Learning objectives
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Classify fetal heart rate patterns according to the three-tiered system of categorization.
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Assess and develop management plans for intrapartum fetal heart rate tracings.
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Assess and develop management plans for uterine contraction patterns.
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Discuss variables that can affect the maternal–fetal oxygen pathway.
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Interpret fetal cord gas results.
Fetal heart rate monitoring is the most common obstetric procedure, and yet it remains a frustrating technology, plagued by false-positive results and miscommunication between providers.
Physiologic Basis for Fetal Heart Rate Monitoring
The goal of fetal heart rate monitoring is to assess fetal well-being. A normal fetal heart rate tracing requires normal oxygenation of the mother and normal placental transfer of oxygen to the fetus. Any process that causes a break in the oxygen pathway can cause fetal heart rate abnormalities. Understanding this physiology is important to intervening to improve fetal oxygenation. Potential breaks in the oxygen pathway are summarized as follows:
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Maternal lungs
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Respiratory depression (narcotics, magnesium)
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Pneumonia/ARDS
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Pulmonary embolus
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Pulmonary edema
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Asthma
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Atelectasis
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Maternal heart
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Reduced cardiac output
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Hypovolemia
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Decreased venous return (compression of vena cava)
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Maternal vasculature
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Hypotension
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Regional anesthesia
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Medications (hydralazine, labetalol, nifedipine)
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Uterus
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Excessive uterine activity
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Placenta
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Placental abruption
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Umbilical cord
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Cord compression
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“True knot” in cord
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Fetal Heart Monitoring Components
Baseline
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Defined as the average heart rate (rounded to the nearest five beats per minute) during a 10-minute segment
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Must be present for a minimum of 2 minutes within the 10-minute segment; if not, baseline is characterized as “indeterminant”
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Normal is 110–160 beats per minute
Variability
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Refers to the beat-to-beat changes in the baseline fetal heart rate ( Fig. 8.1 )
Fig. 8.1
Examples of (A) absent, (B) minimal, (C) moderate, and (D) marked variability.
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Amplitude of variability is classified as follows:
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Absent—no detectable variation
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Minimal—detectable variation, but variation is five beats per minute or less
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Moderate—6–25 beats per minute (this is normal)
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Marked—greater than 25 beats per minute
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Accelerations
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This is an abrupt rise in the fetal heart rate from the baseline
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Defined as follows:
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At 32 weeks gestation or higher-peak of at least 15 beats per minute above the baseline and lasts at least 15 seconds
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Prior to 32 weeks gestation-peak of at least 10 beats per minute above the baseline and lasts at least 10 seconds
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At any gestation—a “prolonged acceleration” lasts 2–10 minutes
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Accelerations indicate that fetal acidemia is not present at that time
Decelerations ( Fig. 8.2 )
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Early decelerations
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Onset to nadir of deceleration is 30 seconds or longer
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Nadir of deceleration coincides with peak of contraction
Fig. 8.2
Examples of (A) early, (B) late, and (C) variable decelerations.
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Late decelerations
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Onset to nadir of deceleration is 30 seconds or longer
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Nadir of deceleration occurs after peak of contraction
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Variable decelerations
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Abrupt decrease in fetal heart rate with onset to nadir of less than 30 seconds
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Nadir must be at least 15 beats per minute below the baseline and deceleration must last at least 15 seconds
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Prolonged decelerations
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Decrease in fetal heart rate of at least 15 beats per minute below baseline
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Lasts 2–10 minute
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Classification of Fetal Heart Rate Tracings
Category I (Normal) ( Fig. 8.3 )
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Baseline rate: 110–160 beats per minute
Fig. 8.3
Example of Category I fetal heart rate tracing. This tracing has a baseline of 145, moderate variability, accelerations, and no decelerations. Contractions are every 2.5–3 minutes.
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Baseline variability: moderate
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Accelerations: may be present or absent
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Early decelerations: may be present or absent
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Late/Variable decelerations: absent
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Interpretation: there is no fetal acidemia at that moment
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Management: continue routine fetal monitoring
Category II (Indeterminate) ( Fig. 8.4 )
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Includes everything not categorized as Category I or Category III. This may include any of the following:
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Bradycardia without absent variability
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Tachycardia
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Prolonged deceleration
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Minimal variability
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Marked variability
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Absent variability without recurrent decelerations
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Moderated variability with recurrent late or variable decelerations
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