Baseline
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The mean FHR
rounded to increments of 5 beats per min during a 10 min segment,
excluding:
- Periodic or episodic changes
- Periods of marked FHR variability
- Segments of baseline that differ by more than 25 beats per min
- The baseline must be for a
minimum of 2 min in any 10-min segment
(NICHD)
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In order to
determine whether there are changes in the fetus' physiologic status, a
baseline heart rate must first be determined as a reference. All
changes are therefore, based on a deviation from that fetus' resting
norm.
The normal FHR
baseline should range between 110 beats/min to 160 beats/min. The
baseline FHR is normally set by the atrial pacemaker and the
beat-to-beat differences in the heart rate are governed by a balance
between the sympathetic and parasympathetic branches of the autonomic
nervous system.
As the central nervous
system matures, the baseline FHR gradually drops, and the normal ranges
for a mid-trimester fetus are between 150-170 beats/min.
Parasympathetic tone becomes more dominant with advancing gestational
age, and a post-term fetus may have a FHR between 110-120 beats/min.
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Basic Pattern Recognition | | Baseline
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