Background
Internal cardiac monitoring with a scalp lead is used to identify fetuses at risk for serious complications from hypoxia during labor. Labor represents a unique stress on the fetus in utero. Uterine contractions reduce the flow of maternal blood to the placenta, in some cases uncovering a deficit in placental capacity. In addition, labor and the associated rupture of the chorioamnion may be associated with various other pregnancy risks, including intrauterine bacterial infection, umbilical cord compression, and placental abruption.
Recording the fetal heart rate in labor was developed in the 1960s as a means of assessing fetal status and documenting fetal well-being. Currently, 2 technologies are commonly available: an “external” monitor that uses Doppler technology to identify fetal heart motion via a sensor applied to the maternal abdomen and an “internal” monitor that directly records the fetal ECG via a single electrode applied to the fetal scalp (a fetal scalp electrode). Inherent differences are observed between the signal recordings via these 2 methods. The fetal scalp electrode detects actual beat-to-beat electrical signals of the fetal heart, whereas the external monitor uses an "averaging algorithm" to smooth the signal generated from the Doppler.
Regardless of the technology, the result is a recorded fetal heart rate. The fetal heart rate pattern is interpreted according to a standard set of criteria [1] with the goal being to identify fetuses at risk for serious complications during labor. The system remains imperfect, as a large number of fetuses display patterns that are neither clearly normal (category 1) or clearly abnormal (category 3). However, the ability to perform continuous fetal heart monitoring in labor is part of the standard of care for an obstetric unit in the United States.
Indications
Fetal heart rate monitoring via a fetal scalp electrode is indicated whenever other monitoring modalities are unsatisfactory in a laboring patient with ruptured membranes. For a low-risk patient, intermittent auscultation of the fetal heart may provide acceptable monitoring. In many cases, however, a continuous record of the fetal heart rate is indicated or is preferred by the patient or her physician. When used for continuous monitoring in labor, an external monitor may not provide a continuous record. Studies confirm that external monitoring yields a more intermittent fetal heart record in labor, especially in the second stage. [2] In the patient with an at-risk fetus, this loss of signal may be unacceptable.
In addition, fetal heart monitoring may be more difficult with an external monitor in some categories of patients, such as very obese mothers or those with fetal arrhythmias. In a study by Frolova et al, 65.4% of women with obesity had internal monitors placed during labor, compared with 50.5% of women without obesity. [3]
In the past, a fetal electrode was necessary to obtain a valid assessment of fetal heart rate beat-to-beat variability (an important component of the interpretation of the intrapartum fetal heart rate pattern). New criteria no longer include a specific assessment of beat-to-beat variability; rather, they incorporate a combined assessment of short-term and long-term variability. Further, current fetal monitors have improved computerized processing of the Doppler signal from the external monitor (“autocorrelation”), which permits more accurate assessment of the heart rate variability and reduces the need for internal heart rate monitoring.
The electrode is removed at the time of delivery. Earlier electrode removal may be performed prior to cesarean or vacuum delivery or if the electrode ceases to function.
Contraindications
Placement of a fetal electrode is relatively or absolutely contraindicated in the following circumstances:
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The fetal membranes are intact.
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Delivery is not planned. The patient is not in labor or labor is to be stopped if possible.
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Any contraindication to cervical examination, such as placenta previa, is present.
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The mother declines the procedure.
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Infectious risks to the fetus (eg, HIV, active herpes) are recognized.
Of note, fetal electrodes may be applied to breech presentations.
Guidelines from the Society for Maternal-Fetal Medicine suggest that internal fetal monitors be avoided in patients with hepatitis C virus infection unless necessary, such as when the fetal heart rate is unable to be traced with external monitors and the alternative is cesarean delivery. [4]
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Parts of a fetal scalp electrode (elements that remain after introduction of the electrode are 2, 3 and 11, marked with a star (*) 1: Overall electrode with introducer 2: Electrode hub* 3: Spiral electrode* 4: Knob (used to turn the electrode assembly during application) 5: Sheath (transmits turning motion to the electrode hub) 6: Connection between sheath and hub 7: Tube (protects maternal tissues during electrode introduction) 8: Direction of motion of tube during introduction 9: Direction of rotation to set electrode 10: Direction of motion for removal of introducer after electrode is seated 11: Electrode wires*
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Application of the scalp electrode: after introduction of the system into the maternal vagina, the introducer is placed against the fetal head, over the parietal or occipital bone (inset). Pressure is applied to the electrode, and a rotary motion is used to drive the needle electrode into the fetal scalp. The introducer is then removed, leaving the electrode in place.
