FHR Terminology / Purpose / Efficacy
Editor: Julian T. Parer, M.D., PhD.
Objectives: Upon the completion of this CNE article, the reader will be able to
- Describe the different definitions that form the basis of fetal heart rate monitoring.
- Accurately discuss the purpose behind using fetal heart rate monitoring.
- Compare and contrast the efficacy of fetal heart rate monitoring versus intermittent auscultation.
- Explain the reasoning behind the importance of correctly knowing the definitions used in fetal heart rate monitoring.
The creation of fetal heart rate monitoring was the result of the work of numerous researchers across the globe. Because of this, different terminology was developed along with slightly different interpretations. Furthermore, the use of fetal monitoring in the United States has greatly increased over the years – 45% in 1980, 62% in 1988, 74% in 1992, to 85% in 2002. It has now become the most common procedure performed in obstetrics in the United States (see ref. 1). Because of its multifactorial development, widespread confusion existed regarding its interpretation, especially in the terminology / definitions, its efficacy, and purpose. This also became an issue regarding uniformity in the language utilized in research.
Therefore, a research planning workshop was organized at the National Institute of Child Health and Human development (NICHD) that met in 1995 and 1996 with the purpose of standardizing and creating unambiguous definitions for fetal heart rate patterns. The results of this workshop were reported in December of 1997 in the American Journal of Obstetrics and Gynecology and the Journal of Obstetric, Gynecologic, and Neonatal Nursing (see ref. 2). In late 2005, early 2006, the American College of Obstetricians and Gynecologists (ACOG) and the Association of Women’s Health Obstetric and Neonatal Nursing (AWHONN) adopted the fetal heart rate monitoring terminology that was published from this NICHD Workshop (see ref. 3).
Prior to creating the definitions that will be discussed below, several preliminary factors were discussed and analyzed. To begin with, the individual interpretations were for visual analysis and included both external Doppler monitoring (with autocorrelation), as well as, internal fetal scalp electrode monitoring. Though not developed for computerized interpretation systems, the definitions were adaptable to those products. As long as the quality of the recording of the fetal heart rate and uterine activity is adequate for visual interpretation, the scaling of the strip and paper speed will not affect the definitions. The most commonly used scaling for the fetal heart rate is 30 beats per min per centimeter in the vertical axis or up and down aspect of the tracing. Some monitors strips have a scaling of 20 beats per min per centimeter in the vertical axis. The most common paper speed (horizontal movement) is 3 centimeters per minute (it is important to know that some monitors have the capability of being changed to 1 centimeter per min, which is slower, making accelerations and decelerations appear much narrower and variability better).
The definitions did not make any deduced assumptions as to the possible etiologies for the patterns described or their relationship to hypoxemia or metabolic acidosis. In the described patterns, "periodic" patterns are those that are associated with uterine contractions, whereas "episodic" patterns are those that are not associated with uterine contractions. For the most part (but clearly not all), decelerations are periodic – related to contractions. Fetal heart rate accelerations primarily occur with fetal movement and thus are mostly episodic. It is important to understand that both fetal heart rate accelerations and decelerations can be both periodic and episodic.
Another area of discussion was variability. There are two basic forms of variability that have been described in the literature – long-term variability and short-term variability. Short-term variability is "beat-to-beat" variability and is the millisecond difference seen between successive heartbeats. The only "true" way to determine short-term variability is by way of an electrocardiogram – fetal scalp electrode or internal monitor. Long-term variability is the larger cycling squiggles seen on a fetal monitor tracing over time excluding periodic changes. The workshop concluded that in clinical usage short-term variability and long-term variability were visually ascertained as a combined unit and thus there was no separate distinction or definition for either of these. Instead variability was defined as a single entity.
Finally, the workshop emphasized that fetal heart rate patterns were gestational age dependent. Likewise, fetal heart rate tracings needed to be analyzed in the context of maternal medication usage or administration (drugs that can stimulate or depress), maternal medical condition (i.e. hypertension, diabetes, cardiopulmonary disease, etc.), and the results of prior fetal heart monitoring (if obtained). The definitions taken directly from the NICHD workshop publication are as follows (see ref. 2):
Is the mean fetal heart rate rounded to increments of 5 beats/min during a 10-minute segment of tracing, excluding
- Periodic or episodic changes (accelerations or decelerations)
- Periods of marked variability (see below)
- Segments of the baseline that differ by more than 25 beats per minute
In any 10-minute window of tracing, the baseline duration must be at least a minimum of 2 minutes or it is considered indeterminate – in which case it may be necessary to refer to the prior 10-minute segment or segments for determination. The normal fetal heart rate baseline is 110 to 160 beats per minute.
Is a fetal heart rate baseline of less than 110 beats per minute.
Is a fetal heart rate baseline of greater than 160 beats per minute.
Is the fluctuation in the fetal heart rate of at least 2 cycles per minute or more and is visually quantified as the amplitude of (or difference between) the peak and trough in beats per minute. These fluctuations are irregular in appearance (the squiggliness of the line).
- Absent variability – the amplitude is undetectable
- Minimal variability – the amplitude range is detectable but < 5 BPM
- Moderate (normal) variability – the amplitude range is 6 to 25 BPM
- Marked variability – the amplitude variation is > 25 beats per minute
The sinusoidal pattern is excluded from the variability definitions because it has a smooth sine wave appearance. Its definition does use (in some respects) the terms short and long-term variability because there is an absence of short-term variability and the regularity of the smooth waves distinguish it from the usual jaggedness of long-term variability.
Are abrupt (defined as the onset of the acceleration to the peak being less than 30 seconds) increases in the fetal heart rate from the most recently determined baseline. The "duration" is defined as the time it leaves the baseline to the time it returns to the baseline.
- In a pregnancy at or beyond 32 weeks gestation the acceleration is an increase in the fetal heart rate that peaks at least 15 beats per minute above the baseline and lasts for at least 15 seconds but less than 2 minutes from the time the fetal heart rate leaves the baseline to when it returns to the baseline.
- In a pregnancy less than 32 weeks gestation the acceleration is an increase in the fetal heart rate that peaks at least 10 beats per minute above the baseline and lasts for at least 10 seconds but less than 2 minutes from the time the fetal heart rate leaves the baseline to when it returns to the baseline.
- A prolonged acceleration is one that lasts for > 2 minutes but < 10 minutes.
- If the acceleration lasts for > 10 minutes it is considered a baseline change.
Are gradual (defined as the onset of the deceleration to the nadir being > 30 seconds) decreases in the fetal heart rate from the most recently determined baseline that then gradually return to the baseline and are associated with uterine contractions. Their timing in relation to the uterine contraction is where the nadir of the deceleration occurs at the same time as the peak of the contraction. In the majority of instances, the onset, nadir, and recovery of the deceleration match the beginning, peak, and ending of the contraction, respectively (they mirror the contraction).
Are gradual (defined as the onset of the deceleration to the nadir being > 30 seconds) decreases in the fetal heart rate from the most recently determined baseline that then gradually return to the baseline and are associated with uterine contractions. Their timing is "delayed" in relation to the uterine contraction where the nadir of the deceleration occurs after the peak of the contraction. In the majority of instances, the onset, nadir, and recovery of the deceleration occur after the beginning, peak, and ending of the contraction, respectively (their timing is "late" in relation to the contraction).
Are abrupt (defined as the onset of the deceleration to the beginning of the nadir being < 30 seconds) decreases in the fetal heart rate from the most recently determined baseline that may or may not be associated with uterine contractions. The decrease in the fetal heart rate must be at least 15 beats per minute or more below the baseline and last for at least 15 seconds but less than 2 minutes from the time the heart rate leaves the baseline to when it returns to the baseline. When variable decelerations are associated with uterine contractions, their onset, depth, and duration vary with successive contractions. The 1997 National Institute of Child Health and Human development (NICHD) Research Planning Workshop did not define "mild", "moderate", or "severe" variable decelerations. Therefore, no standardized definitions for further differentiating variable decelerations into "mild", "moderate", or "severe" (such as depth and duration, etc.) are available.
Are abrupt (defined as the onset of the deceleration to the beginning of the nadir being < 30 seconds) decreases in the fetal heart rate from the most recently determined baseline that may or may not be associated with uterine contractions. The decrease in the fetal heart rate must be at least 15 beats per minute or more below the baseline and last for > 2 minutes but < 10 minutes from the time the heart rate leaves the baseline to when it returns to the baseline. Prolonged decelerations that last for > 10 minutes are considered a baseline change.
If decelerations occur with 50% or more of the uterine contractions in a 20-minute window of fetal heart monitoring, they are defined as "recurrent". In review, "duration" of accelerations and decelerations are defined from when the fetal heart rate leaves the determined baseline to when it returns back to the baseline. Though the primary emphasis of the new terminology was for intrapartum fetal heart rate monitoring, these definitions can also be applied to antenatal fetal heart rate monitoring. The term "reactive" is a commonly used word in Obstetrics and it has primarily been used in antenatal testing. A definition for "reactive" was not formulated at the NICHD workshop. The term is applied to a specified number of fetal heart rate accelerations (as defined above) that occur in a specified time period and this finite definition will often vary from institution to institution. Many institutions use 2 accelerations (as defined) in a 20 minute time window; however, other definitions have been reported in the literature such as 2 accelerations in 10 minutes or 3 accelerations in 20 minutes, etc. The important issue here is to know the definition that is used by the institution or group one associates with.
The purpose of fetal heart rate monitoring is to determine if the fetus is well oxygenated at that point in time. The purpose of fetal heart rate monitoring is not to tell you that the fetus is "normal". Essentially nothing will tell you that the fetus is "normal" until a thorough examination and evaluation occurs following birth. The researchers at the NICHD workshop agreed that the fetal heart rate tracing could be considered normal (again, the tracing not the fetus) and that the fetus was probably well oxygenated at that point in time, if there was a normal baseline, normal or moderate variability, the presence of accelerations, and the absence of decelerations. At the other end of the spectrum, the majority of researchers agreed that a pattern of absent fetal heart rate variability combined with either recurrent late or variable decelerations or a substantial bradycardia, was indicative of impending or ongoing fetal asphyxia that was severe enough that it could lead to fetal neurologic damage or death. As expected, the majority of fetuses have fetal heart rate patterns that fall between these two extremes and thus their management and predictive value are controversial.
The original goal of fetal heart rate monitoring was that it could be used to identify the early stages of developing fetal asphyxia so that intervention could occur prior to damage. Thus, with the use of intrapartum fetal heart rate monitoring, the incidence of newborns with brain damage and cerebral palsy should decline over the years. Unfortunately, it appears that the incidence of these major disorders in term newborns has remained relatively constant at about 1 in 500 births over the past 30 years and that fetal heart rate monitoring did not reduce the incidence (see ref. 4 and 5). In fact, it appears that in 70% to 90% of newborns that suffer from severe neurologic impairment or cerebral palsy, the disorder was in existence prior to the onset of labor. It also appears that intrapartum events only account for 10% or less of the newborns that actually suffer from a significant neurologic disorder.
Because of this, the American College of Obstetricians and Gynecologists (ACOG) and the American Academy of Pediatrics (AAP) adopted new guidelines (see ref. 6) that define an acute intrapartum hypoxic event that was sufficient enough to cause cerebral palsy, and these are:
- Evidence of a metabolic acidosis in the fetal umbilical artery (cord blood gas) obtained at delivery with a pH < 7.0 and a base deficit > 12 mmol/L (or base excess of < –12 mmol/L))
- Early onset of severe or moderate neonatal encephalopathy in infants > 34 weeks of gestation (for example seizures, etc.)
- Cerebral palsy of the spastic quadriplegic or dyskinetic type
- Exclusion of other identifiable etiologies (such as trauma, coagulation disorders, infectious conditions, genetic disorders, pre-existing conditions, etc.)
There are 5 criteria that collectively are suggestive of an intrapartum event (within close proximity to labor and delivery – 0 to 48 hours), but are nonspecific to asphyxial insults and these are: (see ref. 6)
- A sentinel (single) hypoxic event occurring immediately before or during labor (for example a ruptured uterus or umbilical cord prolapse, etc.)
- A sudden sustained fetal bradycardia or the absence of fetal heart rate variability in the presence of persistent late, or variable decelerations, usually seen after a sentinel hypoxic event when the pattern was previously normal
- Apgar scores of 0 to 3 beyond 5 minutes
- Onset of multisystem (multi-organ) involvement within 72 hours of birth (for example, elevated liver function tests, or elevated creatinine, or hypotension requiring dopamine administration, or blood coagulation dysfunction, etc.)
- Early neurologic imaging studies showing evidence of an acute nonfocal cerebral abnormality
Evidence for the efficacy of fetal heart rate monitoring is tentative at best. No randomized studies have been performed that compare the benefits of fetal heart rate monitoring to no fetal monitoring during labor. There have been studies that compare fetal heart rate monitoring to intermittent auscultation and the individual results vary. A meta-analysis of these studies was undertaken and unfortunately the results were not overwhelming in favor of fetal heart rate monitoring (see ref. 7). What was identified was that fetal heart rate monitoring statistically increased the incidence of cesarean section, forceps delivery, and vacuum delivery and did not reduce the overall perinatal mortality rate. It did appear that the incidence of perinatal death from hypoxic events decreased. However, because the number of deaths from hypoxic events was so small (even in this meta-analysis), one less hypoxic event death in the intermittent auscultation group would render this finding insignificant.
The purpose of the tocodynamometer is to identify uterine contractions and depict them on the fetal heart monitor tracing. No standardized terminology currently exists for uterine contractions or their frequency; however, the NICHD workshop publication did state that the recording of the uterine activity needed to be of "adequate quality for visual interpretation". As with fetal heart monitoring, there are two basic ways to record uterine contractions (excluding palpation) and these are an external tocodynamometer and an intrauterine pressure catheter (IUPC). When analyzing the uterine activity portion of the fetal heart monitor tracing, several issues may be addressed and these are the baseline uterine tone, the contraction duration, contraction strength, and contraction frequency. The external tocodynamometer can only supply the contraction frequency. An intrauterine pressure catheter is needed to supply the baseline uterine tone and contraction strength. An external monitor in some instances may closely approximate the contraction duration; however, the true contraction duration also requires an IUPC.
Loose definitions for uterine activity have been summarized in one ACOG publication (see ref. 8). In "normal" labor, the contraction frequency on average is once every 2 to 3 minutes (or 3 to 5 contractions in a 10-minute window) with a baseline uterine tone on average of about 8 to 15 mmHg. "Normal" labor contractions on average range from about 30 to 90 seconds in duration (with an average of about 60 seconds). Excessive contraction frequency in many cases is described as contractions that occur closer than every 2 minutes (or > 6 contractions in 10 minutes for two consecutive 10-minute periods). An elevated baseline uterine tone is often defined as one that exceeds 20 to 25 mmHg and a prolonged, protracted, or extended contraction is one that lasts more than 2 minutes. Again, it is important to understand that there is no universally accepted standard on these definitions and the ones just described above are relative parameters.
In addition, the fetal response in relation to these contraction patterns is also of importance. Uterine hyperstimulation (seen as excessive uterine contractions, a protracted contraction, or an increase in baseline internal uterine pressure) has been given various names over the years and no standardized terminology currently exists for these either. Some of the words that are used include tetanic contractions, uterine hypertonus, tachysystole, polysystole, paired contractions (couplets or triplets), skewed contraction, or a prolonged, protracted, or extended contraction (see ref. 9).
All of this above information would beg the question as to "why use fetal heart rate monitoring?". It is necessary to understand that in all of the studies that compared fetal heart rate monitoring to intermittent auscultation, the intermittent auscultation group utilized one-on-one nursing. In today’s society in the United States with the current healthcare cost crisis, one-on-one nursing is not a reality. Fetal heart rate monitoring allows one nurse to manage and care for more than one patient at a time. This is equal to one-on-one nursing with intermittent auscultation, as long as the nurse is able to adequately recognize the patterns that are visualized and correctly transfer this information to the delivering obstetrician or other healthcare provider. Therefore, at the present time, fetal heart rate monitoring is a reality in the United States and an accurate understanding of the terminology, purpose, and efficacy is crucial.
The following fetal heart rate tracings are supplied to demonstrate some of the definitions described above. Strip #1 is an example of moderate (or normal) variability. Strip #2 is an example of minimal variability, while Strip #3 is an example of marked variability. Finally, Strip #4 is an example of absent variability. Strip #5 depicts accelerations in a gestation < 32 weeks. Strip #6 depicts accelerations in a gestation > 32 weeks.
References or Suggested Reading:
- Martin JA, Hamilton BE, Ventura SJ, et al. Births: final data for 2002. National Vital Statistics Report 2003;52:1-113.
- Electronic fetal heart monitoring: research guidelines for interpretation. National Institute of Child Health and Human Development Research Planning workshop. Am J Obstet Gynecol 1997;177:1385-90.
- ACOG. Intrapartum Fetal Heart Rate Monitoring: Nomenclature, Interpretation, and General Management Principles. American College of Obstetrics and Gynecology Practice Bulletin #106 July 2009 – Reaffirmed 2013.
- Clark, SL, Hankins GD. Temporal and demographic trends in cerebral palsy – fact and fiction. Am J Obstet Gynecol 2003;188:628-33.
- Thacker SB, Stroup D, Chang M. Continuous electronic heart rate monitoring for fetal assessment during labor. The Cochrane Database of Systematic Reviews 2001, Issue 2. Art No.: Cd000063. DOI: 10.1002/14651858. (meta-analysis)
- ACOG. Inappropriate Use of the Terms Fetal Distress and Birth Asphyxia.. American College of Obstetrics & Gynecology Committee Opinion #326 December 2005 – Reaffirmed 2013.
- Vintzileos AM, Nochimson DJ, Guzman EF, Knuppel RA, Lake M, Schifrin BS. Intrapartum electronic fetal heart rate monitoring versus intermittent auscultation: a meta-analysis. Obstet Gynecol 1995;85:149-55.
- The American College of Obstetrics & Gynecology. Prolog on Obstetrics. Fifth Edition. Induction of labor. Page 69. 2003.
- Stookey RA, Sokol RJ, Rosen MJ. Abnormal contraction patterns in patients during labor. Obstet Gynecol 1973;42:359-65.
- Parer JT. Handbook of Fetal Heart Rate Monitoring 2nd Edition. Saunders. Philadelphia, PA 1997.
- Freeman RK, Garite TJ, Nageotte MP, miller LA. Fetal Heart Rate Monitoring 4th Edition. Lippincott Williams and Wilkins. Philadelphia, PA. 2013.
Dr. Parer is currently a Professor in the Department of Obstetrics and Gynecology at the University of California at San Francisco. He was the Co-Chair (along with Edward J. Quilligan, M.D.) of the National Institute of Child Health and Human Development (NICHD) Research Planning Workshop on “Electronic Fetal Heart Rate Monitoring” regarding research guidelines and interpretation that was held in 1995 and 1996. The terminology developed at this workshop has now been adopted by ACOG and AWHONN.
In Dr. Parer’s distinguished career, he has authored numerous peer review articles, has given lectures on a wide variety of obstetrical topics nationwide, and has authored one of the major textbooks regarding fetal heart rate monitoring in the field of obstetrics. Dr. Parer reports no conflicts of interest.