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To study the forces exerted by labour, a rubber bag was inserted into the uterus which was connected with a manometer. In this way it was found that the intra-uterine pressure, in the intervals between the contractions, was represented by a column of mercury 20 millimeters high, 5 of which were due to the tonicity of the walls and 15 to its contents. During the pains, however, the mercury rose considerably, reaching a height of from 80 to 250 millimeters.
—J. Whitridge Williams (1903)
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Little is written in the first edition of this textbook concerning monitoring of the fetus during labor. Much later, periodic auscultation of the fetal heartbeat with a fetoscope was adopted. These practices were eclipsed in the late 1960s and early 1970s by the development of electronic fetal monitoring (Hon, 1958). It was hoped that the continuous graph-paper portrayal of the fetal heart rate was potentially diagnostic in assessing pathophysiological events affecting the fetus.
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When first introduced, electronic fetal heart rate monitoring was used primarily in complicated pregnancies but gradually became used in most pregnancies. Now, more than 85 percent of all live births in the United States undergo electronic fetal monitoring (Ananth, 2013).
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ELECTRONIC FETAL MONITORING
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Internal (Direct) Electronic Monitoring
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Direct fetal heart measurement is accomplished by attaching a bipolar spiral electrode directly to the fetus (Fig. 24-1). The wire electrode penetrates the fetal scalp, and the second pole is a metal wing on the electrode. The electrical fetal cardiac signal—P wave, QRS complex, and T wave—is amplified and fed into a cardiotachometer for heart rate calculation. The peak R-wave voltage is the portion of the fetal electrocardiogram (ECG) most reliably detected.
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An example of the method of fetal heart rate processing employed when a scalp electrode is used is shown in Figure 24-2. Time (t) in milliseconds between fetal R waves is fed into a cardiotachometer, where a new fetal heart rate is set with the arrival of each new R wave. As also shown in Figure 24-2, a premature atrial contraction is computed as a heart rate acceleration because the interval (t2) is shorter than the preceding one (t1). The phenomenon of continuous R-to-R wave fetal heart rate computation is known as beat-to-beat variability.
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