Fetal Scalp Electrode Application

CHAPTER 168 Fetal Scalp Electrode Application

Fetal heart rate monitoring is the most common obstetric procedure in the United States, with approximately 85% of fetuses being assessed with internal or external monitoring during labor. Electronic fetal monitoring (EFM) became possible in the 1950s with Edward Hon’s design of an electrode that directly attached to the fetus. EFM entered widespread clinical use in the late 1960s despite the fact that evidence supporting its benefit was nominal (i.e., limited to case reports and retrospective studies).

Intermittent auscultation and continuous EFM are both available for assessing fetal heart rate trends. To perform intermittent auscultation, a hand-held Doppler ultrasonography transducer is used at specific intervals during labor. However, this requires a one-to-one nurse-to-patient ratio and an explicit auscultation schedule; therefore, intermittent auscultation is both difficult and expensive to perform. Although the majority of professional maternity care societies believe that some type of fetal monitoring is needed during labor, no randomized, controlled trials have compared either intermittent auscultation or continuous EFM with no monitoring. Because continuous EFM is simpler and less expensive and provides more data, it has become the default method of monitoring on most labor units, especially in the United States.

Continuous EFM is available either through external or internal monitoring. With external monitoring, a cardiotocometer measuring fetal heart rate is attached across the maternal abdomen using a belt. Internal EFM technology makes use of a bipolar spiral electrode that attaches directly to the fetal scalp. An electrical circuit is created between the wire electrode that twists into the fetal scalp and the metal wing on the electrode. Vaginal fluids create a saline electrical bridge and close the circuit. The resulting voltage difference (fetal cardiac signal) is amplified and transferred to a cardiotocometer that calculates heart rate. The bipolar wires also connect to a reference electrode on the maternal thigh to minimize electrical interference.

Both internal and external EFM are hindered by poor intraobserver consistency and a high false-positive rate. Continuous EFM is usually performed with the patient in dorsal lithotomy position and likely contributes to dysfunctional labor. Widespread use of EFM is associated with higher rates of cesarean delivery, operative vaginal delivery, and litigation. Continuous EFM has limited ability to identify a truly hypoxic–ischemic fetus and fails to decrease rates of cerebral palsy. In a recent meta-analysis of 13 randomized, controlled trials, the only clinically significant benefit for routine continuous EFM was the prevention of neonatal seizures.

With older EFM technology, internal scalp electrodes provided more accurate information on fetal heart rate trends than external monitoring. Newer-generation equipment allows for better evaluation of beat-to-beat variability (short-term variability) using the external monitor. Scalp electrodes are less frequently used as a result of these improvements in technology. Because certain situations still necessitate the use of an internal monitor, understanding both the application and use of the fetal scalp electrode is important for maternity care providers.

NOTE: Discussion of the subtleties of the abnormal fetal heart rate tracing is beyond the scope of this chapter. The practitioner of maternal and fetal care is referred to any recent comprehensive obstetrics textbook for this information. The entire clinical picture, including stage of labor, concurrent medical problems, current medications, and availability of a physician to perform an operative delivery, should be considered when making management decisions.

Indications

Most healthy, low-risk pregnancies are safely monitored with intermittent auscultation or continuous external monitoring. Higher-risk pregnancies affected by maternal or fetal medical complications often require continuous monitoring. Although continuous external monitoring is usually adequate, the following situations warrant consideration of internal monitoring with a fetal scalp electrode (FSE):

• Ineffective external monitoring or inadequate fetal tracing as a result of

• Maternal body habitus

• Excessive maternal movement

• Excessive fetal movement

• Inadequate staffing for provision of intermittent auscultation

Tags: Pfenninger and Fowlers Procedures for Primary Care Expert Consult

May 14, 2017 | Posted by admin in GENERAL & FAMILY MEDICINE | Comments Off

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