Master Class

Assessing Fetal Heart Rate


 

The majority of women in labor in the United States undergo continuous intrapartum fetal heart rate monitoring. However, the pervasive use of electronic fetal monitoring in obstetric practice has been challenging due to a lack of standardized nomenclature for heart rate assessment and clear guidance about how to interpret and manage various types of tracings.

The issue of nomenclature was a main focus of a 2008 workshop sponsored by the American College of Obstetricians and Gynecologists (ACOG), the Eunice Kennedy Shriver National Institute of Child Health and Human Development, and the Society for Maternal-Fetal Medicine. Workshop participants, myself included, reaffirmed nomenclature for baseline fetal heart rate (FHR) and FHR variability, accelerations, and decelerations. We also recommended new terminology and nomenclature for the description of uterine contractions. We were driven by the need to "speak the same language" – that is, to agree on definitions, lessen ambiguity, and allow for a more evidence-based approach to the management of fetal compromise during labor.

Dr. George A. Macones

In this light, the workshop also recommended adoption of a three-tier system for categorizing FHR patterns. Under this system, Category I FHR tracings are normal and not associated with fetal acidemia. Category II FHR tracings are indeterminate, and Category III FHR tracings are abnormal, or predictive of abnormal fetal acid-based status (Obstet. Gynecol. 2008;112:661-6).

The recommended adoption of this three-tiered classification system was a significant outcome of the workshop and a useful first step in bringing more clarity and meaning to challenges of electronic fetal monitoring.

There was a problem, however: There are very few tracings that do not fall into Category II. Indeed, a growing number of studies have indicated that a significant fraction of all FHR tracings encountered in clinical care – as many as 85% of tracings, it is believed – are of an "indeterminate" nature. Thus, a major task after the 2008 workshop became one of digging more deeply into the Category II tracings to give practicing physicians more meaningful guidance on how to manage this diverse spectrum of abnormal FHR patterns.

The ACOG Practice Bulletin issued in 2010 on "Management of Intrapartum Fetal Heart Rate Tracings" (No. 116) delves into the challenging issue of Category II FHR tracings. It essentially addresses the question, how does one manage the woman who has variable decelerations?

Variable Decelerations

Although intermittent variable decelerations (those that occur with less than 50% of contractions) most often do not require any treatment and are associated with normal perinatal outcomes, recurrent variable decelerations (those occurring with 50% or more of contractions) can be more indicative of impending fetal acidemia.

The evidence is fairly strong that recurrent variable decelerations result from umbilical cord compression. Changing maternal position is a good first step to alleviate some of that compression. Amnioinfusion, that is, the infusion of a solution into the uterine cavity to cushion the umbilical cord – also may be used in managing umbilical cord compression.

A meta-analysis reviewed and reprinted this year in the Cochrane Database of Systematic Reviews concluded that the use of amnioinfusion for "potential or suspected umbilical cord compression" may reduce the occurrence of variable FHR decelerations, improve short-term measures of neonatal outcome, reduce maternal postpartum endometritis, and lower the rate of cesarean delivery (Cochrane Database Syst. Rev. 2012 Jan. 18;1:CD000013).

One important overarching principle is that in FHR tracings with recurrent variable decelerations – as in other types of Category II tracings – the presence of FHR accelerations (either spontaneous or induced) or FHR variability that is good ("moderate" FHR variability) is significantly predictive of a fetus that is not acidemic.

Recurrent variable decelerations should, of course, be evaluated for frequency, depth, and duration; uterine contraction pattern; and other FHR characteristics. If variable decelerations get deeper and/or last for longer periods of time, one has to be more concerned than if decelerations are not lasting as long, or if FHR is not dropping as much.

Other Category II Tracings

Recurrent Late Decelerations. These patterns are believed to reflect uteroplacental insufficiency, which is usually caused by uterine tachysystole, maternal hypoxia, or maternal hypotension, the latter of which often occurs after the administration of regional anesthesia. Measures to improve perfusion to the placenta include the administration of intravenous fluid boluses, maternal oxygen administration, and steps to reduce uterine activity when the uterus is contracting too frequently. One important initial measure is to check the blood pressure, and if it is low (likely due to regional anesthesia), to work with the anesthesiologist on appropriate medical management of hypotension.

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