Variability of Fetal Heart Rate

The baseline FHR fluctuates constantly under normal conditions, described as its variability. This variability is often a good indicator of a healthy nervous system. Variability is the oscillation of the FHR around the baseline with amplitude of 6 to 25 beats/min. Variability reflects vagal efferent impulses only. Uncomplicated loss of variability is often caused by fetal quiescence (sleep state), CNS depressants (e.g., diazepam, morphine, magnesium sulfate), or parasympatholytic agents (e.g., atropine). The uncomplicated loss of variability is associated with no risk or minimal risk of acidosis and low Apgar scores. The presence of decreased variability in combination with late or severe variable deceleration is an ominous finding. Clinically variability is one of the most important indicators of fetal well-being, and the majority of babies with good variability do well regardless of the presence of decelerations.

Recent guidelines do not recommend the use of or differentiation between short-term and long-term variability in the assessment of fetal well-being (NICHHD, 2008). In the assessment of variability, the following categories are now recommended, reflecting the amplitude of the FHR tracing around the baseline (Fig. 21-11): Absent: Amplitude range is undetectable. Minimal: Amplitude range is detectable, but 5 beats/min or less.

Moderate: Amplitude is 6 to 25 beats/min.

Marked: Amplitude range is greater than 25 beats/min.

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