2.2 Electronic Fetal Monitoring (EFM) & NICHD Pattern Interpretation

Electronic Fetal Monitoring (EFM) & NICHD Pattern Interpretation

Intrapartum electronic fetal monitoring (EFM) provides continuous assessment of fetal oxygenation during labor. The National Institute of Child Health and Human Development (NICHD) established standardized definitions for evaluating fetal heart rate (FHR) patterns. RNC-OB candidates must demonstrate expertise in EFM modalities, uterine activity quantification, baseline heart rate assessment, variability classification, and deceleration differentiation.

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EFM Monitoring Modalities: External vs. Internal

Fetal heart rate and uterine contractions can be assessed using external indirect transducers or internal direct electrodes and catheters:

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NICHD Uterine Contraction Terminology

Contraction parameters are evaluated over a 30-minute window:

• Normal Uterine Activity: $\le 5$ contractions in 10 minutes, averaged over 30 minutes.
• Tachysystole: $>5$ contractions in 10 minutes, averaged over 30 minutes. Tachysystole applies to both spontaneous and oxytocin-stimulated labor and must always be qualified by the presence or absence of associated FHR decelerations.

Montevideo Unit (MVU) Calculation

MVUs quantify contraction strength over a 10-minute period using an IUPC. Subtract the uterine resting tone (mmHg) from the peak pressure (mmHg) of each contraction in 10 minutes, then sum the values: $\text{MVU} = \sum_{i=1}^{n} (\text{Peak Pressure}_i - \text{Resting Tone})$ Adequate labor progression typically requires 200 to 250 MVUs.

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NICHD Baseline Fetal Heart Rate & Variability

Baseline FHR

The mean baseline FHR is rounded to increments of 5 beats per minute (bpm) over a 10-minute segment, excluding accelerations, decelerations, and periods of marked variability. The baseline must be observed for at least 2 minutes in any 10-minute window.

• Normal Baseline: $110\text{ to }160\text{ bpm}$.
• Fetal Bradycardia: Baseline $<110\text{ bpm}$ lasting $\ge 10\text{ minutes}$. (Causes: complete heart block, maternal hypothermia, severe fetal hypoxia, beta-blockers).
• Fetal Tachycardia: Baseline $>160\text{ bpm}$ lasting $\ge 10\text{ minutes}$. (Causes: maternal infection/chorioamnionitis, maternal hyperthyroidism, fetal hypoxia, terbutaline, fetal anemia).

[ Baseline FHR Variability Categories (Peak to Trough Amplitude) ]

1. Absent Variability   : Undetectable  --------------------
2. Minimal Variability  : 1 to 5 bpm             ---/\--/\-----------
3. Moderate Variability : 6 to 25 bpm            --/\  /\  /\-------- (Current metabolic acidemia unlikely)
4. Marked Variability   : > 25 bpm               -/\/\/\/\/\/\/\-----

Baseline Variability Definitions

Variability describes irregular amplitude fluctuations in the baseline FHR, quantified as peak-to-trough amplitude:

• Absent: Amplitude range is undetectable.
• Minimal: Amplitude range of $>0\text{ bpm}$ to $\le 5\text{ bpm}$.
• Moderate: Amplitude range of $6\text{ to }25\text{ bpm}$. Moderate variability strongly predicts the absence of current fetal metabolic acidemia; it does not guarantee future status or prove normal oxygenation by itself.
• Marked: Amplitude range greater than 25 bpm. Interpret with the full tracing and clinical picture; marked variability alone does not identify a single cause.

Differential Diagnosis for Minimal/Absent Variability: Fetal sleep cycle (typically 20–40 minutes, up to 60 minutes), maternal opioid administration, magnesium sulfate infusion, central nervous system anomalies, or progressive fetal acidemia.

FHR Accelerations & Deceleration Taxonomy

Fetal Heart Rate Accelerations

Visually apparent abrupt increase in FHR above baseline (onset to peak $<30\text{ seconds}$):

• $\ge 32$ Weeks Gestation: Peak of $\ge 15\text{ bpm}$ above baseline with duration of $\ge 15\text{ seconds}$ (15x15 rule).
• $<32$ Weeks Gestation: Peak of $\ge 10\text{ bpm}$ above baseline with duration of $\ge 10\text{ seconds}$ (10x10 rule).
• Prolonged Acceleration: Duration $\ge 2\text{ minutes}$ but $<10\text{ minutes}$. An acceleration lasting $\ge 10\text{ minutes}$ is a baseline change.

Deceleration Classification & Physiology

1. Early Decelerations:

   • Morphology: Gradual decrease and return to baseline ($\ge 30\text{ seconds}$ from onset to nadir). The nadir of the deceleration mirrors the peak of the uterine contraction.
   • Etiology: Fetal head compression during contractions $\rightarrow$ increased intracranial pressure $\rightarrow$ vagal nerve stimulation $\rightarrow$ transient slowing of FHR.
   • Clinical Action: Benign, physiological pattern. No intrauterine resuscitation required.

2. Variable Decelerations:

   • Morphology: Abrupt decrease ($<30\text{ seconds}$ from onset to nadir). The drop is $\ge 15\text{ bpm}$ lasting $\ge 15\text{ seconds}$ and $<2\text{ minutes}$.
   • Etiology: Umbilical cord compression (vein and artery occlusion). Occlusion of the low-pressure vein causes initial reflex tachycardia ("shoulder"), followed by arterial occlusion raising systemic resistance and triggering vagal deceleration.
   • Non-reassuring Features: Loss of shoulders, slow return to baseline, overshoot, or persistent minimal baseline variability within the trough.

3. Late Decelerations:

   • Morphology: Gradual decrease and return to baseline ($\ge 30\text{ seconds}$ from onset to nadir). The deceleration is delayed in timing, with the nadir occurring after the peak of the contraction and returning to baseline after the contraction ends.
   • Etiology: Uteroplacental insufficiency. Reduced intervillous blood flow during contraction causes transient fetal arterial hypoxemia, triggering chemoreceptor-mediated vagal slowing and myocardial depression.
   • Clinical Action: Evaluate recurrence, variability, uterine activity, and the clinical cause. Recurrent late decelerations require prompt assessment and etiology-directed intervention.

4. Prolonged Decelerations:

   • Visually apparent decrease in FHR $\ge 15\text{ bpm}$ below baseline lasting $\ge 2\text{ minutes}$ but $<10\text{ minutes}$.
   • Etiologies include prolonged umbilical cord prolapse, maternal hypotension following epidural placement, rapid descent, or uterine rupture.

5. Sinusoidal Pattern:

   • A smooth, sine wave-like undulating pattern in baseline FHR with a cycle frequency of 3 to 5 cycles per minute persisting for $\ge 20\text{ minutes}$.
   • Etiology and Action: A true sinusoidal pattern raises urgent concern for severe fetal anemia or hypoxia. Notify the team, evaluate the cause, begin indicated resuscitation, and expedite birth if the pattern persists or the clinical situation requires it.