Ch. 2 Physiologic Basis of Fetal Monitoring

Question 1

Why do we use fetal monitoring?

Answer 1

To assess the adequacy of fetal oxygenation

Question 2

Purpose of the placenta?

Answer 2

• Fetal lung (respiration)
• Kidney (excretion)
• GI tract (nutrition)
• Skin (heat exchange)
• Endocrine organ that produces steroid (estrogen, progesterone) and protein (human chorionic gonadotropin, human placental lactogen) hormones

Question 3

Within the placenta, where does maternal-fetal and fetal-maternal exchange occur?

Answer 3

Between the intervillous space (where maternal blood occupies) and the fetal capillary

Question 4

Blood supply to uterus

Answer 4

Uterine arteries but anastomoses occur between these vessels, other branches of the hypogastric arteries, and ovarian arteries

Question 5

Factors that decrease uterine blood flow (7)

Answer 5

1. Supine position (compression of vena cava > decreased return of blood to the heart > decreased maternal CO > maternal hypotension > decreased uterine blood flow)
2. Anesthesia: Sympathetic blockade > maternal hypotension > ephedrine (mixed alpha- and beta-adrenergic stimulator) used to restore maternal BP after hypotension induced by anesthesia
3. Hypertension: decreased intervillous space bloow flow
4. Diffusion distance: thickness of the placental membrane between intervillous space and fetal capillary may decrease transfer of oxygen (ex: fetal erythroblastosis with placental edema, villous hemorrhage and edema in diabetes)
5. Contractions: spiral arteries that traverse myometrium are subject to collapse as the uterus contracts and intramyometrial pressure exceeds spiral arterial pressure
6. Exercise: diverts blood away from uterus to supply somatic muscle groups (tachycardia following exercise is a sympathetic response to a period of reduced fetal oxygen)
7. Surface area: Anything that decreases effective surface area of placenta will clearly increase potential for fetal hypoxia. Placental abruption!

Question 6

Fetal circulation

Answer 6

Placenta > umbilical vein > liver > ductus venosus IVC > RA > cephalic circulation

SVC > ductus arteriosus > lower body > placenta

Question 7

How much of fetal cardiac output goes to the placenta?

Answer 7

30%

Question 8

How does the fetus exist at a maximum pO2 below that of the uterine vein (about 35 mm Hg)?

The oxygen content of the umbilical venous blood closely approximates that of the uterine venous drainage.

Answer 8

1. Fetal hemoglobin concentration is higher than in the adult, allowing for a much greater oxygen-carrying capacity
2. Fetal cardiac output far exceeds that of the adult on a volume per unit body weight basis
3. Fetal hemoglobin dissociation curve favors higher saturation at a given pO2

So, although the fetal pO2 is lower, the fetus is able to compensate by having an increased oxygen content, due to the high hemoglobin concentration and the characteristic of the fetal hemoglobin dissociation curve.

Question 9

What explains the gradual decrease in FHR that occurs with increasing gestational age?

Answer 9

Increase in parasympathetic tone

Question 10

In a fetus with heart block, what is the FHR?

Answer 10

Usually in the range of about 60 BPM, which represents the intrinsic ventricular or nodal rate

Question 11

FHR variability is largely under the influence of?

Answer 11

the CNS

Parasympathetic impulses and sympathetic impulses originate in the brainstem

Question 12

Early Deceleration

Answer 12

Pressure on fetal head > decrease cerebral blood flow > central vagal stimulation > FHR decel

Will usually see early decels between 4 and 6 cm dilation

If you administer atropine, earlies will disappear (confirms vagal reflex)

Question 13

Variable deceleration

Answer 13

When the umbilical arteries are occluded, there is a sudden increase in total fetal peripheral resistance > sudden fetal HTN > stimulation of fetal baroreceptors occur instantly > vagal response > PS outflow that produces sudden slowing effect on fetal atrial pacemaker

Question 14

What will you see on FHT with mild forms of umbilical cord compression?

Answer 14

Only the umbilical vein may be occluded, resulting in FHR acceleration

Question 15

Mechanism of late deceleration

Answer 15

• Without acidemia:
  
  • UPI > chemoreceptor stimulus > alpha adrenergic response > Fetal HTN > Baroreceptor Stimulus > PS Response > Decel
• With acidemia:
  
  • Myocardial depression > Decel

Question 16

What is sinusoidal FHR pattern associated with?

Answer 16

Chronic fetal anemia associated with erythroblastosis

Question 17

Causes of neurological damage (9)

Answer 17

1. Prematurity
2. IUGR (perinatal infections, teratogens, congenital anomalies, inadequate nutrition, UPI)
3. Traumatic birth
4. Prolonged labor
5. Anesthesia and Analgesia
6. Genetic factors
7. Congenital Infection
8. Toxins
9. Antenatal insults

Question 18

The most common diseases which alter uterine contractions?

Answer 18

1. Polyhydramnios > variable effect on tonus until uterus is severely stretched and baseline tone is often low to low normal
2. Preeclampsia > tonus unaffected but frequency and intensity of contractions increased
3. Abruption > greatest degree of hyperactivity (polysystole, tachysystole, hypertonus)

Question 19

Causes of fetal tachycardia (9)

Answer 19

1. Fetal hypoxia
2. Maternal fever
3. Parasympatholytic drugs (Atropine, Vistaril)
4. Meternal hypothyroidism
5. Fetal anemia
6. Fetal heart failure
7. Amnionitis
8. Fetal cardiac tachyarrhythmia
9. B-sympathomimetic drugs

Question 20

What is fetal bradycardia with good variability (reassuring) often associated with?

Answer 20

Occiput posterior or transverse position (increased vagal tone)

Question 21

What are the components of a reassuring variable deceleration?

Answer 21

1. The decel lasts no more than 30 to 45 sec
2. There is a rapid return to baseline from the nadir of the decel
3. Normal short-term variability of baseline FHR is present (not absent or increased), and normal baseline FHR continues