FETAL HEART RATE (FHR) MONITORING
Normal FHR findings are highly reassuring of fetal well-being. Abnormal FHR findings are poor predictors of fetal compromise. Wide usage of electronic FHR monitoring has not lowered the rate of cerebral palsy (CP) because the antecedents of CP appear not to be intrapartum events but rather antenatal events. The false-positive rate for electronic FHR monitoring for predicting CP is >99%.
Modalities of Labor Monitoring
Both of the following modalities are equivalent in predicting fetal outcome.
41 Intermittent auscultation of FHR is performed with a fetoscope using auditory FHRcounting averaged for 10–15 s.
42 Electronic monitoring measures the milliseconds between consecutive cardiac cyclesgiving an instantaneous FHR continuously.
External Devices
These are placed on the uterine fundus and are the most common devices used. Advantages are utilization before significant cervical dilation and membrane rupture. Disadvantages include poor quality tracing with maternal obesity, and maternal discomfort from the device belts.
• Fetal. A continuous ultrasound transducer picks up fetal cardiac motion but also canregister maternal great vessel pulsations.
• Contractions. A tocographic transducer device senses the change in uterine wall mus-cle tone. It can measure the beginning and ending of contractions but cannot assess contraction intensity.
Figure I-16-1. Electronic Fetal Heart Rate Monitor
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Internal Devices
These are placed through the dilated cervix. Advantages include optimum signal quality, which is unaffected by maternal obesity. Disadvantages include limitation to labor when cervical dila-tion and membrane rupture have occurred.
• Fetal. A direct scalp electrode precisely senses each QRS complex of the fetal cardiaccycle. Complications can include fetal scalp trauma and infection.
• Contractions. An intrauterine pressure catheter (IUPC), placed into the uterine cavity,precisely registers intrauterine hydrostatic changes with each contraction.
Figure I-16-2. Internal Fetal Heart Rate Monitoring
INTRAPARTUM FETAL HEART RATE MONITORING
Electronic Fetal Monitoring (EFM) Definitions
Baseline Fetal Heart Rate (FHR): The mean FHR rounded to increments of 5 beats/min duringa 10-minute segment. Normal FHR baseline: 110–160 beats/minute
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Tachycardia: FHR baseline is >160 beats/min
• Non-hypoxic explanations include:
– Maternal: medications (β-adrenergic agonists [terbutaline], atropine, scopolamine), fever, thyrotoxicosis
– Fetal: repetitive accelerations (from fetal movements), fetal tachyarrythmias, prematurity
Bradycardia: FHR baseline is <110 beats/min
• Non-hypoxic explanations include:
– Maternal medications: β-adrenergic blockers, local anesthetics
– Fetal arrhythmia: congenital heart block (associated with maternal lupus)
Baseline variability: Fluctuations in the baseline FHR that are irregular in amplitude andfrequency. It is a reflection of the autonomic interplay between the sympathetic and parasym-pathetic nervous system.
• Absent amplitude range undetectable
• Minimal amplitude range detectable but<5 beats/min
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• Moderate (normal): amplitude range 6-25 beats/min
• Marked: amplitude range >25 beats/min
Acceleration: A visually apparent abrupt increase (onset to peak in <30 seconds) in the FHR.These are mediated by the sympathetic nervous system in response to fetal movements or scalp stimulation.
• At ≥32 weeks gestation, an acceleration has a peak of >15 beats/min above baseline,with a duration of >15 seconds but < 2 min from onset to return.
• At <32 weeks gestation, an acceleration has a peak of >10 beats/min above baseline,with a duration of >10 sec but <2 min from onset to return.
Early deceleration: A visually apparent usually symmetrical gradual decrease and return of theFHR associated with a uterine contraction. These are mediated by parasympathetic stimulation and occur in response to head compression.
• A gradual FHR decrease is defined as from the onset to the FHR nadir of >30 seconds.
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• The decrease in FHR is calculated from the onset to the nadir of the deceleration.
• The nadir of the deceleration occurs at the same time as the peak of the contraction.
Late deceleration: A visually apparent usually symmetrical gradual decrease and return of theFHR associated with a uterine contraction. These are mediated by either vagal stimulation or myocardial depression and occur in response to placental insufficiency.
• A gradual FHR decrease is defined as from the onset to the FHR nadir of >30 seconds.
• The decrease in FHR is calculated from the onset to the nadir of the deceleration.
• The deceleration is delayed in timing, with the nadir of the deceleration occurring after the peak of the contraction.
Variable deceleration: A visually apparent abrupt decrease in FHR. These are mediatedby umbilical cord compression.
• An abrupt FHR decrease is defined as from the onset of the deceleration to the begin-ning of the FHR nadir of <30 seconds.
• The decrease in FHR is calculated from the onset to the nadir of the deceleration.
• The decrease in FHR is >15 beats per minute, lasting >15 seconds, and<2 minutes in duration.
Sinusoidal pattern:
• A visually apparent, smooth, sine wave-like undulating pattern in FHR baseline with a cycle frequency of 3–5/min which persists for ≥20 min.
OB Triad
Early Decelerations
• Gradual drop of FHR
• Gradual return of FHR
• Mirror image of contraction
OB Triad
Late Decelerations
• Gradual drop of FHR
• Gradual return of FHR
• Delayed in relation to contractions
OB Triad
Variable Decelerations
• Abrupt drop of FHR
• Sudden return of FHR
• Variable in relation to contractions
FHR Categories
A 3-tiered system for the categorization of FHR patterns is recommended. It is important torecognize that FHR tracing patterns provide information only on the current acid–base status of the fetus.
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• Categorization of the FHR tracing evaluates the fetus at that point in time; tracing patterns can and will change.
• FHR tracing may move back and forth between the categories depending on the clini-cal situation and management strategies used.
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