Some have defined intrapartum sine wavelike baseline variation with periods of acceleration as pseudosinusoidal. Murphy and colleagues (1991) reported that pseudosinusoidal patterns were seen in 15 percent of monitored labors. Mild pseudosinusoidal patterns were associated with use of meperidine and epidural analgesia. Intermediate pseudosinusoidal patterns were linked to fetal sucking or transient episodes of fetal hypoxia caused by umbilical cord compression. Egley and associates (1991) reported that 4 percent of fetuses demonstrated sinusoidal patterns transiently during normal labor. hese authors observed patterns persisting for up to 90 minutes in some cases.
The pathophysiology of sinusoidal patterns is unclear, in part due to various deinitions. There seems to be general agreement that antepatum sine wave baseline undulations portend severe fetal anemia. Still, few anti-D alloimmunized fetuses develop this pattern (Nicolaides, 1989). The sinusoidal pattern has been reported to develop or disappear ter fetal transusion (Del Valle, 1992; Lowe, 1984). Ikeda and associates (1999) proposed that the pattern is related to waves of arterial blood pressure, reflecting oscillations in the baroreceptor-chemoreceptor feedback mechanism.
These refer to deviations from baseline that are temporally related to uterine contractions. Acceleration refers to a rise in fetal heart rate above baseline, and deceleration is a drop below the baseline rate. The nomenclature most commonly used in the United States is based on the timing of the deceleration in relation to contractions-thus, eary, late, or variable. he waveform of these decelerations is also significant for pattern recognition.
In early and late decelerations, the slope of fetal heart rate change is gradual, resulting in a curvilinear and uniform or symmetrical waveform. With variable decelerations, the slope of fetal heart rate change is abrupt and erratic, giving the waveform a jag ged appearance. The NICHD Workshop (1997) proposed that decelerations be defined as recurrent if they accompanied �50 percent of contractions in any 20-minute period.
Another system now used less often to describe decelera tions is based on the pathophysiological events considered most likely to underlie the pattern. In this system, early decelera tions are termed head compression, late decelerations are termed uteroplacental insuiciency, and variable decelerations are cord compression patterns.
hese are abrupt heart rate increases above the fetal heart rate baseline and defined by n onset-to-peak rise within 30 seconds (American College of Obstetricians and Gynecologists, 2017a). At 32 weeks' gestation and beyond, an acceleration has a peak � 15 bpm above baseline. Its duration is � 15 sec but <2 minutes from onset to baseline return (see Table 24-1). Before 32 weeks, a peak �10 bpm for 10 seconds to 2 minutes is considered normal. Prolonged acceleration is defined as �2 minutes but < 10 minutes.
According to Freeman and coworkers (2003), accelerations most often occur antepartum, in early labor, and in association with variable decelerations. Proposed mechanisms for intrapartum accelerations include fetal movement, stimulation by uterine contractions, umbilical cord occlusion, fetal stimulation during pelvic examination, scalp blood sampling, and acoustic stimulation. Accelerations are common during labor. These are virtually always reassuring and almost always conirm that the fetus is not acidemic at that time.