- Guidelines on fetal monitoring aim to codify normal, abnormal FHR
Robert L. Barbieri, MD (Editorial, October 2008)
A Cat-I tracing is characterized by:
- baseline rate of 110–160 beats/min
- moderate variability
- no late or variable decelerations
- early decelerations being present or absent
- accelerations being present or absent.
For laboring women who exhibit a Cat-I FHR tracing, FHR monitoring during labor may either be continuous or intermittent.
Category III—highly troubling and damaging
Conversely, the Cat-III tracing is an OB’s nightmare. It is clearly abnormal and is associated with increased risk of fetal acidemia, neonatal encephalopathy, and cerebral palsy.
A Cat-III tracing is characterized by absent variability plus any one of the following:
- recurrent late decelerations
- recurrent variable decelerations
Recurrent late or variable decelerations are defined as those decelerations that occur with 50% or more of contractions. A sinusoidal pattern—characterized by a smooth, sine wave-like, undulating pattern with a cycle frequency of 3–5 waves/min that persists for 20 minutes or longer—is also classified as a Cat-III tracing.
Delivery is often warranted if a Cat-III tracing cannot be resolved, promptly, by means of obstetrical interventions.
The great gray zone—Category II
The label of “Cat-II tracing” is given to all FHR patterns that cannot be assigned to Cat I or Cat III.
A Cat-II tracing is neither normal nor definitively abnormal. Namely:
- If FHR accelerations or moderate variability are detected, the fetus is unlikely to be currently acidemic.
- If fetal heart accelerations are absent and variability is absent or minimal, the risk of fetal acidemia increases.
Cat-II tracings should be monitored closely and evaluated carefully.
Sometimes, when an OB nurse, an OB, or a midwife assigns a tracing to Cat II, she (or he) decides to take no action to improve fetal status. Why not? Because she is temporarily paralyzed with the hope that the tracing will improve. A common rationalization is that a Cat-II tracing is caused by a fetal sleep cycle, and that (again—it’s hoped) the tracing will improve once the fetus awakens.
There is an alternative to such hopeful watching and waiting: you cando something to improve the fetal status so that the FHR tracing converts to Category I.
Suggestions for moving to a better category
In 2010, ACOG offered a number of recommendations for obstetrical maneuvers that might improve Cat-II and Cat-III FHR tracings.2 These include advice to:
- change the position of the mother3 (see below)
- give her an intravenous bolus of 500–1,000 mL of lactated Ringer’s solution over 20 minutes4
- administer O2 to her4
- decrease or stop oxytocin infusion (especially if tachysystole is present)
- discontinue cervical ripening agents
- consider administering a tocolytic, such as terbutaline, if tachysystole is present or if uterine contractions are prolonged or coupled5
- consider amnio-infusion if variable decelerations are present.6
Cervical examination and digital scalp stimulation.These two interventions are not discussed in the 2010 ACOG recommendations, but they are often useful for assessing the progression of labor and fetal status. After digital scalp stimulation, acceleration in the FHR is evidence that the fetus is unlikely to be acidemic.7,8
If you prefer not to perform a vaginal examination, an alternative method of fetal stimulation is to use vibro-acoustic stimulation to assess fetal status.9
Last, when the mother has a regional anesthetic, such as an epidural, and her blood pressure is lower than it was at baseline, interventions to increase blood pressure, such as an IV bolus of ephedrine and fluids, may be effective.10
Changing maternal position may be pivotal
In the case example with which I began this discussion, the Cat-II tracing was occurring while the mother was lying flat on her back. Consider this finding: In a study of more than 900 laboring patients, 14% had an FHR tracing characterized by late decelerations.3 Of that subset of subjects, the late decelerations resolved in approximately 20% when they were moved from a supine to a lateral position. Furthermore, late decelerations recurred in some of those women when they were returned to a supine position.
In that same study, laboring in the supine, as opposed to the lateral, position was associated with lower femoral arterial blood pressure, lower toe capillary pulse pressure, and lower fetal scalp capillary pH. Other studies11,12 have also reported a decrease in fetal oxygen saturation when the mother labors supine.
CASE: Delivery accomplished
The OB snapped out of her reverie and instructed the labor nurse to change the patient’s position, administer a fluid bolus and O2, and recheck the blood pressure to assess for hypotension.
Twenty minutes after these interventions, the FHR tracing became Category I.