Terminology
- Baseline fetal heart rate (FHR): Rate of fetal heartbeats between contractions.
- Normal FHR: 120 to 160 or greater for short periods.
- Tachycardia: Sustained (10 minutes or more) FHR of 160 or greater. Etiology: Early fetal hypoxia, immaturity, amnionitis, maternal fever, and/or terbutaline.
- Bradycardia: Sustained (for 10 minutes or more) baseline FHR below 120. Etiology: Late or profound fetal hypoxia, maternal hypotension, prolonged umbilical cord compression, and/or anesthetics.
Methods
External Monitoring
Contractions and FHR are monitored and recorded through transducers applied to the mother’s abdomen. Quality of tracing is affected by many variables.
Internal Monitoring
FHR is monitored by an electrode attached to the fetal scalp or presenting part, and contractions are measured using a sterile water-filled pressure catheter placed in the uterine cavity alongside the fetus (should not be used if mother tests positive for group B Streptococcus).
Interpretation
Variability
The term variability refers to cardiac rhythm irregularities. Beat to beat variability refers to the difference between successive heartbeats. To the extents described here, variability of cardiac rhythm is considered normal or abnormal:
- No variability:0 to 2 variations per minute (abnormal)
- Minimal variability:3 to 5 variations per minute (abnormal)
- Average variability:6 to 10 variations per minute (normal)
- Moderate variability:11 to 25 variations per minute (normal)
- Marked variability: Above 25 variations per minute (abnormal)
NOTE: Decreased or marked variability may be the first sign of fetal distress. Place mother on her left side and hydrate.
Deceleration
The term deceleration refers to decreases in the fetal heart rate. Deceleration may be further described as follows:
- Early deceleration: Can be detected in the monitor tracing appearance (mirror image of the contraction is seen). Etiology: Usually caused by head compression. Treatment: None (Fig. 3–4A).
- Late deceleration: Can be detected in the monitor tracing appearance (reverse mirror image of the contraction seen late in the contraction). Etiology: Usually uteroplacental insufficiency. Treatment: Change mother’s position from supine to lateral, slow or stop oxytocin, give IV fluids. Give O2 at 6—10 L/min via face mask if fetal distress suspected. C-section may be needed if not corrected (Fig. 3–4B).
- Variable deceleration: Occurs at unpredictable times during contractions. Etiology: Usually cord compression. Treatment: Change mother’s position from supine to lateral or Trendelenburg. Give O2 at 6—10 L/min via face mask. Perform pelvic exam to assess for descent of fetal presenting part or prolapsed cord. If cord is palpable, lift presenting part above it, place mother in Trendelenburg position and avoid manipulating cord. C-section is usually needed (see Fig. 3–4C).