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Fetal heart rate (FHR) monitoring is the most common obstetric procedure. It is performed to detect fetal hypoperfusion, hypoxia, and acidosis. When the FHR tracing is abnormal, resuscitative measures are taken to increase O2 delivery to the placenta and improve umbilical blood flow. Ultimately, a cesarean section might be performed if the FHR tracing continues to be concerning. In 2009, American College of Obstetricians and Gynecologists (ACOG) issued a bulletin to streamline the nomenclature and management of FHR monitoring to decrease inter- and intraobserver variability in interpretation.1,2
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BASICS OF FHR TRACING
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Baseline normal FHR ranges are between 110 and 160 beats per minute (bpm). A sustained change in the heart rate lasting more than 10 minutes constitutes a change in baseline.
FHR variability is fluctuations in the FHR of two or more cycles (visual regions on the tracing) per minute.
Moderate variability ranges from 6 to 25 bpm and is considered normal. Moderate variability is a surrogate for the absence of metabolic acidemia at the time it is observed.3
Minimal and absent variability raises concern for poor fetal cerebral perfusion. It can also be caused by medications that decrease nervous system activity, such as opioids and magnesium, and is normal during the fetal sleep state.
Accelerations are abrupt increases in FHR.
At >32 weeks, an acceleration is 15 beats above baseline, for 15 seconds to 2 minutes.
At <32 weeks, an acceleration is 10 beats above baseline, for 10 seconds to 2 minutes.
Presence of accelerations is also typically a sign of fetal well-being, particularly in the antepartum period. Their relevance is less clear intrapartum.
Decelerations fall into three groups: early, late, and variables. The last two reflect the interruption of oxygen transfer to the fetus at various points along the oxygen delivery pathway.
Early decelerations are characteristically symmetrical to contractions. They are gradual (>30 seconds from onset to nadir) and rarely more than 20 bpm below baseline. They are a normal physiological vagal reflex to the baby’s head being compressed.
Late decelerations are similar to early decelerations in their gradual nature (>30 seconds from onset to nadir); however, they are delayed in timing. The onset, peak, and end of a late deceleration occur after those of the contraction. They are associated with uteroplacental insufficiency and suggest fetal asphyxia. Although not always as sensitive when present alone, they strongly suggest fetal distress when concomitant with decreased or absent FHR variability.
Variable decelerations are abrupt (<30 seconds from onset to nadir) decrease in FHR of 15 or more bpm lasting 15 seconds or more but less than 2 minutes and of variable shapes with each contraction. They are associated with umbilical cord compression. When severe and persistent, they are a sign of fetal hypoxia.
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INTERPRETATION OF FHR
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In 2009, ACOG introduced three categories in which to place FHR tracing to help with ...