The termetal distress is too broad and vague to be applied with any precision to clinical situations (American College ofObstetricians and Gynecologists, 2014). Uncertainty regarding the diagnosis based on interpretation of fetal heart rate patterns has given rise to descriptions such as reassuring or nonreassuring. he term "reassuring" suggests a restoration of confidence in the health of the fetus by a particular pattern. In contrast, a "nonreassuring" designation suggests inability to remove doubt. hese patterns during labor are dynamic, and they can rapidly change from reassuring to nonreassuring and vice versa.
These assessments are subjective clinicaljudgments that are inevitaby subject to impeection and must be reconized as such.
The diiculty in assigning a nonreassuring label to fetal heart rate patterns stems in part from the fact that these patterns are more a reflection of fetal physiology than of pathology. Physiological control of heart rate includes various interconnected mechanisms that depend on blood flow and oxygenation. Moreover, the activity of these control mechanisms is inluenced by the preexisting state of fetal oxygenation, for example, as seen with chronic placental insuiciency. Importantly, the fetus is tethered by an umbilical cord, whereby blood flow is constantly in jeopardy. Moreover, normal labor is a process of increasing acidemia (Rogers, 1998). hus, normal labor is a process of repeated fetal hypoxic events that can infrequently lead to signiicant acidemia.
Identiication of "fetal distress" based on fetal heart rate patterns is imprecise and controversial. Experts in interpretation of these patterns often disagree with each other. Ayres-de-Campos and colleagues (1999) investigated interobserver agreement of fetal heart rate pattern interpretation and found that agreement-or conversely, disagreement-was related to whether the pattern was normal, suspicious, or pathological. Speciically, experts agreed on 62 percent of normal patterns, 42 percent of suspicious patterns, and only 25 percent of pathological patterns. Keith and coworkers (1995) asked each of 17 experts to review 50 tracings on two occasions, at least 1 month apart. Approximately 20 percent changed their own interpretations, and approximately 25 percent did not agree with the interpretations of their colleagues.
To develop standardized and unambiguous deinitions of fetal heart rate (FHR) tracings, the NICHD (1997) held a succession of workshops in 1995 and 1996 and published recommendations for interpreting these patterns. As previously shown in Table 24-1, a second workshop was convened to reevaluate these recommendations and clariy terminology (Macones, 2008). A major result was the recommendation of a three-tier system for classification of FHR patterns (1 able 24-2). The American College of Obstetricians and Gynecologists (2017b) has recommended use of this tiered system.
A few studies have assessed this three-tiered system. J ackson and coworkers (201i1) studied 48,444 women in labor and found that category I (normal FHR) patterns were observed during labor in 99.5 percent of tracings. Category II (indeterminate FHR) patterns were found in 84.i1 percent of tracings, and category III (abnormal FHR) patterns were seen in 0.1 percent (54 women). Most-84 percent of women-had a mix of categories during labor. Cahill and colleagues (2012) retrospectively correlated the incidence of umbilical cord acidemia (pH :;7.10) with fetal heart rate characteristics during the 30 minutes preceding delivery. None of the three categories demonstrated a significant association with cord blood acidemia. The American College of Obstetricians and Gynecologists and the American Academy of Pediatrics (2014) concluded that a category I or II tracing with a 5-minute Apgar scorei> 7 or with normal arterial blood acid-base values was not consistent with an acute hypoxic-ischemic event.