The evolution of standardized fetal heart rate terminology
In 1997, the National Institute of Child Health and Human Development (NICHD) proposed standardized definitions for fetal heart rate tracings [1]. In 2005 and 2006, the American College of Obstetricians and Gynecologists (ACOG), the Association of Women’s Health, Obstetric and Neonatal Nurses (AWHONN) and the American College of Nurse Midwives (ACNM) officially endorsed the NICHD definitions summarized in Table 42.1. The standardized FHR definitions were reaffirmed by a second NICHD consensus report in 2008 [2]. In July, 2009, the definitions were again endorsed in ACOG Practice Bulletin 106 [3].
Table 42.1 Standardized FHR definitions
| Pattern | Definition |
| Baseline | Mean FHR rounded to increments of 5 bpm during a 10 min segment, excluding accelerations, decelerations and periods of marked variability |
| The baseline must be at least 2 min in any 10-min segment (not necessarily contiguous) | |
| Normal baseline FHR range 110–160 bpm | |
| Baseline >160 bpm = tachycardia; baseline <110 bpm = bradycardia | |
| Variability | Fluctuations in the FHR baseline that are irregular in amplitude and frequency |
| Quantitated as the amplitude of peak-to-trough in bpm | |
| Absent – amplitude range undetectable | |
| Minimal – amplitude range detectable ≤5 bpm | |
| Moderate (normal) – amplitude range 6–25 bpm | |
| Marked – amplitude range >25 bpm | |
| Accelerations | Abrupt increase (onset to peak <30 s) in the FHR from the most recently calculated baseline |
| At ≥32 weeks, an acceleration peaks ≥15 bpm above baseline and lasts ≥15 s but <2 min | |
| At <32 weeks, an acceleration peaks ≥10 bpm above baseline and lasts ≥10 s but <2 min | |
| Prolonged acceleration lasts ≥2 min but <10 min; acceleration ≥10 min is a baseline change | |
| Decelerations | |
| Early | Gradual (onset to nadir ≥30 s) decrease in FHR during a uterine contraction |
| Nadir of the deceleration occurs at the same time as the peak of the contraction | |
| Late | Gradual (onset to nadir ≥30 s) decrease in FHR during a uterine contraction |
| Onset, nadir, and recovery occur after the beginning, peak, and end of the contraction | |
| Variable | Abrupt (onset to nadir <30 s), decrease in the FHR ≥15 bpm below the baseline lasting ≥15 s but less than 2 min |
| Prolonged | Deceleration ≥15 bpm below baseline lasting ≥2 min or more but <10 min |
| If a deceleration lasts 10 minutes or longer, it is a baseline change | |
| Sinusoidal pattern | Visually apparent, smooth, sine wave-like undulating pattern in FHR baseline with a cycle frequency of 3–5 per minute which persists for 20 minutes or more |
Evidence-based interpretation of fetal heart rate patterns
This chapter will review standardized FHR terminology and the physiology underlying FHR patterns. Supporting evidence will be stratified according to the method outlined by the US Preventive Services Task Force (Box 42.1) [4]. Level I evidence is considered to be the most robust and level III, the least.
The primary objective of intrapartum FHR monitoring is to assess fetal oxygenation during labor. However, a number of factors can influence the appearance of a FHR tracing via mechanisms unrelated to fetal oxygenation. Some examples are summarized in Table 42.2. If a FHR abnormality is thought to be related to any of these factors, individualized management is directed at the specific underlying process. The following discussion of FHR physiology and interpretation will focus on FHR patterns related specifically to fetal oxygenation.
Terminology, physiology and interpretation of specific fetal heart rate patterns
Baseline rate
Baseline FHR is defined as the mean FHR rounded to increments of 5 bpm during a 10-minute segment, excluding accelerations, decelerations and periods of marked variability. Normal FHR baseline ranges from 110 to 160 bpm.
Physiology
Baseline FHR is regulated by cardiac pacemakers and conduction pathways, autonomic innervation, humoral factors (catecholamines), extrinsic factors (medications) and local factors (calcium, potassium). Autonomic input regulates the FHR in response to fluctuations in PO2, PCO2 and blood pressure detected by chemoreceptors and baroreceptors.
Tachycardia
Baseline FHR in excess of 160 bpm is defined as tachycardia. There are many potential causes of fetal tachycardia (see Table 42.2). One possible cause is recurrent or sustained interruption of fetal oxygenation leading to metabolic acidemia and blunting of parasympathetic cardiac innervation. If interrupted oxygenation is the cause of fetal tachycardia, other FHR changes may be present, including decelerations, loss of variability and loss of accelerations. The scientific evidence supporting a relationship between fetal tachycardia and interrupted oxygenation primarily is level III.
Bradycardia
According to the definitions proposed by the NICHD, “bradycardia” is a baseline rate below 110 bpm, while a “deceleration” is a periodic or episodic fall in heart rate that interrupts the baseline. Decelerations are common and can reflect interrupted fetal oxygenation. On the other hand, true baseline bradycardia is rare and is not specifically related to fetal oxygenation. Causes of fetal bradycardia are summarized in Table 42.2.
Box 42.1 US Preventive Services Task Force stratification of scientific evidence
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