fetal circulation

Basic Cardiac Embryology & Fetal Circulation:

Basic Cardiac Embryology & Fetal Circulation

Timeline of Events:

Timeline of Events Week 3 (21 days) - heart tube forms 23 days- heart beats Week 4 - cardiac loop forms Week 7 - heart fully developed the 1st organ to fully develop in the fetus resembles adult heart except for foramen ovale

Slide 3:

approx. 3wks approx. 4 wks approx. 7 wks Heart tube cardiac loop forms fully developed

Primitive Cardiac Tube (bulboventricular tube):

Primitive Cardiac Tube (bulboventricular tube) Consists of 6 parts functioning as a tubular pump: sinus venosus primitive atria primitive ventricle bulbus cordis conus cordis truncus arteriosus

Cardiac Loop:

Cardiac Loop As the tube grows it bends in an anterior & rightward direction forming the bulboventricular loop . Normal looping = dextro or ‘d-looping’ Abn. looping to left = levo or ‘l-looping’

What forms what:

What forms what Sinus venosus (1) Primitive atria (2) 1 & 2 contribute to SVC, IVC, CS, RA & LA Atrioventricular canal (3) large area of connection between primitive atria & ventricle endocardial cushion will form here; will form parts of MV & TV

More...:

More... Primitive ventricle (4) - forms into LV Bulbus cordis: 3 parts Primitive RV (5) Conus cordis (6) - will form the ventricular outflow tracts (crista supraventricularis) Truncus arteriosus (7) - will form the aorta & pulmonary artery trunk

Aortic arches:

Aortic arches 6 paired sets of arches form about week 4 These develop into the adult arterial system 3rd set forms common & internal carotid arteries bilaterally 4th develops into aortic arch 6th develops into right & left pulmonary arteries & ductus arteriosus

Septation (septal development):

Septation (septal development) Atrial septum: Atrial septum begins from the atrial roof; septum primum & septum secundum separate common atria into rt & lt halves Endocardial cushions: divide the AV canal into 2 orifices & parts of MV & TV; help form part of the atrial primum septum & membranous ventricular septum

Atrial Partitioning I :

Atrial Partitioning I Septum primum grows from atrial roof toward endocardial cushions Foramen primum: shunt that closes Foramen secundum: perforates septum primum, allowing shunt Septum secundum grows down, overlapping foramen secundum

Atrial Partitioning II :

Atrial Partitioning II Septum secundum grows down,overlapping foramen secundum Foramen ovale: between septum primum & septum secundum Remaining portion of septum primum forms valve of foramen ovale

Atrial Partitioning III:

Atrial Partitioning III • Fetus • right side high pressure (high pulmonary resistance, etc.) • well oxygenated blood streams through foramen ovale • valve of foramen ovale closes with left atrial contraction • After birth • right side low pressure (low pulmonary resistance) • valve remains closed (physiological closure) • valve eventually fuses (anatomical closure): fossa ovalis

Ventricular Partitioning • Closes in week 7: not part of fetal circulation • Muscular IV septum grows from floor • Membranous IV septum forms from endocardial cushions and bulbar ridges • Closure of membranous IV is associated with partitioning of truncus arteriosus:

Ventricular Partitioning • Closes in week 7: not part of fetal circulation • Muscular IV septum grows from floor • Membranous IV septum forms from endocardial cushions and bulbar ridges • Closure of membranous IV is associated with partitioning of truncus arteriosus

Partitioning of Truncus Arteriosus • continuous set of ridges in bulbus cordis (bulbar ridges) and truncus arteriosus (truncal ridges) • grow toward each other, spiraling 180º • fuse to form spiraling aorticopulmonary septum, dividing aorta & pulmonary trunk • bulbar ridges involved in formation of IV septum • bulbar & truncal ridges derived from neural crest cells—clinical implications:

Partitioning of Truncus Arteriosus • continuous set of ridges in bulbus cordis (bulbar ridges) and truncus arteriosus (truncal ridges) • grow toward each other, spiraling 180º • fuse to form spiraling aorticopulmonary septum, dividing aorta & pulmonary trunk • bulbar ridges involved in formation of IV septum • bulbar & truncal ridges derived from neural crest cells—clinical implications

Valves:

Valves AV valves (mitral & tricuspid) are formed mainly from the internal ventricular muscular wall Aortic & pulmonic roots are formed from a separation that occurs in the truncus arteriosus. The semilunar valves are formed from small tubercles in the truncus.

Fetal circulation (prenatal):

Fetal circulation (prenatal) In the adult, the lungs provide oxygen and CO2 exchange In the fetus, the lungs are basically collapsed & fluid-filled so there is high resistance to blood flow The placenta provides oxygen for the fetus;delivers nutrients & removes wastes

Placenta and umbilical vessels:

Placenta and umbilical vessels Placenta functions as fetal lungs and the oxygenated blood passes into left umbilical vein which enters the liver. The umbilical arteries(right and left) carry deoxygenated blood from the fetus to the placenta

Umbilical Circulation:

Umbilical Circulation Pair of umbilical arteries carry deoxygenated blood & wastes to placenta. Umbilical vein carries oxygenated blood and nutrients from the placenta.

The left umbilical vein:

The left umbilical vein Highly oxygenated,nutrient rich blood comes from the left umbilical vein. Much of this blood is diverted into ductus venosus, which connects the left umbilical vein to IVC in the liver left umbilical vein

The Placenta:

The Placenta Facilitates gas and nutrient exchange between maternal and fetal blood. The blood itself does not mix.

Umbilical arteries:

Umbilical arteries About 65%of blood in the descending aorta Passes into umbilical arteries(right and left) Which are direct branches of fetal internal iliac arteries(hypo gastric arteries) Remaining 35% of blood supplies the lower half of the body and viscera

Sphincter mechanism in the liver:

Sphincter mechanism in the liver This regulates the flow of remaining blood from umbilical vein into IVC through hepatic veins. It is generally agreed that a physiological sphincter exists and prevents overloading of heart when the venous flow in the left.umb.vein is high(eg.during uterine contractions)

Problems of fetal circulation:

Problems of fetal circulation The lungs do not function in fetal life. Hence the left heart and the systemic circulation are maintained by number of by-pass mechanisms namely; Foramen ovale Ductus arteriosus Ductus venosus

foramen ovale:

foramen ovale Blood is shunted from right atrium to left atrium, skipping the lungs. More than one-third of blood takes this route. Is a valve with two flaps that prevent back-flow.

Ductus arteriosus:

Ductus arteriosus Low oxygenated blood from SVC and some amount of blood from IVC pass into right ventricle and thence into pulmonary artery(trunk). 90% of this blood is by-passed into the aorta by a channel ductus arteriosus

Ductus arteriosus:

Ductus arteriosus Connects the left branch of the pulmonary trunk to arch of aorta(beyond the origin of left subclavian artery) It protects the lungs from circulatory overloading.

Pulmonary vascular resistance:

Pulmonary vascular resistance is high in fetal life and pulmonary blood flow is low as the lungs do not need much blood for their survival but developing brain does.

Postnatal changes:

Postnatal changes Once the child takes the first respiration,pulmonary circulation begins and the right and left hearts become completely independent of each other. All the by-pass channels having served their purpose,obliterate. Foramen ovale is closed and becomes fossa ovalis in the right atrium

Ligamentum arteriosum:

Ligamentum arteriosum The ductus arteriosus becomes a fibrous band called ligamentum arteriosum(which has left recurrent laryngeal nerve hooking around) (By 96 hours after birth 100% closure occurs) Many factors contribute to this process of obliteration called involution(mainly oxygen, and transforming growth factor)

Ligamentum venosum:

Ligamentum venosum Ductus venosus becomes a fibrous band called ligamentum venosum which is seen in continuation with ligamentum teres(obliterated left umbilical vein)

Other changes and clinical aspects:

Other changes and clinical aspects The umbilical arteries become umbilical ligaments attached to the internal iliac arteries upto superior vesical arteries. Any failure of all these closures lead to Patent foramen ovale,ASD,VSD,PDA and coarctation of aorta. The left umbilical vein remains patent for considerable time and can be used for exchanging transfusions.

Fetal Post-natal:

Fetal Post-natal Right-sided pressure higher Higher pulmonary resistance 3 shunts exist Placenta provides oxygenated blood IVC blood is O2 rich SVC,CS is O2 poor Lungs inflate; lowers pulm resistance/ Rt-heart pressures Lt-heart press. rises; closes foramen ovale Ductus arteriosus closes w/in 48-72 hrs; becomes ligamentum Ductus venosus closes as flow ceases; becomes ligamentum

Changes at birth:

Changes at birth When newborn begins to breathe, the baby’s body gets higher levels of O2 Pulmonary vascular resistance decreases, blood flow into the lungs increases LAP rises; closes foramen ovale Increases O2 levels, lower vascular resistance closes ductus arteriosus

Continued..:

Continued.. Clamping umbilical cord ends placental function; closes umbilical vein & ductus arteriosus In premature infants, PFO & PDA are common

Slide 36:

Foramen ovale Closes shortly after birth, fuses completely in first year. Ductus arteriousus Closes soon after birth, becomes ligamentum arteriousum in about 3 months. Ductus venosus Ligamentum venosum Umbilical arteries Medial umbilical ligaments Umbilical vein Ligamentum teres

Problem with persistence of fetal circulation:

Problem with persistence of fetal circulation Patent (open) ductus arteriosus and patent foramen ovale each characterize about 8% of congenital heart defects. Both cause a mixing of oxygen-rich and oxygen-poor blood; blood reaching tissues not fully oxygenated. Can cause cyanosis. Surgical correction now available, ideally completed around age two. Many of these defects go undetected until child is at least school age.

Slide 38:

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