Physiologic Basis of Fetal Heart Monitoring

Disruption of Fetal Oxygenation

    Fetal oxygenation is therefore dependent upon maternal blood pressure and oxygenation, the integrity of the placenta, specifically the amount of surface area for oxygen transfer, and the patency of the umbilical cord.  Anything that disturbs this chain of oxygen transfer will consequently affect fetal oxygenation.  This disturbance in transfer of oxygen from maternal blood to the fetus is called uteroplacental insufficiency (UPI). 

   The concept of fetal oxygen transfer is analogous to someone holding his breath underwater in an indoor swimming pool. Contractions are similar to a  swimmer ducking his head underwater for 30-45 seconds at a time every 2-3 minutes. While the swimmer is underwater, he has only his “cardiopulmonary reserve” to sustain him. When he is above the water, he may restore that reserve. This is not unlike the fetus with its placental reserve When the uterus contracts and relaxes.

   As long as the swimmer is healthy, he can tolerate this repetitive submersion for hours without adverse effects, just as a normal healthy fetus may tolerate repetitive contractions during labor for hours.

   Uteroplacental insufficiency really describes a problem with oxygen transfer in one or more of three compartments, the placenta, the uterus, and the mother’s perfusion of the uterus, leading to fetal hypoxemia.  Using the swimming pool analogy, if the swimmer has poor cardiopulmonary reserve (e.g. lung disease), he may not be able to tolerate continued submersions for very long because his ability to take in and transfer oxygen would be affected. In the same way, if the placenta  is abnormal (e.g. infarcted, too small, or abrupted), the fetus may begin to suffer from hypoxemia secondary to poor oxygen transfer during uterine relaxation .

   If the swimmer is submerged too often underwater or for too long, then he may not have sufficient time to recover between submersions. Similarly, if uterine contractions are occurring too frequently (hyperstimulation) or are lasting too long (uterine tetany), then the placenta may not have time to absorb oxygen from maternal blood between contractions.

   Lastly, if the atmosphere surrounding the swimming pool is smoky or has decreased oxygen tension, then even if the swimmer is healthy and has adequate time to recover, he may not be able to absorb oxygen when he is out of the water. In the same way, if the mother is hypotensive, hypoxic, or for any other reason underperfusing the placenta, then the baby will suffer in spite of the fact that the placenta may be healthy and the contractions are within normal limits.

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