Fetus

FHR Variability

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FHR Variability, FHT Variability, Fetal Heart Tone Variability

  • See Also
  1. Fetal Assessment
  2. Fetal Testing Indications
  3. Fetal Heart Tracing
  • Classification of FHT Variability
  1. Absent variability
  2. Minimal variability (<5 bpm change)
  3. Moderate variability (6 to 25 bpm change)
  4. Marked variability (>25 bpm change)
  • Evaluation
  1. Continuous monitoring
  • Interpretation
  1. Normally FHR varies 10-15 bpm from baseline
  2. Variability is related to fetal cerebral activity
  3. Evaluation based on one of two techniques
    1. Continuous Electronic Fetal Monitoring (CEFM)
    2. Structured Intermittent Auscultation (SIA)
      1. Count FHR in five second intervals over 60 seconds
      2. Examples (12 number sets)
        1. Good: 10,12,13,12,10,12,13,11,11,13,12,13
        2. Poor: 11,11,10,11,11,11,11,10,10,10,11,10
  • Efficacy
  • Loss of Variability
  1. Most accurate with internal scalp electrode
  2. Low sensitivity: 17%
  3. High Specificity
    1. Reassuring if normal variability
    2. Most specific for fetal asphyxia
    3. Negative Predictive Value >98%
  • Causes
  • Decreased FHT Variability
  1. Normal causes
    1. Fetal sleep cycle (usually lasts 20-40 minutes)
    2. Extreme prematurity
  2. Medications
    1. Opioids
    2. Benzodiazepines
    3. Anticholinergic Medications
    4. Barbiturates
    5. Phenothiazines
    6. Parasympatholytic medications
    7. General Anesthesia
    8. Maternal Cocaine use
    9. Magnesium Sulfate
    10. Betamethasone (does not occur with Dexamethasone)
  3. Fetal Hypoxia or Metabolic Acidosis
    1. Especially concerning if other findings of distress
      1. Late Decelerations
      2. Variable Decelerations
    2. Associated with decreased APGAR Scores if uncorrected
  4. Other abnormal causes
    1. Fetal neurologic anomalies (Anencephaly)
    2. Chorioamnionitis
    3. Fetal Heart Block
    4. Fetal Tachycardia
  • References
  1. Bailey (2000) ALSO, E:1-13
  2. Gabbe (2002) Obstetrics, p. 395
  3. Arnold (2020) Am Fam Physician 102(3): 158-67 [PubMed]
  4. Rylander (2001) Clin Fam Pract 3(2):287-305 [PubMed]