Basic FHR pattern

 

Changes of FHR pattern

Sequence of events in response to fetal distress

How to use and interpret Fetal Heart Rate (Cardiotocography)

Use of Non-stress test as part of Biophysical profile

Not in common use clinically

Fetal ECG monitoring

Fetal heart rate pattern:

Electronic fetal heart "rate” monitoring is the plotting of fetal heart rate over time, (it is not an ECG, it is just the fetal heart rate). Fetal heart rate is characterized by having a basal pattern and shows changes to this basal pattern depending on fetal environment changes:

1-  Basal Fetal heart rate pattern:   The pattern shows a basal "rate" of fetal heart and  basal satate of "varaibility".

Unlike adult heart rate, fetal heart rate (FHR) is much faster and irregular so it is constantly moving up and down an imaginary "basal heart rate”, which is normally from 120 to 160 bpm.

The oscillations up and down this imaginary line are called variability, and, there are 2 types of variability seen during the normal tracing of the fetal heart rate:

a.      The first is called short term variability or beat to beat variability, this is seen as the small oscillations in the range of 5 to 10 bpm. This reflect that every heart beat is in a different rate than the following and the previous beat.

b.      The second type is called long-term variability in the range of 10 to 25 bpm. Consecutive beats are trending up or down the basal FHR line, they are in the range of 3 to 10 cycles per minute.

Variability reflects the interaction between 3 forces influencing fetal heart rate:

i.      Sympathetic causing  tachycardia (excitatory influence)

ii.      Parasympathetic causing bradycardia (inhibitory influence)

iii.      Pacemaker favouring a stable basal fetal heart

So what happens is that the sympathetic system will drive the fetal heart rate up till it reaches a threshold at which the parasympathetic system starts driving the rate downwards and in the middle of this up and down struggle the pace-maker is struggling to keep the fetal heart steady at what is interpreted as "basal FHR”

2-    Changes to basal FHR pattern: The fetal heart rate undergoes constant and minute adjustments in response to the fetal environment and stimuli, these adjustments are seen as change of the usual pattern; the basal FHR and the 2 variability types.

Changes from the usual pattern are what we look for to predict presence or absence of fetal hypoxia.

PS: "Fetal well-being” is a term to describe a fetus with normal growth pattern + no congenital malformations diagnosed + not in state of hypoxia or acidosis, "fetal distress” refers to fetal hypoxia and acidosis.

Changes (from the usual pattern of basal FHR and Variability) can be transient or persistent.

A-    Transient changes to the FHR, of 2 types:

a.      Accelerations:

Transient rise of FHR by > 15bpm for > 15 seconds

They are usually associated with fetal Stimulation of the peripheral nerves of the fetus by its own activity.

In cases of chronic hypoxia with CNS depression there will be absent accelerations and reduced variability.

b.      Decelerations: Transient fall

of FHR by > 15bpm for > 15

seconds.

According to its relation to the uterine contractions during labor they are of 3 types, each with a different interpretation:

i. Variable decelerations: The only type of deceleration which may occur during pregnancy as it is not related to uterine contractions, frequently seen during labor especially in the second stage, assumed to be due to cord compression.

Do not signify fetal distress unless frequent and associated with other non-reassuring changes.

ii. Early Decelerations: Occur during a contraction and FHR drops with the onset of contraction and recovers with the cessation of uterine contraction.

Signify respiratory acidosis with accumulation of carbon dioxide, unlike metabolic acidosis, carbon dioxide is volatile and has increased permeability, so, it accumulates rapidly with onset of contraction and is also washed out rapidly by the placenta with cessation of contraction.

iii. Late decelerations: Occur during a contraction, FHR drops after the contraction starts and recovers after cessation of the contraction.

Signify metabolic acidosis occurring with the uterine contraction, metabolites accumulate after the contraction starts and are slowly washed away when the placental bed opens after the contraction ends. So typically the onset is after the start of contraction (metabolites accumulate) and recovery is after the contraction

ends (metabolites slowly washed away since they are not as permeable as carbon

dioxide). Point where the FHR recovers (with or after cessation of contraction) is much more important than the onset of the deceleration.

B-    Persistent changes to FHR (lasting more than 15 minutes), there are 3 types:

a.Tachycardia: FHR more than 160 bpm more than 15 minutes

b. Bradycardia: FHR less than 120 bpm more than 15 minutes (had been changed

to less than 110 bpm by some authorities)

c. Loss of variability more than 15 minutes, which may be:

i. Loss or reduced of long term variability (<3 cycles/min)

ii. Loss of short term variability (< 5 bpm range)

SO WHAT IS THE SEQUENCE OF EVENTS IN CASE OF FETAL HYPOXIA?

1-     There will be no accelerations, due to mild depression of CNS with decreased fetal activity.

2-     There may be tachycardia, but this is not common, tachycardia is usually due to arrhythmia, maternal fever or medications rather than hypoxia.

3-     Then there will be early decelerations due to respiratory acidosis

4-     With more depression of the autonomic nervous system there will be loss of variability.

5-     With development of metabolic acidosis late decelerations will set in.

6-     Then with further acidosis there will be myocardial depression with persistent fetal bradycardia.

7- With more depression of the myocardium there will loss of short term variabiliity together with bradycardia, in such omnious condition the late decelerations are obscured with the bradycardia (Sinusoidal rhythm)

 

How to use the information listed above?

May be used to reflect fetal well-being either through pregnancy or during fetal birth, however the concept is not the same:

1-     Antepartum:

2-     Intrapartum:

a.      We want to assure fetus is not suffering acute distress during uterine contractions and transient drop of placental blood supply.

b.      The main idea is to trace FHR continuously during labor and look for non-reassuring signs in the form of:

              i.      Decelerations (especially late decelerations)

              ii.      Persistent changes in FHR patterns (Tachycardia, bradycardia and loss of variability for > 15minutes)

 

Interpretation and terms used to communicate results of fetal CTG trace:

-Antepartum Non-Stress Test (NST) (20minute trace) is either "reactive” or "non-reactive”.

-Intra-partum (Cardiotocography) CTG is either  "reassuring” or "non-reassuring”

Reassuring trace:

• Basal FHR is between 120-160 bpm

• Long term variability preserved

• No decelerations (except occasional variable decelerations, infrequent early decelerations)

• Accelerations are an additional bonus.

Non-reassuring trace:

View Strips of CTG:

Antepartum NST:

- Reactive NST

- Non-reactive NST

Intrapartum CTG trace:

-Reassuring CTG

-Non-reassuring CTG

How to perform (NST)?

During pregnancy in a healthy fetus the FHR pattern will be normal, there will be fetal movements perceived by the mother and when recorded in the Non-Stress test will show response of FHR by acceleration.

Presence of 2 accelerations in 20 minute trace is called reactive Non-Stress test

In cases of fetal distress (Hypoxia) the fetus will show decreased activity and movement, and there will be no response to movement by an acceleration, there may also be loss of variability. The trace can be extended for further 20 minutes or repeated after 2 hours or the fetal activity provoked by acoustic stimulation (sound), palpation of maternal abdomen, intake of juice (glucose), depending on the clinical situation. 

Non-stress test is also used as one item in the Biophysical profile.

Biophysical Profile (BPP):

Biophysical profile is another more accurate method to exclude fetal distress antenatally.

It is the simple recording of 5 items each scores either 2 if normal or 0 if absent or abnormal, it is recommended that the score is done after the mother drinks some juice or has a snack to avoid a sleepy fetus, also some medications may affect the result of BPP such as Magnesium administration as well as antihistaminics.

The original 5 items scored in the biophysical profile are:

Not all items of the biophysical profile are of the same importance. Indeed NST and AF volume assessment are the 2 most important items. Amniotic fluid volume reflects a rather chronic form of fetal distress while NST reflects acute distress.

In view of this information a Modified biophysical profile had been proposed, it consists of NST together with amniotic fluid index (not vertical depth of largest pocket) if score is 4  (reactive NST and AFI >5) then it is considered reassuring.

*Recommended management based on the original BPP score and items would be:

- Socre of 8 is generally reassuring, unless the zero is the score due to olighydramnious, further investigations should be done or plan delivery if GA is beyond 38 weeks.

- Score 6 is equivocal and should be repeated unless the items with 0 score are a non-reactive NST and oligohydramnios, fetus may be in danger of hypoxia and further investigation (as doppler assessment of UA flow is warranted).

- Score 4 is non-reassuring fetus is in danger.

- Score 2 or 0 is ominous and fetus is in eminent danger.

Decision of birthing should be based on the clinical criteria as a whole not just the BPP. Elderly primigravida or diabetic patient, also patient with previous bad obstetric history as well as a fetus suffering FGR are all scenarios that may change you action for the same BPP score depending on the patient's clinical situation.

Fetal ECG change analysis:

The changes in fetal ECG can be "automatically" traced using a STAN monitor (ST segment analysis). The Monitor will automatically alert for:

• Episodic rise in T:QRS ratio

• Baseline rise in T:QRS ration

• Recurrent biphasic (depressed) ST segments

These changes reflect hypoxic changes more accurately than FHR monitoring, STAN specifically reflects:

Interpretition with FHR + STAN

- STAN is a handy tool in cases of non-reassuring FHR traces, but in reassuring trace or obviously omnious (sinusoidal pattern or lost varaibility) trace it would not alter the management decision.

- In case of a non-reassuring trace (Tachycardia, bradycardia, decelerations but with good variability) the decision to wait for natural birth or proceed to a caesarean birth is equivocal, then STAN alerts can guide the decision:

• Absent ST alerts favors expectant managment

• Presence of ST alerts favors expedited delivery.