Umbilical Cord Blood Gases and Hypoxic-Ischemic Encephalopathy | Birth Injury Diagnosis

Michigan birth injury attorneys serving clients across the U.S.

Umbilical cord blood gas tests can be very important in diagnosing birth injuries such as hypoxic-ischemic encephalopathy (HIE), a form of neonatal brain damage that can occur when the baby’s brain does not receive enough oxygenated blood.

At ABC Law Centers: Birth Injury Lawyers (Reiter & Walsh, P.C.), we aim not only to provide unparalleled legal services to our clients, but to make complex information on birth injury easier to access. Throughout this page, we’ll discuss fetal circulation, oxygen deprivation/birth asphyxia, neonatal brain damage, and the role of umbilical cord blood gas tests in birth injury diagnoses and litigation. Should you have any legal questions or case inquiries as you read through this page, please contact our team for a free consultation.

In the womb, a fetus does not breathe in the same way humans do outside the womb (although “practice breathing” occurs from time to time [1]). Rather, oxygen molecules must traverse a pathway involving the mother’s lungs, heart, vasculature, uterus, and placenta, before finally making its way through the umbilical cord and into the fetus (2). Carbon dioxide and other fetal waste products travel back through the umbilical cord and placenta, into the mother’s circulatory system (3). At any point along this pathway, gas exchange between mother and baby can be dangerously interrupted. In many cases, birth asphyxia and subsequent issues (such as hypoxic-ischemic encephalopathy, or HIE) stem from an interruption in one of the final three steps: the uterus, placenta, or umbilical cord. Umbilical cord blood gas tests can provide valuable information on how such interruptions affect individual babies.

After birth, umbilical cord blood tests can provide valuable information on the baby’s status. However, it is important to note that results may vary depending on what part of the umbilical cord blood is drawn from.

The umbilical cord contains one large vein, which carries blood from the placenta to the fetus, and two small arteries, which carry blood from the fetus back to the placenta (4). Blood in the umbilical vein primarily reflects placental status. This does of course influence fetal status (because the fetus gets oxygen and nutrients from the placenta), but blood in the umbilical arteries reflects fetal status more directly. Therefore, it may be more useful to sample arterial blood if seeking information about whether the baby has experienced a hypoxic-ischemic injury. If umbilical artery blood has a low pH (is acidic), that means that the baby experienced fetal acidosis/anaerobic metabolism (5). Anaerobic metabolism occurs when oxygen is not readily available, and is therefore an indicator that an hypoxic-ischemic or anoxic event occurred. These oxygen-depriving events can cause a condition called hypoxic-ischemic encephalopathy (HIE), which is a form of neonatal brain damage that can lead to cerebral palsy, seizure disorders, and other permanent conditions.

Different studies have reported slightly different averages and ranges for umbilical artery blood samples. The following data came from a 1993 study by Anthony R. Gregg and Carl P. Weine (6):

Normal values in an umbilical arterial sample in a term newborn:

• Mean pH: 7.28 (standard deviation 0.07)
• Mean PCO2: 49.9 (standard deviation 14.2)
• Mean PO2: 23.7 (standard deviation 10.0)
• Mean HCO3: 23.1 (standard deviation 2.8)
• Mean base deficit: -3.6 (standard deviation 2.8)

Normal umbilical arterial values in a preterm newborn:

• Mean pH: 7.29 (standard deviation 0.07)
• Mean PCO2: 49.2 (standard deviation 9.0)
• Mean PO2: 23.6 (standard deviation 8.9)
• Mean HCO3: 23.0 (standard deviation 3.5)
• Mean base deficit: -3.3 (standard deviation 2.4)

*The “P” in PCO2 and PO2 stands for “partial pressure,” which is how these umbilical cord blood gases are measured.

The most important measurements in a blood gas test for evaluating a baby’s current condition and prognosis are the pH and the base deficit.

Some experts define fetal acidemia  as a pH of less than 7.1. However, there may be negative consequences even with a higher pH (this is discussed more below; see “Severity of acidosis and brain damage”).

While acidosis can be recognized using only pH, the base deficit (calculated from the pH and the partial pressure of carbon dioxide, or PCO2) can help to determine whether that acidosis was respiratory or metabolic. With either type of acidosis, outcomes can vary; some children will have no lasting neurological issues, while others may develop serious complications. However, metabolic acidosis is more likely to be associated with negative outcomes than is respiratory acidosis. If there is a base deficit of 12 mmol/L or above, this suggests metabolic acidosis. Infants with metabolic acidosis are at higher risk for the following complications:

• Multiorgan failure
• Death
• Moderate-to-severe neonatal encephalopathy (brain damage)
• Long-term neurological conditions

Lactate levels may also serve as a marker of fetal acidosis (5).

Fetal hypoxemia occurs in three ways, which we will explain in detail below.

1. Reduced delivery of oxygen to the placenta

The following complications can cause the placenta to receive insufficient oxygen, and lead to fetal acidosis:

• Hypotension from maternal hemorrhage or spinal anesthesia
• Uterine tachysystole/hyperstimulation
• Prolonged labor and delivery
• Preeclampsia/hypertension
• Diabetes
• Maternal respiratory or cardiac problems
• Connective tissue diseases

2. Reduced transfer of oxygen across the placenta from the maternal to the fetal side

When reduced levels of oxygen transfer from the mother to the fetus through the placenta, fetal acidosis can occur. Conditions that can cause this include the following:

• Placental abruption
• Placental insufficiency and intrauterine growth restriction (IUGR)
• Placenta previa

3. Reduced transport of oxygen by the fetal circulation

Decreased fetal oxygen transport can occur when the following conditions are present:

• Umbilical cord complications (such as a nuchal cord, pictured below) leading to cord compression
• Fetal anemia from rhesus disease, parvovirus infection, α-thalassaemia or feto-maternal haemorrhage
• Fetal tumors
• Serious cardiac problems (7, 8)

In 2010, authors of an important review paper (9) concluded that low arterial pH in umbilical cord blood was strongly associated with long-term adverse outcomes. These outcomes included hypoxic-ischemic encephalopathy (HIE), periventricular leukomalacia (PVL), intraventricular hemorrhages, cerebral palsy (CP), and death.

What was unclear, at that point, was what pH constituted clinically significant acidemia. A 2012 study (10) of singleton, term neonates found the ideal cord arterial pH to be 7.26 – 7.30 for all outcomes. The risk for adverse neurological outcomes starts to rise below a pH of 7.10, with the risk being highest below a pH of 7.0. Another key finding of this study is that most neonates with adverse outcomes, even that of seizures in the first 24 hours, are not born acidemic. In addition, the authors concluded that “it appears that the lowest risk of any adverse outcome occurs at 7.26 – 7.30, rather than ‘the higher the better’, for there may be a higher risk at higher pH levels.”

The American College of Obstetricians and Gynecologists (ACOG) suggests that the arterial cord blood’s pH should be less than 7.0 if it is going to be used as a factor establishing a link between birth asphyxia and neurological injury (11). This conclusion, however, is debatable. Multiple studies confirm that birth asphyxia can cause brain damage even when umbilical cord arterial pH is greater than 7.0. Yeh et al (whose study excluded infants with major congenital abnormalities) found that infants with a pH between 7.0 and 7.11 accounted for 10 – 15% of adverse neurological outcomes in their study. Even when considering cases involving seizures in the first 24 hours of life, at least half had a pH above 7.10, and 39 percent had a pH over 7.20.

Moreover, Yeh et al. noted the following (10):

• Many babies with birth asphyxia (who often have low Apgar scores) often have a normal pH. (A 2012 study by Joseph J. Volpe also noted this [12])
• In some cases, neonates with low Apgar scores who are acidemic may do better in the long term than those who are not.
• Catastrophic intrapartum events can occur without acidemia.

The aforementioned events can occur because newborns who experienced hypoxia may have been unable to develop acidemia as a response. Yeh et al. suggest that this can occur if cerebral circulation is disrupted, for reasons such as shoulder dystocia or umbilical cord occlusion.

Severely asphyxiated newborns may have poor perfusion around the time of birth; this can include umbilical circulation. If blood flow from the fetus through the umbilical cord has been disrupted, the umbilical cord blood will not accurately reflect fetal status. As the baby is resuscitated, circulation improves and tissue lactic acid is cleared into the central circulation. For this reason, postnatal blood gas tests may reveal worse acidosis than was found in the umbilical cord blood test. These postnatal tests can be important predictors of neurological outcome (13).

Umbilical cord blood gases are frequently used in birth injury litigation. A hospital may attempt to use normal umbilical cord gas results to defend their case on causation. There are many reasons why a fetus who suffered asphyxia or hypoxia can have a normal cord gas. The fetus may have had very poor circulation and perfusion right before birth, the fetus may have suffered a severe and sudden head injury during delivery that caused ischemia (lack of blood flow) in the brain, or the results may be invalid due to error in drawing, storing or analyzing the blood. A 2002 study suggested that up to 19 percent of umbilical cord blood samples may be invalid due to technical errors (14).

Due to the importance umbilical cord gas results play in litigation, it is imperative to have an attorney with experience in this area. A skilled birth injury attorney will know what to look for in medical records to determine if a sample is valid and an accurate reflection of the baby’s condition at the time of birth. They will also work with the best experts to find the cause and timing of the birth injury.