Fetal and Neonatal Circulation Flashcards
1
Q
Four fetal shunts
A
- oxygenated blood returning from the placenta via the umbilical vein passes through the ductus venosus, mainly bypassing the liver, then passes into the interior vena cava
- 40% of inferior vena cava flow passes through the foramen ovale into the left atrium
- 3/4 or more of blood from the right ventricle is pumped through ductus arteriosus into descending aorta bypassing the lung
- blood in the descending aorta passes primarily through hypogastric arteries into the two umbilical arteries and into the placenta
2
Q
Cardiopulomary adjustments at/after birth
A
- loss of placental circulation requires the newborn to breath on its own
- dramatic increase in pulmonary blood flow
- closure of the ductus venosus, foramen ovale, and ductus arteriosus
3
Q
First breath
A
- triggered by mild hypoxia, hypercapnia, tactile stimuli and cold skin
- first inspiratory effort requires a transpulmonary pressure of 60 cm H20 to increase the lung volume by 40 ml
- breathing becomes easier once the alveoli are open and type II alveolar pneumocytes deliver surfactant to the air-water interface
- respiratory distress syndrome is caused by deficiency of surfactant often found in premature births
4
Q
Effects of birth on pulmonary vascular resistance
A
-in fetus pulmonary vascular resistance is high, pulmonary blood flow is low, and mean pulmonary arterial pressure is high
-at birth each of these three situations rapidly reverses
-the primary event is the fall in resistance, which occurs because of:
1- the pulmonary blood vessels are no longer being crushed
2- breathing causes increased PO2, which, in turn causes vasodilation
3- local prostaglandins causes vasoldilation
-the reason the pressure falls after birth is that the fall in pulmonary vascular resistance is greater than the rise in blood flow
5
Q
Placenta functions
A
- lungs: gas exchange
- GI- nutrition
- liver- nutrition and waste removal
- kidneys- fluid and electrolyte balance, waste removal
- insuffiency causes type II intrauterine growth restriction
- any insult that interferes with gas exchange
6
Q
When do fetal organs develop
A
- the fetal heart begins beating approximately one month following fertilization
- HR increases over the course of pregnancy from 65 to 140 bpm near birth
- at one month all the organs of the fetus have been formed and over the next 2-3 months minute details of each organ are established and become grossly the same as those of the newborn
- nervous system, kidneys and liver require more than 9 months for full development
7
Q
Intervillous space
A
- the intravillous space contains a pool of maternal blood for gas exchange
- the outer surface of mature chorionic villus is covered with a thin layer of syncytiotrophoblast
- under this are cytotrophoblasts, mesenchyme, and fetal blood vessels
- maternal blood is trapped in the intervillous space, between the endometrium on the maternal side of the villi on the fetal side
- in the mature placenta, spiral arteries from the mother empty directly into the intervillous space, which is drained by maternal veins
- the villi look like a thick forest of trees arising from the chorionic plate which is the analog of the soil from which trees sprout
8
Q
Maternal and fetal oxygen levels
A
- uterine artery- 100 PO2, Hb saturation 97.5%
- intervillous space- 30-35 PO2 Hb saturation 57-67
- uterine vein- 30 PO2 Hb sat 57
- umbilical arteries 23 PO2 Hb sat 60
- umbical vein 30 PO2 Hb sat 85%
9
Q
Closure of placental circulation
A
- increases the pressure in the aorta
- the placental receives 50% of the CCO in fetal circulation
- closure causes peripheral resistance to double causing an increase in aortic and left ventricle pressure
10
Q
Closure of foramen oval
A
- caused by reversal of right/left atrial pressure
- increased pulmonary circulation causes increased venous return to the left atrium
- there is a decrease in right atrial pressure
- the reversal of pressure closes foramen ovale’s valve
- permanent seal forms from in a few months or years. It becomes fossa ovalis
- failure of closure: patent foramen ovale
11
Q
Closure of ductus venosus
A
- in the fetus, large portion of blood in the portal vein enters the ductus venosus bypassing the liver
- within 3 hrs of birth, constriction of the vascular smooth muscle within the ductus venosus completely occlude the shunt. It becomes ligamentum venosum
- mechanism: possibly increased PO2 and reduced prostaglandins
- failure of closure: portosystemic shunt
12
Q
Closure of ductus arteriosus
A
- within a few hours, ductus arteriosis closes because of constriction of the wall
- thrombosis obliterate the lumen in a month. It beomces ligamentum arteriosum
- mechanisms: increased PO2 and decreased circulating prostaglandin cause constriction. Also bradykinin from the lung is involved
- failure of closure: patent ductus arteriosus. It leads to pulmonary hypertension, and possibly congestive heart failure and cardiac arrythmias
13
Q
Changes in circulation at birth
A
- at first the ductus arteriosus and foramen ovale both close and establish separate right and left circulatory systems
- as the pressure in the left atrium rises higher than the pressure in the right atrium-owing to the large decrease in pulmonary vascular resistance the flap of the foramen ovale pushes against the septum, thus preventing blood flow from the left to right atrium
- the flap seals shut
- as aortic pressure exceeds pressure of pulmonary artery, blood flow through the ductus arteriosus reverses
- well-oxygenated aortic blood now flows through the ductus arteriosus
- this high PO2 causes vasoconstriction, which functionally closes the ductus arteriosus within a few hours
- fall prostaglandin levels also contribute to the rapid closure
- eventually, the lumen of the ductus becomes anatomically obliterated
- the elimination of the fetal shunts and the oxygenation of the blood in the lungs lead to major increases in the O2 saturation and PO2 in the circulation
14
Q
Neonatal circulation
A
- characterized by a reversed flow through the ductus arteriosis to the lung- enhances O2 uptake
1) Loss of blood through placenta produces a doubling of the systemic vascular resistance. This results in an increase in aortic pressure as well as increased pressures in the left ventricle and left atrium
2) Resp activity decrease pulmonary vascular resistance (no longer compressed and higher blood O2 levels produce arteriolar vasodilation as does release of prostaglandins
3) Reduced the pulmonary arterial pressure and right ventricular and right atrial pressures
4) Following lung inflation a large amount of blood is now directed to left atrium from the lungs. Increased left atrial pressure
5) Increase in left atrial and decrease in right atrial close foramen ovale and flow from IVC is now limited to right atrium
6) 1-3 hours ductus venosus contracts and closes
7) Increased aortic pressure and decrease pulmonary pressure blood flow through ductus arteriosus reverses.. increases blood flow to lung, increasing oxygen uptake
8) Increased plasma PO2 and decreases prostagland ductus arteriosus contracts, 2-3 days it will stop blood flow
15
Q
Patent Ductus Arteriosus
A
- 1 of every 2000
- small opening no symptoms but murmur exists
- large opening breathing is difficult, feeding poor, HR and sweating elevated, weight low, endocarditis risk- need surgery
