Intrauterine Asphyxia: Clinical Implications for Providers of Intrapartum Care

Jenifer Fahey, CNM, MSN, MPH; Tekoa L. King, CNM, MPH

Disclosures

J Midwifery Womens Health. 2005;50(6):498-506. 

In This Article

Cellular Respiration and Acid-Base Balance

As carbohydrates, fat, and proteins are metabolized in the cell to produce energy, hydrogen ions and carbon dioxide are released as waste products. Metabolism in the presence of oxygen is called aerobic metabolism. In this process, hydrogen ions combine with oxygen to form water, and carbon dioxide combines with water to form carbonic acid. In adults, carbonic acid is transformed back into carbon dioxide in the lungs and excreted on exhalation; thus, the lungs are the main regulator of acid-base balance in the body. The fetus, however, depends on maternal and placental circulation for the delivery of oxygen and the removal of the waste products of metabolism, including carbon dioxide.

In the absence of oxygen, cells can continue to produce energy, but this process, called anaerobic metabolism, cannot be maintained over the long term. When oxygen is not present to accept hydrogen ions, they form organic acids such as lactic acid. Buildup of organic acid changes the pH within the cells, and if this process continues too long, the pH will decrease to levels that result in cellular death. Normal adult cell function depends on the maintenance of a plasma pH within the range of 7.35 to 7.45 (slightly alkaline). Mean umbilical artery pH in newborns is between 7.25 and 7.30, indicating that cell function in the fetus can be maintained at a lower pH.[8,9] Table 1 includes definitions of terms related to respiration and acid-base balance that are used in this article.

When carbon dioxide is not removed from circulation, there is a drop in blood pH as well. Any change in pH secondary to changes in the partial pressure of carbon dioxide (PCO2) is termed a respiratory change. Therefore, a drop in blood values of pH resulting from an increase in the PCO2 is classified as a respiratory acidemia. Similarly, if too much carbon dioxide is removed (e.g., during hyperventilation), hydrogen ion concentration may fall and cause an increase in blood pH. This is termed respiratory alkalemia.

The blood pH level is usually kept in balance via the interaction of nonvolatile acid with bicarbonate (HCO3), which is produced by the kidneys. Any changes in pH due to changes in bicarbonate concentration are termed metabolic. If the production of lactic acid and other metabolic acids outstrips the body's ability to produce enough bicarbonate as a buffer, a decrease in blood pH may result, creating a metabolic acidemia, which, if unimpeded, will progress to a drop in tissue pH (metabolic acidosis).

Asphyxia occurs when gas exchange is impaired enough to cause significant metabolic acidosis. As asphyxia progresses, the fetus loses the ability to protect vital organs. Eventually, there is a decrease in cardiac output. This, in turn, leads to marked hypotension and a subsequent further decrease in blood flow to the heart and the brain. Resultant central nervous system damage depends on a variety of factors, including duration and severity of compromised gas exchange, the underlying condition of the fetus, and the ability of noncirculatory mechanisms to protect brain cells and tissue from hypoxic injury and death. If prolonged and unrelieved, asphyxia will lead progressively to cellular death, tissue damage, organ and organ system failure, and, ultimately, fetal death.

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