How is the fetal heart rate baseline determined in labor?

Determination of the Fetal Heart Rate Baseline in Labor

The fetal heart rate (FHR) baseline is determined by counting the FHR between contractions for at least 60 seconds to establish the average baseline rate, which should normally fall between 110-160 beats per minute (bpm). 1

Definition and Methodology

The baseline FHR is a critical component of fetal monitoring during labor and is defined as:

• The mean FHR rounded to increments of 5 bpm during a 10-minute segment
• Measured between contractions
• Excludes periods of marked FHR variability, accelerations, or decelerations
• Requires at least 2 minutes of identifiable baseline in a 10-minute segment

Structured Approach to Determining Baseline

When using structured intermittent auscultation:

1. Palpate the abdomen to determine fetal position (Leopold maneuvers)
2. Place the Doppler over the area of maximal intensity of fetal heart tones
3. Differentiate maternal pulse from fetal pulse
4. Palpate for uterine contraction during FHR auscultation to determine relationship
5. Count FHR between contractions for ≥60 seconds to determine average baseline rate 1

When using continuous electronic fetal monitoring (EFM), the baseline is assessed as part of the DR C BRAVADO approach:

• Baseline RAte: Bradycardia (<110 bpm), normal (110-160 bpm), or tachycardia (>160 bpm) 1

Normal vs. Abnormal Baseline

• Normal baseline: 110-160 bpm 1, 2
• Tachycardia: >160 bpm (some guidelines use >150 bpm) 2
• Bradycardia: <110 bpm 2

Clinical Significance of Baseline Values

It's important to note that in fetuses at or beyond 40 weeks gestation, a baseline between 150-160 bpm, while technically within normal range, has been associated with:

• Higher incidence of meconium-stained amniotic fluid
• Increased risk of cesarean section for fetal distress
• Higher rates of neonatal acidemia 3

Potential Pitfalls in Baseline Determination

1. Signal ambiguity: Maternal heart rate can sometimes be mistaken for FHR, leading to erroneous interpretation. Maternal heart rate typically has:

   • Lower baseline rate
   • More marked beat-to-beat variability
   • More accelerations and fewer decelerations 4

2. Baseline changes: The baseline can change over time, requiring ongoing reassessment. Sleep cycles of 20-40 minutes may cause normal decreases in FHR variability 1

3. Medications: Certain medications including analgesics, anesthetics, barbiturates, and magnesium sulfate can affect FHR variability 1

Timing of Monitoring

While continuous EFM is widely used in North American hospitals, evidence suggests that starting continuous FHR monitoring when cervical dilation reaches 4-5 cm (second phase of first stage of labor) may be sufficient in low-risk pregnancies, allowing for:

• Greater maternal comfort
• Lower resource utilization
• No difference in detection of significant FHR abnormalities 5

Interpretation Framework

The baseline FHR should not be interpreted in isolation but as part of a comprehensive assessment using frameworks such as:

1. DR C BRAVADO approach:

   • Determine Risk
   • Contractions
   • Baseline RAte
   • Variability
   • Accelerations
   • Decelerations
   • Overall assessment 1

2. NICHD Three-Category System:

   • Category I (normal): Normal baseline (110-160 bpm), moderate variability, no late/variable decelerations, may have accelerations
   • Category II (indeterminate): All patterns not categorized as I or III
   • Category III (abnormal): Absent variability with recurrent late/variable decelerations, bradycardia, or sinusoidal pattern 1

By systematically determining and interpreting the FHR baseline as part of a complete assessment, clinicians can better identify fetuses at risk for hypoxic injury and intervene appropriately to improve neonatal outcomes.