Bradycardia. In the third trimester, the normal mean baseline fetal heart rate has generally been accepted to range between 120 and 160 bpm. But, pragmatically, a rate between 100 and 119 bpm, in the absence of other changes, usually is not considered to represent fetal compromise. Such low but potentially normal baseline heart rates also have been attributed to head compression from occiput posterior or transverse positions, particularly during second-stage labor (Young, 1976). Such mild bradycardias were observed in 2 percent of monitored pregnancies and averaged approximately 50 minutes in duration. Freeman and associates (2003) have concluded that bradycardia within the range of 80 to 120 bpm and with good variability is reassuring. Interpretation of rates less than 80 bpm is problematic, and such rates generally are considered nonreassuring.
Some causes of fetal bradycardia include congenital heart block and serious fetal compromise 0aeggi, 2008; Larma, 2007). Figure 24-5 shows bradycardia in a fetus dying from placental abruption. Maternal hypothermia under general anesthesia for repair of a cerebral aneurysm or during maternal cardiopulmonary bypass for open-heart surgery can also cause fetal bradycardia. Sustained fetal bradycardia in the setting of severe pyelonephritis and maternal hypothermia also has been reported (Hankins, 1997). Involved fetuses apparently are not harmed by several hours of such bradycardia.
Tachycardia. Fetal tachycardia is deined as a baseline heart rate greater than 160 bpm. The most common explanation for fetal tachycardia is maternal fever from chorioamnionitis, although fever from any source can produce this. In some cases, fetal tachycardia may precede overt maternal fever (Gilstrap, 1987). Fetal tachycardia caused by maternal infection typically is not associated with fetal compromise unless there are associated periodic heart rate changes or fetal sepsis.
Other causes of fetal tachycardia include fetal compromise, cardiac arrhythmias, and maternal administration of parasympathetic inhibiting (atropine) or sympathomimetic (terbutaline) drugs. Prompt relief of the compromising event, such as correction of maternal hypotension caused by epidural analgesia, can result in fetal recovery. he key feature to distinguish fetal compromise in association with tachycardia seems to be concomitant heart rate decelerations.
Wandering Baseline. This baseline rate is unsteady and "wanders" between 120 and 160 bpm (Freeman, 2003). This rare finding is suggestive of a neurologically abnormal fetus and may occur as a preterminal event. In contrast, changes of the normal baseline are common in labor and do not predict morbidity (Yang, 2017).
Baseline variability is an important index of cardiovascular function and appears to be regulated largely by the autonomic nervous system (Kozuma, 1997). That is, a sympathetic and parasympathetic "push and pull" mediated via the sinoatrial node produces moment-to-moment or beat-to-beat oscillation of the baseline heart rate. Such heart rate change is deined as baseline variability. Variability can be further analyzed over the short term and long term, although these terms have fallen out of use. Short-term variabiliy relects the instantaneous change in fetal heart rate from one beat-or R wave-to the next. This variability is a measure of the time interval between cardiac systoles (Fig. 24-6). Short-term variability can most reliably be determined to be normally present only when electrocardiac cycles are measured directly with a scalp electrode. Long-term variability is used to describe the oscillatory changes during 1 minute and result in the waviness of the baseline (Fig. 24-7).