Fetal Physiology and the Newborn

Question 1

Structural Features over the Fetal Heart

Answer 1

R, L ventricles = equal size, wall thickness – pump into common circulation

High intrinsic compliance, reduced myocardial stiffness

Floppy, stretchy hearts that don’t contract well

Question 2

Significance of high intrinsic compliance, reduced myocardial stiffness with fetal hearts?

Answer 2

Produces adequate output even with reduced filling pressures (3-4mm Hg) in utero

High compliance: intrinsic stretch of sarcomeres – triggers proliferation, growth of cardiac structures

Question 3

MOA Reduced contractility in the fetal heart?

Answer 3

reduced number of sarcomeres, lower Ca stores within T tubules, lower distribution of beta R within immature SNS

Question 4

Where is the most oxygenated blood in the fetus?

Answer 4

–Umbilical veins immediately after drainage from placenta, prior to entrance of fetal liver/DV
–PaO2 = 30mm Hg

Question 5

Where is the least oxygenated blood in the fetus?

Answer 5

Cranial VC: PaO2 = 14mm Hg

Question 6

What is the PaO2 in the fetal circulation following mixture of blood from the DV, hepatic vein, portal vein and inferior VC?

Answer 6

25mm Hg

Question 7

What is the PaO2 in the brachial cephalic artery traveling to the brain, head?

Answer 7

25mm Hg

Question 8

What is the PaO2 in the descending aorta?

Answer 8

22mm Hg

Question 9

Why is the PaO2 in the descending aorta lower than the BCA?

Answer 9

–Blood in the descending aorta has mixed with deoxygenated blood from the DA - decreases PaO2

Question 10

What are the important structures in the fetal circulation?

Answer 10

–Umbilical veins: carry oxygenated blood to the fetal liver
–Ductus venous: bypass liver via low resistance channel
–Crista dividens: directs oxygenated blood from cd VC through foramen ovale
–Foramen ovale: connection btw RA, LA
–Ductus arteriosus: allows deoxygenated blood to bypass pulmonary circulation and directly mix with oxygenated blood in descending aorta
–Umbilical arteries: carry oxygenated blood from descending aorta to caudal tissues

Question 11

Direction of shunting through DA

Answer 11

In fetal circulation: R to L shunt, mixing of deoxygenated blood with oxygenated blood

In persistent DA: L to R shunt, mixing of oxygenated blood back into pulmonary circulation

Question 12

Does Frank Starling apply to the neonate?

Answer 12

NO!

Question 13

Gas Transport Across the Placenta

Answer 13

inefficient, placental in parallel with tissue circulation

Question 14

What direction is the fetal O2 curve shifted?

Answer 14

curve shifted LEFT
 Fetal blood = hypoxic, needs RBCs with high oxygen affinity

Question 15

Mother to fetus transport depends on:

Answer 15

 Placental BF
 O2 partial pressure gradient
 Placental diffusion capacity
 Maternal, fetal hgb affinities
 Bohr effect (double Bohr effect)

Question 16

Three MOA for increasing oxygen affinity?

Answer 16

1. Fetal hgb with high intrinsic O2 affinity
2. Primates: decreased interaction with 2,3 DPG in fetal hgb, no difference in affinities
3. No fetal hgb: low 2,3-DPG in fetal erythrocytes

Question 17

Fetal Hgb with high o2 affinity

Answer 17

• Fetal hgb unresponsive to 2,3-DPG
• After birth, gradual replacement of fetal hgb with adult hgb

Ruminants

Question 18

Fetal hgb: decreased interaction with 2,3 DPG

Answer 18

Primates

no difference in O2 affinity btw fetal, maternal hgb

Question 19

No Fetal Hgb

Answer 19

 Horses, Pigs, Dogs, Cats – no fetal hgb
* Also mice, rabbits, GPs, chickens
* Low concentration of 2,3-DPG in fetal erythrocytes – high O2 affinity

Question 20

Double Bohr Effect

Answer 20

In the fetus: CO2 down concentration gradient into maternal blood, decreases PaCO2 in fetal blood –> increases pH, increased affinity of hgb for O2

Question 21

Contents of the fetal lung

Answer 21

Fetal lung not collapsed: inflated with liquid to 40% of TLC

Question 22

Things that stimulate first breath in the fetus?

Answer 22

hypoxemia, hypercapnia, high chemoreceptor sensitivity

Stimulation from mother, evaporative cooling

Question 23

What happens when fetus takes first breath?

Answer 23

Resp m generates IMMENSE negative intrathoracic pressure, (-40 to -100cm H2O), surfactant stabilizes open alveoli –> reduction in PVR –> reduced PA, RV, RA pressures
o Umbilical vessels rupture, loss of low-resistance placental circulation

Increases SVR, increased pressure in aorta, LV, LA

Fall in RA pressure as umbilical flow ceases, PVR decreases

Rise in LA pressure with drop in RA pressure reverses flow through foramen ovale
 Mechanical closure via flap

Reverse flow through DA, closes few minutes after birth DT local O2 concentration, decreased circulating prostaglandins

Question 24

Immediate Post Delivery Care

Answer 24

o Clear membranes from head, fluid from oropharynx
o Umbilical vessels milked toward neonate, clamped, ligated 2-5cm from body wall
o Gently rubbed, dried with a towel (avoid vigorous motion)
o Support head and neck, flow by O2, active warming
o Reverse opioids with naloxone sublingual
o Oral dextrose can be considered

Question 25

CPR Efforts in the Neonate

Answer 25

o Intubate with small IVC if necessary, ventilated with a syringe with a 3-way stopcock
 Last ditch effort – consider doxapram (OTM), see below - airways MUST be clear
o Thoracic compressions initiated if bradycardic

Question 26

Drug Support for the Newborn

Answer 26

 Epinephrine 1:1000 – 1 drop sublingual
 Naloxone sublingual: 1 drop
 Atropine – no effect on HR before 11-14d

Question 27

Consequences if Premature

Answer 27

potential for pulmonary surfactant disruption

Question 28

Morbidity/Mortality Parameters

Answer 28

o Umbilical vein lactate >5 mmol/L: increased neonatal morbidity within 48hr
* Apgar scores also prognostic indicators: 7-10 range had greater 48 hour survival