As with beat-to-beat variability, accelerations represent intact neurohormonal cardiovascular control mechanisms linked to fetal behavioral states. Krebs and colleagues (1982) analyzed electronic heart rate tracings in nearly 2000 fetuses and found sporadic accelerations during labor in 99.8 percent. Fetal heart rate accelerations during the irst or last 30 minutes during labor, or both, were a favorable sign for fetal well-being. The absence of such accelerations during labor, however, is not necessarily an unfavorable sign unless coincidental with other nonreassuring changes. he chance of acidemia in the fetus that fails to respond to stimulation in the presence of an otherwise nonreassuring pattern approximates 50 percent (Clark, 1984; Smith, 1986).
his physiological response shows a gradual fetal heart rate decline and then return to baseline associated with a contraction (Fig.
Freeman and associates (2003) defined early decelerations as those generally seen in active labor between 4 and 7 cm cervical dilation. In their deinition, the degree of deceleration is generally proportional to the contraction strength and rarely falls below 100 to 110 bpm or 20 to 30 bpm below baseline. Such decelerations are common during active labor and not associated with tachycardia, loss of variability, or other fetal heart rate changes. Importantly, early decelerations are not associated with fetal hypoxia, acidemia, or low Apgar scores.
FIGURE 24-11 Features of early fetal heart rate deceleration. Characteristics include a gradual decline in the heart rate with both onset and recovery coincident with the onset and recovery of the contraction. The nadir of the deceleration is 30 seconds or more after the deceleration onset.
Head compression probably causes vagal nerve activation as a result of dural stimulation, and this mediates the heart rate deceleration (Paul, 1964). Ball and Parer (1992) concluded that fetal head compression is a likely cause not only of the deceleration shown in Figure 24-11 but also of those shown in Figure 24-12, which typically occur during second-stage labor.
FIGURE 24-12 Two diferent fetal heart rate patterns during second-stage labor that are likely both due to head compression (upper pane0. Maternal pushing eforts (lower pane0 correspond to the spikes with uterine contractions. Fetal heart rate deceleration () is consistent with the pattern of head compression shown in Figure 24-1o1. Deceleration (B), however, is "variable" in appearance because of its jagged configuration and may alternatively represent cord occlusion.
FIGURE 24-13 Features of late fetal heart rate deceleration. Characteristics include gradual decline in the heart rate with the contraction nadir, and recovery occurring after the end ofthe contraction. The nadir ofthe deceleration occurs 30 seconds or more after the onset of the deceleration.
Indeed, they observed that head compression is the likely cause of many variable decelerations classically attributed to cord compression.