Wednesday, March 10, 2010

Cord blood gases: Here's all you need to know

Every respiratory therapist dreads having to draw cord blood gases, and all OB nurses dread the circumstances that require them to be drawn. So, that said, what are the indications for drawing cord blood gases, what is the significance of drawing them, and why do we draw them in the first place?

Basically, the reason we draw cord blood gases is in case their is a lawsuit that might take place years down the road accusing the delivering doctor of causing an anoxic brain injury that resulted in diseases such as cerebral palsy.

The cord blood can prove that neurological deficits that develop in infants were caused by an anoxic brain injury that occurred after delivery or before delivery and was not the result of an anoxic episode at birth. The cord blood gas has been shown to be proof positive in about 80% of the cases (According to PubMed.com), and has in many cases cleared physicians from litigation.

A cord blood gas does not need to be drawn unless a baby is born has a low APGAR score within 5 minutes of delivery, such as a 3 or less. When the APGAR score is low a cord blood gas should automatically be drawn.

When we refer to cord blood we are referring to blood drawn from the placenta after delivery. If you look at a placental cord (see picture) you will see one large vein surrounded by two arteries that wrap around the vein.

According to PubMed.com, the Umbilical Vein delivers freshly oxygenated blood from the mom to the baby. Since an anoxic brain injury in baby in not likely to change the pH of the Umbilical Vein, this is not where you will want to draw a cord gas from.

The Umbilical Artery is where the baby's venous circulation dumps unoxygenated blood. This is blood that was on its way back to the mom's heart and lungs to pick up oxygen. Thus, when you draw a cord gas for litigation purposes you will want to draw from one of the two Umbilical Arteries.

Blood from the Umbilical Artery is called a Cord Arterial Blood Gas (CABG), and basically shows how the baby was doing prior to birth.

From this blood we want to watch for acidosis. Since anaerobic metabolism occurs during the absence of oxygen, the acid base balance (pH) of the baby's body increases due to increase in the amount of lactic acid produced. Therefore pH is the most important indicator in the CABG.

If the pH of the CABGis above 7.10, then we know that the baby was not hypoxic during the delivery, and if there was a hypoxic episode it occurred prior to the delivery process. It may have occurred weeks or months prior to birth, or it may have occured hours before birth. Either way, this proves the episode did not occur as a result of the delivery and should clear the physician of litigation.

If the pH is less than 7.10 the episode was more likely acute and the episode may have occurred during the delivery. If the pH is greater than 7.10, the episode typically occurred before the delivery.

According to obgyn.org, Some experts believe a pH of 7.0 with a significant metabolic component is a more significant sign of asphyxia at birth, and may lead to significant neurological dysfunction during life, or possibly even death.

Also according to obgy.org, "Even when this low pH threshold is used to define significant acidemia, most newborns in this category will be neurologically normal, with no apparent morbidity."

The baby's at greatest risk of anoxic brain injury are premature infants, according to obgyn.org. They are at higher risk of "intracranial hemorrhage and subsequent neurological dysfunction, such as cerebral palsy. Without umbilical cord blood gas analysis, these neurological complications could be incorrectly attributed to intrapartum or birth asphyxia, especially if the latter is solely based on APGAR scores. Normal umbilical cord blood values in the premature infant virtually eliminate the diagnosis of significant intrapartum hypoxia or birth asphyxia."

So, ideally, you will want the pH to be normal. If it is normal and there is an anoxic brain injury the doctor can prove by the CABG results that since the pH had time to return to normal the injury occurred prior to delivery and the injury did not occur as a result of delivery. If the pH less than 7.1 chances are the injury occurred during delivery.

Once a CABG has been drawn it can be set aside. Most studies now show that a CABG does not need to be placed on ice, and is good for up to an hour.

  • pH: 7.28 (+/-.5)
  • pCO2: 49 (+/-8)
  • pO2: 18 (+/- 6.2)
  • HCO3: 2.5-3.5
  • BE: 10
Critical values that might show anoxic brain injury during birth (acidosis):
  • pH less than 7.0
  • CO2 greater than 50
  • PO2 variable (remember this is the baby's venous blood, so the PO2 is relatively low)
  • BE is normal or low (10 or less)
Critical values that might show injury due to metabolic cause:
  • pH less low (less than 7.25, critical is 7.10 as mentioned above)
  • PO2 less than 20
  • CO2 is normal or high
  • BE greater than 10 (Best indicator of metabolic cause
The following are conditions that would warrant a CABG:
  • Any abnormality during delivery process
  • Low 5 minutes APGAR score (less than 3)
  • Any abnormality in patient condition that occurs within 1st 5 minutes after birth
  • Premature birth
  • Post term birth
  • Meconium in amniotic fluid
  • Intubation
  • Positive pressure ventilation (Neo-puff or bag mask ventilation)
  • Suctioning
  • Cesarean-section
  • Severe growth retardation
  • abnormal fetal heart rate tracing
  • maternal thyroid disease
  • intrapartum fever
  • multifetal gestation
The following are sources used for this post:

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