Co-published in Obstetrics & Gynecology, Vol. 112, No. 3, September 2008.
The workshop was jointly sponsored by the American College of Obstetricians and Gynecologists, the Eunice Kennedy Shriver National Institute of Child Health and Human Development, and the Society for Maternal-Fetal Medicine.
The Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) convened a series of workshops in the mid- 1990s to develop standardized and unambiguous definitions for fetal heart rate (FHR) tracings, culminating in a publication of recommendations for defining fetal heart rate characteristics (NICHD, 1997). The goal of these definitions was to allow the predictive value of monitoring to be assessed more meaningfully and to allow evidence-based clinical management of intrapartum fetal compromise.
The definitions agreed upon in that workshop were endorsed for clinical use in the most recent American College of Obstetricians and Gynecologists (ACOG) Practice Bulletin in 2005 and also endorsed by the Association of Women's Health, Obstetric and Neonatal Nurses (ACOG, 2005). Subsequently, the Royal College of Obstetricians and Gynaecologists (RCOG, 2001) and the Society of Obstetricians and Gynaecologists of Canada (SOGC, 2007) convened expert groups to assess the evidence-based use of electronic fetal monitoring (EFM). These groups produced consensus documents with more specific recommendations for FHR pattern classification and intrapartum management actions (Liston, Sawchuck, & Young, 2007; RCOG, 2001). In addition, new interpretations and definitions have been proposed, including terminology such as “tachysystole” and “hyperstimulation” and new interpretative systems using three and five tiers (Liston et al., 2007; Parer & Ikeda, 2007; RCOG, 2001). The SOGC Consensus Guidelines for Fetal Health Surveillance presents a three-tier system (normal, atypical, abnormal), as does RCOG (Liston et al., 2007; RCOG, 2001). Parer and Ikeda (2007) recently suggested a five-tier management grading system. Recently, the NICHD, ACOG, and the Society for Maternal-Fetal Medicine jointly sponsored a workshop focused on EFM. The goals of this workshop were 1) to review and update the definitions for FHR pattern categorization from the prior workshop; 2) to assess existing classification systems for interpreting specific FHR patterns and to make recommendations about a system for use in the United States; and 3) to make recommendations for research priorities for EFM. Thus, while goals 1 and 3 are similar to the prior workshop, a new emphasis on interpretative systems (goal 2) was part of the recent workshop.
As was true in the prior publication, (NICHD, 1997) before presenting actual definitions and interpretation, it is necessary to state a number of assumptions and factors common to FHR interpretation in the United States. These were defined in the initial publication (NICHD, 1997) and were affirmed and/or updated by the panel:
- A.The definitions are primarily developed for visual interpretation of FHR patterns. However, it is recognized that computerized interpretation is being developed and the definitions must also be adaptable to such applications.
- B.The definitions apply to the interpretations of patterns produced from either a direct fetal electrode detecting the fetal electrocardiogram or an external Doppler device detecting the fetal heart rate events with use of the autocorrelation technique.
- C.The record of both the FHR and uterine activity should be of adequate quality for visual interpretation.
- D.The prime emphasis in this report is on intrapartum patterns. The definitions may also be applicable to antepartum observations.
- E.The characteristics to be defined are those commonly used in clinical practice and research communications.
- F.The features of FHR patterns are categorized as either baseline, periodic, or episodic. Periodic patterns are those associated with uterine contractions, and episodic patterns are those not associated with uterine contractions.
- G.The periodic patterns are distinguished on the basis of waveform, currently accepted as either “abrupt” or “gradual” onset.
- H.Accelerations and decelerations are generally determined in reference to the adjacent baseline FHR.
- I.No distinction is made between short-term variability (or beat-to-beat variability or R–R wave period differences in the electrocardiogram) and long-term variability, because in actual practice they are visually determined as a unit. Hence, the definition of variability is based visually on the amplitude of the complexes, with exclusion of the sinusoidal pattern.
- J.There is good evidence that a number of characteristics of FHR patterns are dependent upon fetal gestational age and physiologic status as well as maternal physiologic status. Thus, FHR tracings should be evaluated in the context of many clinical conditions including gestational age, prior results of fetal assessment, medications, maternal medical conditions, and fetal conditions (eg, growth restriction, known congenital anomalies, fetal anemia, arrhythmia, etc).
- K.The individual components of defined FHR patterns do not occur independently and generally evolve over time.
- L.A full description of an EFM tracing requires a qualitative and quantitative description of:
- 1Uterine contractions.
- 2Baseline fetal heart rate.
- 3Baseline FHR variability.
- 4Presence of accelerations.
- 5Periodic or episodic decelerations.
- 6Changes or trends of FHR patterns over time.
Uterine contractions are quantified as the number of contractions present in a 10-minute window, averaged over 30 minutes. Contraction frequency alone is a partial assessment of uterine activity. Other factors such as duration, intensity, and relaxation time between contractions are equally important in clinical practice.
The following represents terminology to describe uterine activity:
- A.Normal:≤5 contractions in 10 minutes, averaged over a 30-minute window.
- B.Tachysystole:>5 contractions in 10 minutes, averaged over a 30-minute window.
- C.Characteristics of uterine contractions:
- •Tachysystole should always be qualified as to the presence or absence of associated FHR decelerations.
- •The term tachysystole applies to both spontaneous or stimulated labor. The clinical response to tachysystole may differ depending on whether contractions are spontaneous or stimulated.
- •The terms hyperstimulation and hypercontractility are not defined and should be abandoned.
Fetal heart rate patterns are defined by the characteristics of baseline, variability, accelerations, and decelerations.
The baseline FHR is determined by approximating the mean FHR rounded to increments of 5 beats per minute (bpm) during a 10-minute window, excluding accelerations and decelerations and periods of marked FHR variability (>25 bpm). There must be at least 2 minutes of identifiable baseline segments (not necessarily contiguous) in any 10-minute window, or the baseline for that period is indeterminate. In such cases, it may be necessary to refer to the previous 10-minute window for determination of the baseline. Abnormal baseline is termed bradycardia when the baseline FHR is<110 bpm; it is termed tachycardia when the baseline FHR is>160 bpm.
Baseline FHR variability is determined in a 10-minute window, excluding accelerations and decelerations. Baseline FHR variability is defined as fluctuations in the baseline FHR that are irregular in amplitude and frequency. The fluctuations are visually quantitated as the amplitude of the peak-to-trough in bpm.
Variability is classified as follows: Absent FHR variability: amplitude range undetectable. Minimal FHR variability: amplitude range>undetectable and≤5 bpm. Moderate FHR variability: amplitude range 6 bpm to 25 bpm. Marked FHR variability: amplitude range>25 bpm.
An acceleration is a visually apparent abrupt increase in FHR. An abrupt increase is defined as an increase from the onset of acceleration to the peak in<30 seconds. To be called an acceleration, the peak must be ≥15 bpm, and the acceleration must last ≥15 seconds from the onset to return. A prolonged acceleration is ≥2 minutes but <10 minutes in duration. Finally, an acceleration lasting ≥10 minutes is defined as a baseline change. Before 32 weeks of gestation, accelerations are defined as having a peak ≥10 bpm and a duration of ≥10 seconds.
Characteristics of Decelerations
|• Visually apparent usually symmetrical gradual decrease and return of the fetal heart rate (FHR) associated with a uterine contraction.|
|• A gradual FHR decrease is defined as from the onset to the FHR nadir of ≥30 seconds.|
|• The decrease in FHR is calculated from the onset to the nadir of the deceleration.|
|• The deceleration is delayed in timing, with the nadir of the deceleration occurring after the peak of the contraction.|
|• In most cases, the onset, nadir, and recovery of the deceleration occur after the beginning, peak, and ending of the contraction, respectively.|
|• Visually apparent, usually symmetrical, gradual decrease and return of the FHR associated with a uterine contraction.|
|• A gradual FHR decrease is defined as one from the onset to the FHR nadir of ≥30 seconds.|
|• The decrease in FHR is calculated from the onset to the nadir of the deceleration.|
|• The nadir of the deceleration occurs at the same time as the peak of the contraction.|
|• In most cases the onset, nadir, and recovery of the deceleration are coincident with the beginning, peak, and ending of the contraction, respectively.|
|• Visually apparent abrupt decrease in FHR.|
|• An abrupt FHR decrease is defined as from the onset of the deceleration to the beginning of the FHR nadir of<30 seconds. The decrease in FHR is calculated from the onset to the nadir of the deceleration.|
|• The decrease in FHR is ≥15 beats per minute, lasting ≥15 seconds, and<2 minutes in duration.|
|• When variable decelerations are associated with uterine contractions, their onset, depth, and duration commonly vary with successive uterine contractions.|
Decelerations are classified as late, early, or variable based on specific characteristics (see the box, “Characteristics of Decelerations”). Variable decelerations may be accompanied by other characteristics, the clinical significance of which requires further research investigation. Some examples include a slow return of the FHR after the end of the contraction, biphasic decelerations, tachycardia after variable deceleration(s), accelerations preceding and/or following, sometimes called “shoulders” or “overshoots,” and fluctuations in the FHR in the trough of the deceleration.
A prolonged deceleration is present when there is a visually apparent decrease in FHR from the baseline that is ≥15 bpm, lasting ≥2 minutes, but<10 minutes. A deceleration that lasts ≥10 minutes is a baseline change.
A sinusoidal fetal heart rate pattern is a specific fetal heart rate pattern that is defined as having a visually apparent, smooth, sine wave–like undulating pattern in FHR baseline with a cycle frequency of 3–5/min that persists for ≥20 minutes.