Nonreassuring fetal heart rate patterns

1. Nonreassuring FHR patterns are nonspecific and require further evaluation. The fetus may not be acidotic initially; however, continuation or worsening of the clinical situation may result in fetal acidosis.

2. Late decelerations are characterized by a smooth U-shaped fall in the fetal heart rate beginning after the contraction has started and ending after the contraction has ended. The nadir of the deceleration occurs after the peak of the contraction. Mild late decelerations are a reflex central nervous system response to hypoxia, while severe late decelerations suggest direct myocardial depression.

3. Sinusoidal heart rate is defined as a pattern of regular variability resembling a sine wave with a fixed periodicity of three to five cycles per minute and an amplitude of 5 to 40 bpm. The sinusoidal pattern is caused by moderate fetal hypoxemia, often secondary to fetal anemia.

4. Variable decelerations are characterized by the variable onset of abrupt slowing of the FHR in association with uterine contractions. Mild or moderate variable decelerations do not have a late component, are of short duration and depth, and end by rapid return to a normal baseline FHR. They are usually intermittent. This pattern is not associated with acidosis or low Apgar scores. Severe variable decelerations have a late component during which the fetal pH falls. They also may display loss of variability or rebound tachycardia and last longer than 60 seconds or fall to less than 70 bpm. They tend to become persistent and progressively deeper and longer lasting over time.

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