Intrapartum fetal monitoring
Fetal Heart Rate (FHR) Monitoring
- Purpose: To assess fetal oxygenation and well-being during labor. A three-tiered system is used to categorize FHR tracings (Category I, II, III) to guide management.
Components of FHR Tracing
- Baseline FHR:
- Normal: 110-160 bpm.
- Tachycardia: >160 bpm for ≥10 minutes.
- Causes: Maternal fever/infection (chorioamnionitis), maternal sympathomimetic drugs, fetal tachyarrhythmia, or fetal hypoxia.
- Bradycardia: <110 bpm for ≥10 minutes.
- Causes: Maternal hypotension, β-blocker use, congenital heart block, or late fetal hypoxia.
- Variability:
- The single most important predictor of fetal acid-base status. It reflects a healthy, oxygenated autonomic nervous system (ANS).
- Classification:
- Absent: Amplitude range undetectable (ominous).
- Minimal: ≤5 bpm amplitude (can be due to fetal sleep, medications, or hypoxia).
- Moderate: 6-25 bpm amplitude (reassuring).
- Marked: >25 bpm amplitude (less clear significance, can be response to acute hypoxia).
- Accelerations:
- Abrupt increase in FHR of ≥15 bpm lasting ≥15 seconds.
- Reassuring sign of fetal well-being. Their presence strongly argues against fetal acidemia.
- Two accelerations in 20 minutes constitutes a reactive Non-Stress Test (NST).
Decelerations
- This is a critical, high-yield area often remembered by the VEAL CHOP mnemonic.
| Mnemonic | FHR Pattern | Pathophysiology | Clinical Significance |
|---|---|---|---|
| V | Variable Deceleration | Cord Compression | - Most common deceleration. - Appearance: Abrupt, sharp drop in FHR (<30 sec to nadir), V- or U-shaped, and can occur with or without contractions. - Management: Maternal repositioning. If persistent/severe, consider amnioinfusion. |
| E | Early Deceleration | Head Compression | - Appearance: Gradual, uniform drop in FHR that mirrors the uterine contraction. Nadir of deceleration occurs at the peak of the contraction. - Significance: Benign, physiologic vagal response. No intervention needed. |
| A | Acceleration | Okay | - Appearance: Abrupt FHR increase. - Significance: Reassuring, indicates adequate fetal oxygenation. |
| L | Late Deceleration | Placental Insufficiency | - Appearance: Gradual, uniform drop in FHR where the onset and nadir occur after the beginning and peak of the contraction. - Significance: Ominous sign of uteroplacental insufficiency and fetal hypoxia. Always pathologic. |
Special Patterns
- Sinusoidal Pattern:
- Smooth, sine wave-like undulating pattern in FHR baseline.
- Ominous sign associated with severe fetal anemia (e.g., Rh isoimmunization, vasa previa rupture) or severe hypoxia.
- Requires immediate intervention and often expedited delivery.
FHR Categories & Management
- Category I (Normal):
- Criteria: Baseline 110-160 bpm, moderate variability, no late/variable decelerations, accelerations present or absent.
- Action: Reassuring. Continue routine monitoring.
- Category II (Indeterminate):
- Criteria: Any tracing not fitting into Category I or III. This is the most common category.
- Action: Requires increased surveillance and may benefit from intrauterine resuscitation measures.
- Category III (Abnormal):
- Criteria: Absent baseline variability PLUS recurrent late decelerations, recurrent variable decelerations, or bradycardia; OR a sinusoidal pattern.
- Action: Non-reassuring. Prompt evaluation and intrauterine resuscitation. If the pattern does not resolve, expedited delivery (e.g., C-section) is necessary.
- Intrauterine Resuscitation Measures:
- Stop uterotonic agents (e.g., oxytocin).
- Maternal repositioning (left lateral decubitus position is common).
- Administer IV fluids and supplemental O₂.
- Consider tocolysis for uterine tachysystole.