NST Teaching

Resources

Abbreviations

FHR - fetal heart rate

US - ultrasound

IUPC - intrauterine pressure catheter

NST - non stress test

CEFM - continuous electronic fetal monitoring

IA - intermittent auscultation

bpm - beats per minute

Definitions

Electronic Fetal Monitoring (EFM)

• Uses Doppler ultrasound to measure the fetal heart rate (FHR) and record it on a graph

Non Stress Test (NST)

• When EFM is used for a discrete period of time (usually 20 min) to check fetal status, of a patient who is *not in labor*

Continuous Electronic Fetal Monitoring (CEFM)

• Uninterrupted tracing of FHR of a woman who is in labor

Intermittent Auscultation (IA)

• Brief monitoring of FHR after contractions (for 30-60 seconds) to ensure normal baseline, rather than continuously monitoring a woman in labor

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What are we looking at? How are we measuring it?

1) Fetal Heart Rate

• External FHR Monitor (Doppler/US)
• Internal Lead

2) Uterine Activity

• External Tocometer
• Intrauterine Pressure Catheter

1 min

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2

Breaking it down… How to approach an NST

1. Assess FHR Baseline
2. Assess Baseline Variability
3. Assess Changes in FHR

a) Accelerations

b) Decelerations

• Assess Uterine Activity

1. FHR Baseline

• Approximate mean FHR during a 10 min segment of tracing
• Should be at least 2 minutes to define baseline
• New baseline once sustained for 10 minutes

Normal Baseline:

• 110 – 160 bpm at term
• Baseline decreases with increasing GA

Tachycardia:

• Mild: 160-180 bpm
• Severe: >180

Bradycardia:

• Mild: 90 – 110
• Severe: <90

2. FHR Variability

• Fluctuations in the baseline FHR is a normal, physiologic characteristic of the FHR, largely controlled by vagus nerve
• So… normal variability is a good indicator of a healthy fetus!

Definitions:

Absent: undetectable variability

Minimal: <= 5 bpm

Moderate: 6-25 bpm

Marked: >=26 bpm

How would you classify this variability?

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2

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3. Changes in FHR

Accelerations:

• Abrupt increases in FHR from baseline
• Onset to peak < 30 sec
• Return to baseline within 2 min
• Criteria:
• >32 weeks: 15 bpm above baseline for 15s
• <32 weeks: 10bpm above baseline for

10s

Decelerations:

• Decreases in FHR
• Can be benign or pathologic depending on type and what the rest of the NST looks like
• Three kinds:
• Early – fetal head compression
• Variable – umbilical cord compression
• Late – fetal hypoxia, acidemia

Early and Late Decelerations

Smooth, gradual

onset decrease in FHR from

baseline (no depth criteria, can be subtle)

How would you describe this deceleration?

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2

Variable Decelerations

Abrupt

decreases in FHR due to

umbilical cord compression,

lasting 15 seconds, and >15 bpm below the baseline

Think of “Variables” as “Vs”

When are variables concerning?

Complicated Variables

(Rule of 60s):

• Lasting >60 seconds
• Down to 60 bpm
• Greater than 60 bpm below baseline
• Baseline fetal tachycardia or bradycardia
• Loss of baseline variability
• Slow return to baseline

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What makes these variables “complicated?”

VEAL CHOP

A Friendly Mnemonic!

To remember the cause of

FHR changes…

Variable

Early

Acceleration

Late

Cord Compression

Head Compression

Okay

Placental Insufficiency

4. Uterine Activity

• Contraction Frequency, Duration, Resting Tone
• Contraction Patterns:
• Regular, doubling, or tripling
• Tachysystole (≥5/10 min, contraction >90s, no resting tone)
• Hyperstimulation (tachysystole with atypical or abnormal FHR changes)
• Usually reported as N contractions in 10 minutes (“N in 10”)
• Recall: May be traced internally (tocometer) or externally (IUPC)

How would you describe this contraction pattern?

Putting it all together…

Based on baseline, variability, accelerations, decelerations, and uterine contraction pattern, we classify a FHF as “normal, atypical, or abnormal.”

Classification of a FHR tracing of a patient who is…

*NOT IN LABOR*

Ex) “Non-Stress Test

Classification of a FHR tracing of a patient who is…

*IN LABOR*

Ex) An “Intrapartum EFM Classification”

Example Tracings