What are the four types of fetal heart rate (FHR) variability and their causes?

Four Types of Fetal Heart Rate Variability and Their Causes

Fetal heart rate (FHR) variability is classified into four distinct types: absent, minimal, moderate, and marked variability, each reflecting different physiological states of fetal oxygenation and autonomic nervous system function. 1

Types of FHR Variability and Their Causes

1. Absent Variability

• Definition: No visible fluctuations in the baseline FHR (amplitude range undetectable)
• Causes:
  • Severe fetal hypoxia/acidemia leading to myocardial depression
  • Fetal central nervous system anomalies
  • Medications (narcotics, magnesium sulfate, general anesthetics)
  • Pre-terminal fetal state
  • Deep fetal sleep cycle (transient)

2. Minimal Variability

• Definition: Barely detectable fluctuations in baseline FHR with amplitude range of ≤5 beats per minute (bpm)
• Causes:
  • Early or mild fetal hypoxemia
  • Medications (narcotics, beta-blockers, magnesium sulfate)
  • Fetal sleep cycles (physiologic, usually transient)
  • Prematurity (<32 weeks gestation)
  • Early stages of fetal compromise

3. Moderate Variability

• Definition: Clearly visible fluctuations in baseline FHR with amplitude range of 6-25 bpm 1, 2
• Causes:
  • Normal fetal oxygenation and acid-base status
  • Intact autonomic nervous system function
  • Represents normal physiologic state and reassuring fetal well-being

4. Marked Variability

• Definition: Fluctuations in baseline FHR exceeding 25 bpm in amplitude
• Causes:
  • Acute hypoxic stress (early compensatory response)
  • Umbilical cord compression
  • Fetal stimulation
  • Recovery from previous hypoxic episode
  • Catecholamine surge

Clinical Significance of FHR Variability

Variability is one of the most important parameters in interpreting fetal heart rate patterns because it directly reflects fetal central nervous system integrity and oxygenation status 1, 3:

• Moderate variability is highly predictive of normal fetal acid-base status and is classified as a Category I (normal) pattern by the National Institute of Child Health and Human Development (NICHD) 1

• Absent or minimal variability, especially when persistent or combined with recurrent late decelerations, indicates potential fetal compromise and is classified as Category II (indeterminate) or Category III (abnormal) depending on other associated features 1, 4

• Loss of beat-to-beat variability for >50% of observation time is strongly associated with neonatal depression and low Apgar scores 4

Special Consideration: Sinusoidal Pattern

A special pattern of variability that warrants mention is the sinusoidal pattern, characterized by:

• Smooth, sine wave-like oscillations
• Amplitude of 5-15 bpm
• Frequency of 2-5 cycles per minute
• Fixed or flat short-term variability
• No areas of normal FHR variability or reactivity 5

True sinusoidal patterns are associated with:

• Severe fetal anemia (Rh sensitization, fetomaternal hemorrhage)
• Severe hypoxia/asphyxia
• Fetal infection
• Impending fetal demise 5

Clinical Management Based on Variability

• Moderate variability: Continue routine monitoring as it indicates normal fetal acid-base status 1

• Minimal variability: Increase surveillance, consider position changes, hydration, oxygen administration, and evaluate for potential causes 1, 3

• Absent variability: When combined with recurrent late decelerations or bradycardia, requires immediate intervention including discontinuing oxytocin, changing maternal position, administering oxygen, giving IV fluid bolus, and considering expedited delivery 1, 3

• Marked variability: May represent early compensatory response to stress; requires close monitoring for progression to decreased variability which would indicate decompensation 3

Common Pitfalls in Interpreting Variability

1. Failing to distinguish between short-term (beat-to-beat) and long-term variability
2. Not accounting for the effect of medications on variability
3. Misinterpreting transient periods of minimal variability during fetal sleep cycles as pathological
4. Overlooking the significance of progressive loss of variability over time
5. Confusing pseudo-sinusoidal patterns (often benign) with true sinusoidal patterns (ominous)

Proper interpretation of FHR variability requires consideration of the complete clinical context, including gestational age, maternal medications, and presence of other FHR features such as accelerations and decelerations 1, 3.