Fetal Monitoring
last authored: Oct 2011, David LaPierre
last reviewed:
Introduction
Fetal monitoring is the assessment of various parameters of the fetus - normally the heart rate - to provide reassurance of well-being and identify potential causes for concern. Increasec concern often prompts the health care team to make decisions regarding the speed and nature of delivery, such as through induction, augmentation, assisted delivery, or Caesarean section.
The fetal heart rate (FHR) increases and decreases largely due to autonomic signals from the brainstem, which also controls variability from beat to beat (Matsurra et al, 1996). A healthy fetus displays transient and repetitive hypoxia, and a physiological drop in pH may also be seen. In fact, up to 80% of all labours result in abnormal or atypical FHR tracings at one time or another, without signifying life-treatening situations (Umstad, Permezel, and Pepperell, 1994). However, if a fetus is compromised for any reason, it's ability to withstand normal hypoxia is reduced. An analogy to describe this is for an adult to swim across a pool while holding their breath (transient hypoxia). Normally this is not difficult. However, if the adult has just raced across a field and is gasping for breath (some compromise), he will have significant difficulty swimming under water across the pool.
Fetal oxygenation is affected by many factors:
Maternal
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Uteroplacental
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Fetal
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The fetus responds to hypoxia in a number of ways:
- redistribution of blood flow to vital organs
- decreased movement, tone and breathing movement
- tachycardia, often preceded by transient bradycardia
- anaerobic metabolism
The goal of fetal monitoring is to identify evidence of worsening hypoxia and metabolic acidosis before organ damage occurs. This can include kidney failure, respiratory distress syndrome, necrotizing enterocolitis, liver damage, leukopenia, cardiomyopathy, and brain damage.
Descriptors of Fetal Heart Rate
The normal fetal heart rate is 110-160. Below this is bradycardia, and above this is tachycardia.
Baseline
The baseline should be counted for at least 30 seconds, and preferably 60 seconds, between contractions. This provides a reference. The baseline should be reassessed between contractions throughout labour.
Variability
Variability describes fluctuation above or below the baseline. It depends on intact CNS and largely reflects vagal tone on the heart.
Decreased variability can be seen with:
- sleep cycles (the fetus can be stimulated using fundal pressure or acoustic or vibratory stimulation)
- acidosis
- sedative/narcotic/steroid medications
- cigarette smoking
- preterm infants
- fetal growth restriction
- oligohydramnios
- meconium
- placental infarction
- fetal CNS abnormalities
Accelerations
A normal acceleration is defined as 15 bpm above baseline, lasting between 15-120 seconds in a fetus older than 32 weeks. For younger than 32 weeks, it is defined as 10 bpm above baseline for at least 10 seconds.
Decelerations
Decelerations are normally seen with fetal movement, and need not be worrisome. However, repetition of at least x declerations in 20 minutes does appear to carry increased risk. Decelerations lasting >1min are even more ominous.
Early decelerations are gradual decreases in heart rate, usually coinciding with the contraction. These are thought to be due to head compression and are almost always benign.
Late decelerations have their onset after the peak.
While false-positives are common, there are some rhythms that reliably signify fetal distress. A beat-to-beat variation in less than 5 bpm, absent accelerations, and late decelerations following uterine contractions is very worrying.
Variable decelerations have an abrupt onset, are a decrease of more than 15 bpm, and last for >15 sec. They can suggest cord compression. Complicated variable decelerations can be biphasic or prolonged.
Contractions
Contractions: 5 or less in 10 min; fixed frequency and duration are rare.
duration <90 sec
configuration: regular, symmetrical
intensity: mild, moderate or strong (compare with the consistency of the nose, chin, forehead)
resting tone: soft, <15mmHg for >30 sec
Intermittent Auscultation
FHR monitoring is done in over 80% of births in North America.
Intermittent auscultation (IA) does not measure baseline variability.
uterine activity by palpation: frequency, duration, strength, resting tone
FHR:
- baseline between contractions, for 60 sec
- post-contraction - immediately post contraction for 30-60 sec
- increase or decrease in FHR
- rhythm
- changes or trends over time
FHR tracing should be reviewed:
- every 30 minutes in the first stage of labour, every 15 min in the second stage before pushing and every 5 minutes while pushing
Common patterns include:
- acceleration (fetal movements)
- early deceleration (head compression from uterine contraction)
- late decleration (uteroplacental insufficiency and fetal hypoxemia)
- variable decleration (umbilical cord compression)
- bradycardia (congentital heart disease, severe hypoxemia)
- tachycardia (hypoxia, maternal fever, anemia)
Results need to be interpreted in the context of the total clinical picture.
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abnormal |
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Electronic Fetal Montioring (EFM)
FHR
- baseline approximate mean, rounded to nearest 5 during a 10 min period; avoid changes with varibility
- variability
- accels or decels
Nonstress Test
The nonstress test (NST) is an ultrasound assessment of fetal heart rate, measured through the mother's abdomen using a Doppler ultrasound. A tocometer is also used to measure uterine contractions.
The NST is the most widely-used test for fetal well-being. In many countries it is carried out routinely as term approaches. Testing was initially carried out one week apart, but this frequency has been increased in women who are postterm, carrying multiple fetuses, have diabetes or gestational hypertension, or have demonstrated fetal growth restriction. It is also indicated acutely if uteroplacental insufficiency or fetal distress is suspected.
A normal NST has at least two accelerations within 20 minutes. However, the test should proceed for at least 40 minutes before being labelled abnormal, in order to rule out sleep.
An abnormal (nonreactive) response is less than two accelerations over 20 minutes.
While normal NSTs are reassuring, 'insufficient acceleration' has a very high rate of false-negatives - up to 90%, by some studies (Devoe et al, 1986). However, it also warrants further testing via a biophysical profile, if available.
Intrauterine Rescuscitation
If fetal monitoring of any type shows significant cause for concern, immediate action is required. Steps that should be taken include:
- get help
- position mom on her side
- provide a fluid bolus
- stop oxytocin if it is running
- examine and rule out a cord prolapse
- consider oxygen for the mother
- consider amnioinfusion or tocolytics
Monitor the mother's vitals, the fetal heart rate, and the fetal scalp pH or lactate if available. Once the infant is born, perform cord blood sampling (both arterial and venous) to assess fetal pH and well being during labour.
Resources and References
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