Uterine Activity

Table 11-1 describes normal uterine activity (UA) during labor.

Fetal Compromise

The goals of intrapartum FHR monitoring are to identify and differentiate the normal (reassuring) patterns from the abnormal (nonreassuring) patterns, which can be indicative of fetal compromise. Although the 2008 National Institute of Child Health and Human Development workshop (Macones et al., 2008) and a recent ACOG Practice Bulletin (2009) both recommend use of the terms normal and abnormal to describe FHR tracings, the terms reassuring and nonreassuring are still frequently used clinically.

Abnormal FHR patterns are those associated with fetal hypoxemia, which is a deficiency of oxygen in the arterial blood. If uncorrected, hypoxemia can deteriorate to severe fetal hypoxia, which is an inadequate supply of oxygen at the cellular level. Examples of abnormal FHR patterns include the following (Macones et al., 2008).

Absent baseline FHR variability and any of the following:

Intermittent Auscultation

Intermittent auscultation (IA) involves listening to fetal heart sounds at periodic intervals to assess the FHR. IA of the fetal heart can be performed with a Pinard stethoscope, a Doppler ultrasound device (Fig.11-1, A) an ultrasound stethoscope (Fig. 11-1, B) or a DeLee-Hillis fetoscope (Fig. 11-1, C). The fetoscope is applied to the listener’s forehead because bone conduction amplifies the fetal heart sounds for counting. The Doppler ultrasound device and ultrasound stethoscope transmit ultrahigh-frequency sound waves reflecting movement of the fetal heart and convert these sounds into an electronic signal that can be counted. Box 11-1 describes how to perform IA.

IA is easy to use, inexpensive, and less invasive than EFM. It is often more comfortable for the woman and gives her more freedom of movement. Other care measures, such as ambulation and the use of baths or showers, are easier to carry out when IA is used. On the other hand, IA may be difficult to perform in women who are obese. Because IA is intermittent, significant events may occur during a time when the FHR is not auscultated. Also, IA does not provide a permanent documented visual record of the FHR and cannot be used to assess visual patterns of the FHR variability or periodic changes (Albers, 2007; Tucker et al., 2009). By using IA the nurse can assess the baseline FHR, rhythm, and increases and decreases from baseline. The recommended optimal frequency for IA in low risk women during labor has not been determined (Nageotte & Gilstrap, 2009).

Every effort should be made to use the method of fetal assessment the woman desires, if possible. However, adherence to the frequency guides can make using IA difficult in today’s busy labor and birth units because when used as the primary method of fetal assessment, auscultation requires a 1 : 1 nurse-to-patient staffing ratio. If acuity and census change such that auscultation standards are no longer met, then the nurse must inform the physician or nurse-midwife that continuous EFM will be used until staffing can be arranged to meet the standards.

The woman can become anxious if the examiner cannot readily count the fetal heartbeats. The inexperienced listener often needs time to locate the heartbeat and find the area of maximal intensity. To allay the mother’s concerns, she can be told that the nurse is “finding the spot where the sounds are loudest.” If the examiner cannot locate the fetal heartbeat, assistance should be requested. In some cases, ultrasound can be used to help locate the fetal heartbeat. Seeing the FHR on the ultrasound screen will be reassuring to the mother if locating the best area for auscultation was initially difficult.

When using IA, UA is assessed by palpation. The examiner should keep his or her hand placed over the fundus before, during, and after contractions. The contraction intensity is usually described as mild, moderate, or strong. The contraction duration is measured in seconds, from the beginning to the end of the contraction. The frequency of contractions is measured in minutes, from the beginning of one contraction to the beginning of the next. The examiner should keep his or her hand on the fundus after the contraction is over to evaluate uterine resting tone or relaxation between contractions. Resting tone between contractions is usually described as soft or relaxed.

Accurate and complete documentation of fetal status and UA is especially important when IA and palpation are being used because no paper tracing record or computer storage of these assessments is provided, as is the case with continuous EFM. Labor flow records or computer charting systems that prompt notations of all assessments are useful for ensuring such comprehensive documentation.

Electronic Fetal Monitoring

The purpose of electronic FHR monitoring is the ongoing assessment of fetal oxygenation. The goal is to detect fetal hypoxia and metabolic acidosis during labor so that interventions to resolve the problem can be implemented in a timely manner before permanent damage or death occur (Garite, 2007).

The two modes of EFM include the external mode, which uses external transducers placed on the maternal abdomen to assess FHR and UA, and the internal mode, which uses a spiral electrode applied to the fetal presenting part to assess the FHR and an intrauterine pressure catheter (IUPC) to assess UA and pressure. The differences between the external and internal modes of EFM are summarized in Table 11-2.

External monitoring

Separate transducers are used to monitor the FHR and UCs (Fig. 11-2). The ultrasound transducer works by reflecting high-frequency sound waves off a moving interface: in this case the fetal heart and valves. Reproducing a continuous and precise record of the FHR is sometimes difficult because of artifacts introduced by fetal and maternal movement. The FHR is printed on specially formatted monitor paper. The standard paper speed used in the United States is 3 cm/min. Once the area of maximal intensity of the FHR has been located, conductive gel is applied to the surface of the ultrasound transducer, and the transducer is then positioned over this area and held securely in place using an elastic belt.

The tocotransducer (tocodynamometer) measures UA transabdominally. The device is placed over the uterine fundus and held securely in place using an elastic belt (see Fig. 11-2). UCs or fetal movements depress a pressure-sensitive surface on the side next to the abdomen. The tocotransducer can measure and record the frequency and approximate duration of UCs but not their intensity. This method is especially valuable for measuring UA during the first stage of labor in women with intact membranes or for antepartum testing. Because the tocotransducer of most electronic fetal monitors is designed for assessing UA in the term pregnancy, it may not be sensitive enough to detect preterm UA. When monitoring the woman in preterm labor, remember that the fundus may be located below the level of the umbilicus. The nurse may need to rely on the woman to indicate when UA is occurring and to use palpation as an additional way of assessing contraction frequency and validating the monitor tracing.

The external transducer is easily applied by the nurse, but it must be repositioned as the woman or fetus changes position (see Fig. 11-2, B). The woman is asked to assume a semi-sitting or a lateral position. Use of an external transducer confines the woman to bed. Portable telemetry monitors allow observation of the FHR and UC patterns by means of centrally located electronic display stations. These portable units permit the woman to walk around during electronic monitoring.

Internal monitoring

The technique of continuous internal FHR or UA monitoring allows a more accurate appraisal of fetal well-being during labor than external monitoring because it is not interrupted by fetal or maternal movement or affected by maternal size (Fig. 11-3). For this type of monitoring the membranes must be ruptured and the cervix sufficiently dilated (at least 2-3 cm) to allow placement of the spiral electrode or IUPC or both. Internal and external modes of monitoring may be combined (i.e., internal FHR with external UA or external FHR with internal UA) without difficulty.

Internal monitoring of the FHR is accomplished by attaching a small spiral electrode to the presenting part. For UA to be monitored internally, a solid IUPC is introduced into the uterine cavity. The catheter has a pressure-sensitive tip that measures changes in intrauterine pressure. As the catheter is compressed during a contraction, pressure is placed on the pressure transducer. This pressure is then converted into a pressure reading in millimeters of mercury. The average pressure during a contraction ranges from 50 to 85 mm Hg. The IUPC can measure the frequency, duration, and intensity of UC, as well as uterine resting tone.

The FHR and UA are displayed on the monitor paper, with the FHR in the upper section and UA in the lower section. Fig. 11-4 contrasts the internal and external modes of electronic monitoring. Note that each small square on the monitor paper or screen represents 10 seconds; each larger box of six squares equals 1 minute (when paper is moving through the monitor at the rate of 3 cm/min).

Baseline Fetal Heart Rate

The intrinsic rhythmicity of the fetal heart, the central nervous system (CNS), and the fetal autonomic nervous system control the FHR. An increase in sympathetic response results in acceleration of the FHR, whereas an increase in parasympathetic response produces a slowing of the FHR. A balanced increase of sympathetic and parasympathetic response usually occurs during contractions, with no observable change in the baseline FHR.

The baseline fetal heart rate is the average rate during a 10-minute segment that excludes periodic or episodic changes, periods of marked variability, and segments of the baseline that differ by more than 25 beats/min (Macones et al., 2008). The normal range at term is 110 to 160 beats/min. The baseline rate is documented as a single number, rather than a range (Tucker et al., 2009).

Variability

Variability of the FHR can be described as irregular waves or fluctuations in the baseline FHR of two cycles per minute or greater (Macones, et al., 2008). It is a characteristic of the baseline FHR and does not include accelerations or decelerations of the FHR. Variability is quantified in beats per minute and is measured from the peak to the trough of a single cycle. Four possible categories of variability have been identified: absent, minimal, moderate, and marked (Fig. 11-5). In the past, variability was described as either long term or short term (beat to beat). The NICHD definitions do not distinguish between long- and short-term variability, however, because in actual practice, they are visually determined as a unit (NICHD, 1997).

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You may also need

• Fetal Assessment During Labor
• Labor and Birth Processes
• Nursing Care of the Family during Labor and Birth
• Management of Discomfort
• Assessment of High Risk Pregnancy
• Maternal Physiologic Changes
• Pregnancy at Risk: Gestational Conditions
• Postpartum Complications

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