The fetal heart rate response to uterine contractions can reflect uterine perfusion or placental function. A late deceleration is a smooth, gradual, symmetrical decline in fetal heart rate beginning at or after the contraction peak and returning to baseline only after the contraction has ended. his deceleration reaches its nadir within 30 seconds of its onset. In most cases, the onset, nadir, and recovery of the deceleration occur after the beginning, peak, and ending of the contraction, respectively (Fig. 24-13). The magnitude of late decelerations is seldom more than 30 to 40 bpm below baseline and typically not more than 10 to 20 bpm. Late decelerations usually are not accompanied by accelerations. Myers and associates (1973) studied monkeys in which they compromised uteroplacenral perfusion by lowering maternal aortic blood pressure. The interval or lag from the contraction onset until the late deceleration onset was directly related to basal fetal oxygenation. hey demonstrated that the length of the lag was predictive of the fetal P02 but not fetal pH. he lower the fetal P02 before contractions, the shorter the lag to the onset of late decelerations. his lag relected the time necessary for the fetal P02 to fall below a critical level necessary to stimulate arterial chemoreceptors, which mediated the decelerations.
Murata and coworkers (1982) also showed that a late deceleration was the irst fetal heart rate consequence ofuteroplacental-induced hypoxia. During the course of progressive hypoxia that led to death over 2 to 13 days, monkey fetuses invariably exhibited late decelerations before development of acidemia. Variability of the baseline heart rate disappeared as acidemia developed.
Generally, any process that produces maternal hypotension, excessive uterine activity, or placental dysfunction can induce late decelerations. The two most common sources are hypotension from epidural analgesia and uterine hyperactivity from onset : sece: 1 ...
FIGURE 24-14 Features ofvariable fetal heart rate decelerations. Characteristics include an abrupt decline in the heart rate, and onset that commonly varies with successive contractions. The deceleration measures : 15 bpm for : 15 seconds and has an onset-to-nadir phase of <30 seconds. Total duration is <2 minutes.
oxytocin stimulation. Maternal diseases such as hypertension, diabetes, and collagen vascular disorders can cause chronic placental dysfunction. Placental abruption can produce acute late decelerations.
he most frequent deceleration patterns encountered during labor are variable decelerations attributed to umbilical cord occlusion. In a study of more than 7000 monitor tracings, variable decelerations were identified in 40 percent when labor had progressed to 5 cm dilation and in 83 percent by the end of first-stage labor (Melchior, 1985). A variable deceleration is defined as an abrupt drop in the fetal heart rate beginning with the onset of the contraction and reaching a nadir in less than 30 seconds. he decrease must last between 15 seconds and 2 minutes and must be :::15 bpm in amplitude. The onset of deceleration typically varies with successive contractions (Fig. 24-14).
Hon (1959) tested the efects of umbilical cord compression on fetal heart rate (Fig. 24-15). In experimental animals, complete occlusion of the umbilical cord produces abrupt, jagged-appearing deceleration of the fetal heart rate 24-16). Concomitantly, fetal aortic pressure rises. Itskovitz and colleagues (1983) observed that variable decelerations in fetal lambs occurred only after umbilical blood flow was reduced by at least 50 percent.