Newborns with mild HIE usually survive with little or no long-term consequences. Newborns with moderate to severe HIE either die during the neonatal period or survive with severe and permanent neuropsychological deficit. Cerebral palsy is an outcome for some.

The reference range for arterial cord blood pH is 7.12-7.35, and for arterial cord BD it is +9.3 to –1.5 mmol/L. In obstetrics, significant metabolic acidosis is often defined as cord arterial blood pH <7.0 and BD >12.0 mmol/L. Some institutions have adopted a higher pH threshold of <7.1. Using these criteria, significant metabolic acidosis occurs in approximately 0.5 to 1% of deliveries. Since the incidence of HIE is much lower (0.15%), it can be concluded that most babies with significant metabolic acidosis do not suffer permanent neurological damage. Significant cord metabolic acidosis is necessary, but not sufficient to confirm that an acute intrapartum hypoxic event was the cause of HIE.

Currently, the only effective treatment for HIE is controlled cooling of the baby to a rectal temperature of 34 ± 0.5 °C for 48 to 72 hours. Efficacy depends on initiating this hypothermic treatment within 6 hours of birth. Significant metabolic acidosis is one parameter for initiation of hypothermic therapy.

Sampling of Cord Blood

The standard technique of obtaining umbilical cord blood for blood gas analysis includes:

  • clamping a segment of the cord
  • removing the clamped cord segment
  • needle aspiration of two blood samples (one venous, one arterial)
  • Transfer of cord blood into two preheparinized syringes

Timing of isolating a sample for analysis is crucial for accurate analysis of cord blood gas results. Immediately after birth, ideally before the baby’s first breath, an approximate 20-cm segment of cord should be isolated between two sets of two clamps. Delay in clamping by as little as 45 seconds after birth results in significant change in acid-base parameters including a progressive decrease in pH and increase in base deficit, pCO2 and lactate.

Once isolated from the maternal/neonatal circulation, acid-base parameters of clamped cord blood are stable at room temperature for 60 minutes. Blood gas values of cord blood stored in a capped heparinized syringe remain sufficiently stable for an hour at room temperature. The Clinical and Laboratory Standards Institute (CLSI) recommends that arterial blood specimens should be analyzed within 30 minutes of sampling.

Arterial blood should be sampled for analysis. Unfortunately the close juxtaposition of the umbilical artery and veins and the larger size of the veins predisposes to cord blood sampling errors. Given these difficulties, guidelines recommend submitting samples from both artery and vein, so that arterial blood results can be validated as truly arterial.

The validation of paired arterial and venous samples is based on minimum arterio-venous (A-V) differences for pH and pCO2. For pH, the A-V difference should be >0.02 pH units, and for pCO2 the A-V difference should be >3.75 mmHg. So long as these minimum differences in pH and pCO2 between the two samples are evident, it can be assumed that the two samples came from different vessels, and that the one with lowest pH and highest pCO2 came from an artery. If the two samples have smaller differences in pH and pCO, then they probably came from the same blood vessel and the results cannot be assumed to represent arterial blood.

Some institutions perform cord blood gas analysis for all births, while others practice a selective approach. The American College of Obstetricians and Gynecologists (ACOG) favors a selective approach of performing cord blood testing for the following situations:

  • Cesarean delivery for fetal compromise
  • Low 5-minute Apgar score
  • Severe intrauterine growth restriction
  • Abnormal fetal heart rate tracing
  • Maternal thyroid disease
  • Intrapartum fever
  • Multifetal gestations

 

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