The interpretation of fetal heart rate patterns can be problematic without deinitions and nomenclature. In one example, Blackwell and colleagues (2011) asked three .1aternal-Fetal Medicine specialists to independently interpret 154 fetal heart rate tracings. Interobserver agreement was poor for the most ominous tracings and moderate for less severe patterns.
The National Institute of Child Health and Human Development (NICHD) Research Planning Workshop (1997) brought together investigators with exper tise in the ield to propose standardized, tion of fetal heart rate patterns during labor. This workshop reconvened in 2008.
he deinitions proposed as a result of this second workshop are used in this chapter and have been adopted by the American
College of Obstetricians and Gynecolo
Importantly, interpretation of electronic fetal heart rate data is based on the visual pattern of the heart rate as portrayed on chart recorder graph paper. Thus, the choice of vertical and horizontal scaling greatly afects the appearance of the fetal heart rate. Scal ing factors recommended by the NICHD
Workshop are 30 beats per minute (beats/ min or bpm) per vertical cm (range, 30 to paper speed. Fetal heart rate variation is falsely displayed at the slower 1 cm/ min paper speed compared with that of the smoother baseline recorded at 3 cm/min. hus, pattern recognition can be con siderably distorted depending on the scaling factors used.
This refers to the modal characteristics that prevail apart from periodic accelerations or decelerations associated with uterine contractions. Descriptive characteristics of baseline fetal heart activity include rate, beat-to-beat variabili, etal arrhythmia, and distinct patterns such as sinusoidal or saltatory fetal heart rates.
With increasing fetal maturation, the heart rate decreases. This continues postnatally such that the average rate is 85 bpm by age 8 years (Tintinalli, 2016). Pillai and James (1990) reported that the baseline fetal heart rate declined an average of 24 bpm between 16 weeks' gestation and term, or approximately 1 bpm per week. his normal gradual slowing of the fetal heart rate is thought to correspond to maturation of parasympathetic (vagal) heart control (Renou, 1969).
The baseline fetal heart rate is the approximate mean rate rounded to increments of 5 bpm during a 10-minute tracing segment. In any 10-minute window, the minimum interpretable baseline duration must be at least 2 minutes. If the baseline fetal heart rate is less than 110 bpm, it is termed bradycardia . If the baseline rate is greater than 160 bpm, it is called tachycardia. The average fetal heart rate is considered the result of tonic balance between accelerator and decelerator influences on pacemaker cells. In this concept, the sympathetic system is the accelerator influence, and the parasympathetic system is the decelerator factor mediated by vagal slowing of heart rate (Dawes, 1985). Heart rate also is under the control of arterial chemoreceptors such that both hypoxia and hypercapnia can modulate rate. More severe and prolonged hypoxia, with a
TABLE 24-' . Electronic Fetal Monitoring Definitions
Baseline • The mean FHR rounded to increments of 5 bpm during a 1 O-min segment, excluding: -Periods of marked FHR variability -Segments of baseline that difer by more than 25 bpm
The baseline must be for a minimum of 2 min in any 1 O-min segment or the baseline for that time period is indeterminate. In this case, one may refer to the prior 1 O-min window for determination of baseline.
Normal FHR baseline: 11s0-1s60 bpm
Tachycardia: FHR baselines is greater than 160 beats per minute
Bradycardia: FHR baseline is less than 11s0 beats per minute