Intrapartum Fetal Heart Rate Monitoring

Introduction

Normal human labor involves regular uterine contractions and transient interruptions of fetal oxygenation. While most fetuses withstand the rigors of labor without issue, a subset may experience distress. Intrapartum fetal heart rate (FHR) monitoring emerges as a pivotal tool in assessing fetal well-being during labor, enabling timely interventions to mitigate the risk of neurological injury or death.

Goal of Intrapartum FHR Monitoring

• To ascertain fetal oxygenation status during labor
• To facilitate the timely implementation of interventions when necessary

Candidates for Intrapartum Fetal Monitoring

• High-risk Pregnancies: Continuous electronic FHR monitoring is typically advocated to closely monitor fetal well-being.
• Low-risk Pregnancies: Options include either intermittent or continuous electronic FHR monitoring.

Exceptions to intrapartum FHR monitoring arise when potential FHR abnormalities would not alter management strategies, such as in extremely preterm labors below the viability threshold or when fetal anomalies incompatible with life are present.

Continuous vs. Intermittent Monitoring

• Continuous Electronic FHR Monitoring: real-time, continuous feedback on fetal heart rate patterns, allowing for immediate identification of abnormalities that may signify distress.
• Intermittent Auscultation: regular checks with a Doppler or fetoscope.

External vs. Internal Monitoring

• External FHR Monitoring: This noninvasive approach is generally as reliable as internal monitoring for most cases. It involves the use of a Doppler ultrasound device placed on the maternal abdomen to detect fetal heart motion and calculate the heart rate. Autocorrelation technology minimizes signal variation, offering a clear tracing of FHR.
  
• Internal FHR Monitoring: Preferred in situations where external tracing quality is compromised (due to early gestational age, maternal habitus, or multiple gestation, among others). It involves placing a bipolar spiral electrode on the fetal scalp to obtain a more direct and clear signal of the FHR.
  

Frequency and Duration of Monitoring

• Initial Assessment
  • Upon admission, a standard assessment of 20 to 30 minutes is recommended to identify fetuses at risk for abnormal FHR patterns.
• Continuous Monitoring
  • Low-risk Pregnancies: Continuous monitoring is advised when feasible, with FHR review every 30 minutes during the first stage of labor and every 15 minutes during the second stage.
  • High-risk Pregnancies: Continuous monitoring with more frequent reviews—every 15 minutes during the first stage and every 5 minutes during the second stage.
• Intermittent Auscultation
  • An alternative for low-risk pregnancies, it involves listening to the FHR for ≥60 seconds and record the rate during and immediately after a uterine contraction at least every 30 minutes during the active phase of the first stage of labor and at least every 15 minutes during the second stage .
  • If risk factors for fetal compromise are present, the FHR is determined, evaluated, and recorded preferably before, during, and after a uterine contraction at least every 15 minutes during the active phase of the first stage of labor and at least every 5 minutes during the second stage.

Assessment of Uterine Activity

• The interpretation of FHR patterns should be conducted alongside an assessment of uterine activity, considering factors like contraction frequency, intensity, duration, and resting tone.
• The NICHD defines normal uterine contraction frequency as five or fewer contractions in 10 minutes, averaged over 30 minutes, with higher frequencies categorized as tachysystole.

   

Assessment of fetal heart

Baseline FHR

The baseline rate is the mean bpm over a 10-minute interval —- indicating a healthy fetal autonomic and cardiovascular system.

• Bradycardia = below 110 bpm — may suggest maternal beta-blocker use, hypothermia, hypoglycemia, hypothyroidism, fetal heart block, or interruptions in fetal oxygenation.
• Normal = 110 to 160 bpm — healthy fetal autonomic and cardiovascular system.
• Tachycardia = over 160 bpm — can be caused by maternal fever, infections, certain medications, hyperthyroidism, increased catecholamines, fetal anemia, arrhythmia, or oxygenation interruptions.
  
     
  
  

FHR Variability

Fluctuations in baseline that are irregular in amplitude and frequency , Measured in a 10-minute window , The amplitude is measured peak to trough — Is the result of integrated activity between the sympathetic and parasympathetic branches of the autonomic nervous system.

• Absent = amplitude undetectable — poor predictors of fetal metabolic acidemia or hypoxic injury.
• Minimal = amplitude 0 to 5 bpm — poor predictors of fetal metabolic acidemia or hypoxic injury.
• Moderate = amplitude 6 to 25 bpm —- Indicates a well-oxygenated fetus and a functional autonomic nervous system, suggesting the absence of significant hypoxia or metabolic acidemia.
• Marked = amplitude over 25 bpm — It may be a normal variant or an exaggerated autonomic response to transient or sustained interruption of fetal oxygenation.
  
  
  
  

Accelerations

• An abrupt increase in the FHR. often linked to fetal movements, a positive sign of fetal well-being, reflecting adequate oxygenation and absence of metabolic acidemia.
• An abrupt increase in the FHR. Before 32 weeks of gestation, accelerations should last ≥10 sec and peak ≥10 bpm above baseline. As of 32 weeks gestation, accelerations should last ≥15 sec and peak ≥15 bpm above baseline.
• A prolonged acceleration is ≥2 minutes but less than 10 minutes. An acceleration of 10 minutes or more is considered a change in baseline.
  
  

Decelerations

An abrupt decrease in FHR below the baseline. The decrease is ≥15 bpm, lasting ≥15 secs and <2 minutes from onset to return to baseline.

• Early Deceleration
  
  • A gradual decrease and return to baseline of the FHR associated with a uterine contraction. The nadir of the FHR and the peak of the contraction occur at the same time. The deceleration's onset, nadir, and termination are usually coincident with the onset, peak, and termination of the contraction.
     
    
    
• Variable Deceleration
  • The onset, depth, and duration of variable decelerations commonly vary with successive uterine contractions.
  • Results from transient umbilical cord compression
    
    
    
• Late Deceleration
  • A gradual* decrease and return to baseline of the FHR associated with a uterine contraction. The deceleration is delayed in timing, with the nadir of the deceleration occurring after the peak of the contraction. The onset, nadir, and recovery usually occur after the onset, peak, and termination of a contraction.
  • Usually a reflex response to transient fetal hypoxemia, indicating reduced oxygen delivery to the fetus.
    
    
    
• Prolonged Deceleration
  • Defined by a drop in FHR of ≥15 bpm lasting 2 to 10 minutes, prolonged decelerations are critical signs that may indicate more severe or extended interruptions in fetal oxygenation.
  • A decrease in FHR below the baseline of 15 bpm or more, lasting at least 2 minutes but <10 minutes from onset to return to baseline. A prolonged deceleration of 10 minutes or more is considered a change in baseline.
    
    
    
• Sinusoidal Pattern
  • A rare, smooth, sine wave-like pattern in FHR, historically associated with severe fetal anemia. The exact pathophysiology remains unclear, but its presence demands careful evaluation and consideration of fetal well-being.
    

Interpretation of an FHR tracing

Management of intrapartum fetal heart rate tracings