Blood in the umbilical vein reflects uteroplacental status, which is determined primarily by maternal conditions such as anemia, hypoxia, hypertension, hypotension, ruptured uterus, placental abruption or inadequate placental size. To determine if a baby has suffered an oxygen-depriving event, called an anoxic or hypoxic-ishemic event, it is best to examine umbilical artery blood because this is the blood coming from the baby (as opposed blood going to the baby through the umbilical veins).
High PCO2: Respiratory acidosis (or alkalosis) defines the contribution of PCO2 to the acid-base status. Metabolic acidosis: The majority of conditions in which fetal acidosis is present are associated with fetal hypoxemia and the accumulation of lactic acid in the fetal tissues and blood.
Mixed acidemia: Mixed acidemia is metabolic acidosis that develops when respiratory acidosis is prolonged. Many blood gas analyzers do not accurately calculate HCO3- in the presence of a PCO2 that significantly deviates from the normal value. Base excess is defined as the amount of acid required to titrate the blood gas sample to a normal pH (7.4) at a normal PCO2 (40) at normal body temperature. Research shows that the probable mechanism for acidosis in the normal term fetus is most likely decreased perfusion (blood flow) of the intervillous space. Tetanic uterine contractions, tachysystole, hyperstimulation or hypertonic contractions: During labor, uterine contractions cause pressure increases in the uterus (womb), and this pressure is exerted on the vessels in the placenta that supply blood to the baby. Prolonged second stage of labor: More than 30 minutes in the second stage of labor is associated with a predominantly metabolic contribution to the fall in pH. Delivery by vacuum extraction or low forceps has been shown to result in lower pH and higher PCO2 in umbilical cord blood, but these changes were associated with the indication for instrument delivery (maternal exhaustion, prolonged second stage of labor, medications given to mother that inhibit her effort to push, breech delivery) and not primarily with the instrument delivery. In 2010, authors of an important systematic review that analyzed data from multiple studies concluded that low arterial pH in umbilical cord blood was strongly associated with long-term adverse outcomes. Neonates with low Apgar scores who are acidemic may do better in the long term than those who are not. The aforementioned events can occur because neonates without acidemia might still have been hypoxic but may have been unable to develop acidemia as a response. In almost all severely asphyxiated newborns, perfusion at the time of birth is poor to nonexistent. Due to the importance umbilical cord gas results play in litigation, it is imperative to have an attorney with experience in this area. At Reiter & Walsh ABC Law Centers, our award-winning birth injury and HIE attorneys have over 60 years of joint experience specifically and exclusively handling birth injury cases. Because of the severity and complexity of these injuries, confronting a loved one’s new hypoxic ischemic encephalopathy (HIE or birth asphyxia) diagnosis can feel overwhelming. 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). On the other hand, blood in the umbilical arteries reflects uteroplacental status as well as fetal status. If umbilical artery blood is acidemic (often called acidosis, which refers to acid in the tissues), it typically suggests that anaerobic metabolism occurred. An elevated PCO2 means that the fetus is producing more carbon dioxide than can be eliminated through circulation. When the cells are not receiving adequate oxygen, they revert to anaerobic metabolism, which produces acidic byproducts, such as lactic acid; when too many acid byproducts are in the blood, acidosis occurs.


What happens with mixed acidemia is that initially, the baby is not getting rid of enough carbon dioxide, which causes acidosis (respiratory acidosis).
Thus, the base excess or base deficit should be examined instead of HCO3- when the PCO2 is abnormal. Some of these outcomes included HIE (hypoxic ischemic encephalopathy), periventricular leukomalacia (PVL), intracranial hemorrhages, cerebral palsy and death. This brief but intense insult may cause brain injury with comparatively low acidemia or no acidemia at all. A hospital may attempt to use normal umbilical cord gas results to defend their case on causation.
A skilled and experienced 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. Fetal tissue oxygenation is influenced by umbilical venous blood flow and problems intrinsic to the fetus, such as heart failure and anemia. Anaerobic metabolism occurs when oxygen is not available, and is therefore an indicator that an anoxic event occurred.
In other words, carbon dioxide is not readily diffusing from the umbilical artery and capillaries into the maternal placenta and maternal circulation.
However, if HCO3- increases, it can buffer the carbon dioxide and prevent the blood from being acidic (acidemia).
While respiratory acidosis means the acidosis is due to impaired gas exchange (elevated carbon dioxide), metabolic acidosis is acidosis caused by metabolic reasons, such as a low HCO3- or the occurrence of anaerobic metabolism. Then, the prolonged oxygen deprivation causes anaerobic metabolism, which produces a metabolic acidosis. In between contractions, blood flows freely and oxygen-rich blood is delivered to the baby.
Multiple studies confirm that birth asphyxia can cause brain damage even when umbilical cord arterial pH is greater than 7.0.
If a fetus has poor circulation and perfusion (blood flow), the acid products that occurred due to anaerobic metabolism will not be moving through the baby’s body via normal circulation and blood flow.
The umbilical arteries will only reflect fetal tissue status up until the flow in them stops. There are many reasons why a fetus who suffered asphyxia or hypoxia can have a normal cord gas. An experienced attorney will work with the best experts to find the cause and timing of the birth injury.
We will fight to obtain the compensation you and your family deserve for lifelong care, treatment and therapy of your child. The relationship between umbilical cord arterial pH and serious adverse neonatal outcome: analysis of 51 519 consecutive validated samples.
Throughout this page, we’ll discuss everything you need to know about fetal circulation, hypoxic ischemic encephalopathy and neonatal brain damage. The placenta is an organ that connects the developing fetus to the uterine wall to allow for nutrient uptake, waste elimination and gas exchange via the mother’s blood supply.
An accumulation of carbon dioxide is most commonly seen in cases of umbilical cord compression, which hinders or prevents the movement of blood to and from the baby.


If a cord gas shows an elevated PCO2 with a normal or close to normal pH, the normal pH will have been caused by an increase in HCO3-; this is called compensated or partially compensated respiratory acidosis. In simplest terms, HCO3- represents the metabolic component and PCO2 represents the respiratory component of acid base status. If there is a base deficit of 25 (which would be written as base excess of –25), for example, it means there is a metabolic component to the acidosis.
Labor induction drugs such as Pitocin (oxytocin) and Cytotec can cause contractions to be too fast and strong (hyperstimulation), leaving little or no time for the placenta to recharge with oxygen-rich blood for the baby. A look back at the baby’s fetal heart tracings will often show heart problems that can cause decreased circulation and perfusion. Lactic acid produced from hypoxia or anoxia at the tissue level will not be cleared to the central circulation and subsequently to the umbilical arteries. 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. When this occurs, both venous and arterial cord blood PCO2 will typically be high, although the venous PCO2 will be more in the high normal range. When contractions are excessively strong, too frequent, too long, or if there is poor resting tone between contractions, the flow of oxygen-rich blood to the baby can be substantially decreased, and this can cause fetal hypoxia. The blood may have been drawn from the wrong vessel, or it may have been improperly stored in the collection tube. PO2 will typically be low, and if this condition persists, there will be progression to metabolic acidosis with accumulation of lactic acid.
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.
As the baby is resuscitated, circulation improves and tissue lactic acid is cleared into the central circulation. There are a number of technical errors that can affect the validity of umbilical cord gas results, and approximately 18 – 20% of cord gas results are, in fact, invalid due to technical error. Strength of association between umbilical cord pH and perinatal and long term outcomes: systematic review and meta-analysis. The vein in the umbilical cord, called the umbilical vein, picks up this oxygenated blood from the capillaries, and carries it to the baby’s heart, which pumps the blood throughout the baby’s body. Tetanic uterine contractions occur when stimulation causes normal muscle twitches to run together, which produces what is essentially a continuous contraction or constant contracted state of the uterus.
Due to the lactic acid entering the central circulation, the postnatal base deficit obtained from an asphyxiated newborn within the first hour after delivery is frequently found to be higher (worse) than in the umbilical cord blood gas.
Once the fetus uses this blood, it is carried away from the heart and back to the placenta by both umbilical arteries. In the intervillous space, carbon dioxide diffuses into the mother’s circulation so the mother can eliminate it by exhalation, and oxygen diffuses into the baby’s circulation.



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