Physiology of FHR monitoring

Question 1

What percentage of total fetal cardiac output is distributed to the placental circulation?

Answer 1

40%

Question 2

What percentage of umbilical blood flow passes through the ductus venosus?

Answer 2

50%

Question 3

What percentage of patients will have category II FHR patterns?

Answer 3

84%

Question 4

Does the placenta have the ability to “autoregulate” blood flow?

Answer 4

No, the volume of blood of delivered to the intervillous space is dependent on adequate blood flow to the uterus.

Question 5

How can blood flow to the placenta be increased or decreased?

Answer 5

Flow can be decreased by vasoconstriction due to uterine contractions, hypotension or hypertension, and sympathomimetic drugs.

Flow can be increased not through vasodilation, but by providing maximal blood flow to the uterus (lateral decubitus position, tocolytics to decrease contractions).

Question 6

How is the diffusion of oxygen and CO2 across the placenta facilitated?

Answer 6

By the differential between the higher O2 pressure in the maternal blood and the lower O2 pressure in the fetal capillaries (and vice versa for CO2).

Question 7

What is the PO2 of blood in the umbilical vein?

Answer 7

Approximately 35 mmHg, the same as the maternal uterine veins.

Question 8

How does the fetus survive with less PO2 relative to maternal values?

Answer 8

1. Fetal blood has more Hgb than adult blod, allowing it to extract maximal amounts of O2.
2. Fetal Hgb has incr O2 affinity - binds to oxgyen more easil than adult Hgb, favoring transfer of O2 from maternal blood to fetal blood
3. Fetal blood flow patterns allow for overperfusion of organs with higher O2 requirements (eg, brain)
4. Fetus has increased cardiac output and heart rate
5. Fetus has more capillaries per unit of tissue than adults do

Question 9

What happens in the fetus when PO2 falls significantly?

Answer 9

1. Redistribution of blood to vital organs (heart, blood, adrenals)
2. Fetal oxygen consumption declines - FHR decreases, myocardium uses less O2
   - If these 2 mechanisms are not sufficient for supplying O2 for metabolic needs, then anaerobic metabolism will result.

Question 10

What is the difference between aerobic and anaerobic metabolism?

Answer 10

Both transform glucose into ATP. The aerobic cycle occurs in the presence of O2 and produces 38 ATP, water, and CO2 per one molecule of glucose. The anaerobic cycle produces 2 ATP molecules, water, and lactic acid per one molecule of glucose. Obviously, anaerobic metabolism is inefficient compared to aerobic.

Question 11

Other than the inefficiency of anaerobic metabolism, what is the consequence?

Answer 11

Lactic acid lowers the pH, creates a metabolic acidosis. Lactic acid diffuses across the placenta more slowly than CO2 diffuses. Over time, ATP depletion and fetal accumulation of lactic acid leads to acidemia, which leads to acidosis and then cell death if O2 isn’t restored.

Question 12

What physiologic factors contribute to the FHR baseline and variability?

Answer 12

Sympathetic, parasympathetic, chemoreceptors, baroreceptors, and higher cortical functions in the brain.

Question 13

How does the autonomic nervous system control the FHR?

Answer 13

The parasympathetic NS (PNS) influences the heart via the vagus nerve, which conducts impulses from the medulla oblongata to the SA node in the fetal atrium. Stimulation of the PNS slows the FHR by overriding the intrinsic rate generated within the SA node.

The sympathetic NS has fibers that terminate throughout the muscle of the heart. Stimulation of these nerves -> increased HR and stroke volume.

Some believe that the constant interplay between sympathetic and parasympathetic causes the timing of each successive heartbeat to vary. Others believe that small changes in the rate of firing of the vagus controls variability directly. During gestation, the PNS matures in the 2nd trimester and the vagus nerve gradually becomes dominant over sympathetic, which explains the slow drop in the normal baseline rate.

Question 14

How do chemoreceptors modulate the FHR?

Answer 14

Chemoreceptors found in the aortic arch and CNS are sensitive to changes in O2 and CO2 in the blood. Increased CO2 -> chemoreceptors signal the medulla oblongata -> vagus nerve stim -> slows FHR gradually (late or prolonged decel)

Question 15

How do baroreceptors modulate the FHR?

Answer 15

Baroreceptors are found in the aortic and carotid arches, where they detect changes in pressure. When BP rises, a vagal reflex slows the heart rapidly. For example, when the umb cord is compressed, flow within the umb vein slows/stops (more compressible than the umb art), this causes an upstream increase in fetal BP -> rapid drop in FHR (variable).

Question 16

What are the causes of minimal FHR variability?

Answer 16

Fetal sleep
Medications
Beginning hypoxia
Almost never caused by hypoxia if decels are absent (intrapartu)

Question 17

What are the causes of absent FHR variability?

Answer 17

Can be related to hypoxia or not related
Other causes:
CNS depressants
Anencephaly
Defective cardiac conduction system
Congenital neuro abnl

Question 18

What are the causes of fetal tachycardia?

Answer 18

Maternal fever
Fetal infection
Fetal cardiac tachyarrhythmia
Can be seen transiently when the fetus recovers from acute hypoxial stress (due to adrenal stimulation)
Tachycardia signifying a fetal acidemia will usu be associated w/ absent variability & decels

Question 19

What are the causes of fetal bradycardia?

Answer 19

Uncomplicated bradycardias of > 80 bpm with nl variability are not associated with fetal acidemia.
Nonhypoxic causes:
Complete heart block
Beta-blockers
Occasionally, a fetus with an idiopathic baseline below 110 w/o pathologic indications

Question 20

What is the pathophysiology of late FHR decelerations w/ retained variability?

Answer 20

Nonasphyxial or “reflex lates” w/ retained variability - neurogenic in origin. When a well-oxygenated fetus has an acute reduction in oxygenation (eg, tetanic contraction), the reduced blood flow falls below the capacity to extract sufficient O2 in the intervillous space. Chemoreceptors detect the hypoxemia and initiate the vagal bradycardic response. The bradycardia is “late” because the chemoreceptor has to first detect the hypoxemia. Bc the fetus is centrally well-oxygenated, variability is retained.

Question 21

What is the pathophysiology of late FHR decelerations with decreased or absent variability?

Answer 21

Asphyxial lates occur when there is insufficient oxygen for myocardial metabolism or normal cerebral function. Likely to occur in chronic placental insufficency.

Question 22

What is the pathophysiology of variable FHR decelerations?

Answer 22

During the 2nd stage or rapid descent in the 1st stage, variables can be the FHR response to vagal stimulation induced by sudden increases in pressure on the fetal cranium.

Cord compression causes an increase in BP -> increased BP -> stimulation of baroreceptors in the aortic arch -> quick bradycardial response. Variable decels are associated with transient hypoxia, and will, if repetitive, lead to acidemia.

Question 23

What is a sinusoidal FHR pattern?

Answer 23

Regular, smooth sine wave-like baseline, w/ a frequency of approx 3-6 per minute and amplitude range of up to 30 bpm. Waves are extremely regular and smooth, and there is absent beat-to-beat or short-term variability. Noted in anemic and severely asphyxiated infants.

Pseudosinusoidal variants of long-term variability are sometimes seen after maternal narcotic administration.

Question 24

What is the lower limit of nl for umbilical artery blood pH?

Answer 24

7.1, using 2 SDs below average

Question 25

What is base excess and what is the mean?

Answer 25

Base excess calculates the metabolic contribution to the acidemia by correcting for the elevated bicarb due to the presence of high CO2. Most of the base excess seen in fetal hypoxemia is lactic acid. The mean is -4 milliequivalents/L, the 2.5th percentile is -11 milliequivalents/L.

Question 26

4 criteria to associate “overshoots” with acidemia

Answer 26

These are ominous, and rare
Follows, but does not preceed, the decel
Accel takes a long time to return to baseline
Associated with “blunted” variable decels

Question 27

Fetal pH is normally how much lower than maternal?

Answer 27

0.1

Question 28

What are the mean 5th% and 95% of umbilical artery pH measurements for term kids?

Answer 28

Mean: 7.28
5%: 7.14
95%: 7.4