Cardiotocograph

Cardiotocography is:

Foetal Heart Rate

Contributors (of unknown relative importance) include:

  • Sympathetic/parasympathetic tone
  • Baroreceptor effects
  • Chemoreceptor effects
  • Cord compression
  • Placental function
  • Uteroplacental function
  • Contractions
    Contractions in uncomplicated labour result in short and long-term reduction in oxygenation; and this effect is markedly ↑ with excessively frequent contractions.

Interpretation

Principles:

  • Accurate interpretation relies on context
    Considerations:
    • Gestational age
    • In labour/not in labour
    • Other symptoms
      e.g. Pain, bleeding.
    • Risk factors
    • Cardiac disease
      Foetal heart rate may not vary in the setting of a cardiac lesion (e.g. heart block).
  • Requires accurate foetal trace
    Maternal trace may be being measured in error. Confirm foetal heart rate using:
    • Maternal SpO2 for heart rate
      Useful if foetal heart rate and maternal heart rate differ.
    • Foetal scalp electrode

Key features:

  • Baseline FHR
    Mean FHR in bpm, when FHR is stable, when there is no contraction over 5-10 minutes.
    • Required to accurately assess the remainder of the CTG
  • Baseline variability
    Minor fluctuation in baseline FHR; as assessed by the difference in peak and trough FHR over one minute. Variability is important.
    • Normal 6-25 bpm
    • ↑ > 25 bpm
      Unclear signifiance - may not be bad.
    • Reduced 3-5 bpm
      Loss of variability is always a bad sign.
      • Absent variability is <3 bpm
  • Accelerations
    Transient ↑ of >15 bpm in FHR above baseline for at least 15s.
    • Normal to have >2 in 20 minutes when:
      • Awake
        Foetal sleep phases last ~45 minutes.
      • Not in labour
  • Decelerations
    Transient ↓ of >15 bpm in FHR below baseline for at least 15s. Subdivided into:
    • Early decelerations
      • Benign
        Usually head compression during a contraction.
      • Usually at 4-8cm cervical dilation
      • Uniform in shape
      • Start and finish with the contraction
    • Variable decelerations
      • Rapid onsent and recovery
        Deep ‘V-shaped’ decelerations.
      • Variable relationship with the contraction
        Most are simultaneous with contraction.
      • Signifiance depends on other findings
      • Complicated variable decelerations are non-reassuring, and occur with:
        • Rising baseline FHR
        • Foetal tachycardia
        • Reduced/absent baseline variability
        • Slow return to baseline FHR following contraction
        • Deceleration nadir following peak contraction
        • Large amplitude (60bpm) or long duration (60s) of deceleration
        • Post-deceleration overshoots
    • Late decelerations
      • Uniform, repetitive deceleration with:
        • Slow onset in second half of contraction
        • Nadir >20s following peak of contraction
        • End after contraction finishes
      • Indication that contraction is occurring in the presence of foetal hypoxia
        Therefore, will lack reasurring signs such as normal baseline variability, early decelerations.
      • May be <15 bpm if foetus is profoundly hypoxic
      • May not be distinguishable from variable decelerations however this is usually academic
    • Prolonged decelerations
      Deceleration lasting 90s to 5 minutes.
  • Sinusoidal
    Smooth, regular oscillations at 2-5 cycles per minute with an amplitude of 5-15bpm. Indicates severe anaemia.

Normal Intrapartum CTG

Low probability of foetal compromise. Features include:

  • Baseline FHR
    110-160bpm.
  • Normal baseline variability
    6-25bpm.
  • Early or absent decelerations

Non-Reassuring Features

May be associated with significant foetal compromise, and requires further action:

  • Baseline foetal tachycardia
    >160 bpm.
  • Reduced baseline variability
    3-5 bpm.
  • Rising baseline FHR
  • Complicated variable foetal decelerations
  • Late decelerations
  • Prolonged decelerations

Likely to be associated with significant foetal compromise, and requires immediate intervention or urgent delivery:

  • Prolonged bradycardia
    100bpm for >5 minutes.
  • Absent baseline variability
  • Complicated variable decelerations with reduced or absent variability
  • Late decelerations with reduced or absent variability

References

  1. The Royal Women’s Hospital. Guideline: Cardiotocograph Interpretation and Response. The Royal Women’s Hospital Melbourne. 2017.
  2. Woodward, A. CTG interpretation for the Anaesthetist. RWH Anaesthetic Tutorial Program.
  3. Thurlow, J.A., and S.M. Kinsella. Intrauterine Resuscitation: Active Management of Fetal Distress. International Journal of Obstetric Anesthesia 11, no. 2 (April 2002): 105–16. https://doi.org/10.1054/ijoa.2001.0933.