CVS S3 - Foetal Heart Development & Congenital Heart Defects Flashcards
What are “acyanotic” heart defects?
Heart defects which do not result in a lower than normal blood oxygen concentration
(So babies aren’t blue)
What are the most common congenital heart defects?
Ventricular septal defects (VSD) are the most common, followed by atrial septal defects (ASD)
What occurs after looping of the primitive heart tube?
Septation
Describe septation
The junction between the ventricle and atrium narrows and the atrioventricular canal is formed, providing framework for septation
Endocardial cushions form and grow superiorly/inferiorly to form the septa
This divides the heart into right and left channels
Describe atrial septation
The septum primum grows down towards the fused endocardial cushions
The ostium primum is the hole just before the septum primum fuses with the endocardial cushions
Before this hole closes, a second hole, the ostium secundum, forms above by apoptosis
A crescent shaped second septum then forms, the septum secundum, which has a hole called the foramen ovale
Why is atrial septation so complex?
There need to be two septa and two holes so that:
a) Blood can flow through both holes from one side of the heart to the other before birth
b) After birth, the different pressures in the atria cause the septa to come together, meaning blood can no longer pass between the left and right sides of the heart
What about the foramen ovale and ostium secundum allows blood flow between left and right atria to stop after birth?
Holes are slightly offset, so that when increased pressure in the left ventricle occurs when the first breath is taken and the septa are forced closed, there is essentially no longer a hole for the blood to pass through
What is the adult remnant of the foramen ovale?
The fossa ovalis
Describe the muscular component of ventricular septation
Has muscular and membranous components
Muscular component forms most of the septa and grows upwards towards the fused endocardial cushions
Leaves a hole at the top called the primary inter ventricular foramen
Describe the ductus venosus and explain why it is necessary
Allows oxygenated blood from the umbilical vein to bypass the liver and go straight to the inferior vena cava
It’s necessary because if the blood went to the heart via the liver, much of the oxygen and nutrients would be removed, meaning there is not enough for the brain and the rest of the body
Describe the ductus arteriosus and explain why it’s necessary
Shunt between the pulmonary trunk and the aorta
Necessary to bypass foetal non functional lungs
Describe the membranous component of ventricular septation
‘Fills the gap’ of the primary interventricular foramen
Derived from the spiral septum which divides the truncus arteriosus into the main outflow vessels
Why is the foramen ovale necessary?
Foetal lungs are non functional and to develop correctly, it’s important they don’t receive much blood, which could damage them
What are some important foetal shunts which allow different blood flow before and after birth?
Ductus venosus
Foramen ovale
Ductus arteriosus
How common are congenital heart defects?
About 6-8 per 1000 births
Give some examples of acyanotic heart defects
Atrial septal defect Patent foramen ovale Ventral septal defect Patent ductus arteriosus Coarction of the aorta
Describe atrial septal defects (ASD)
Acyanotic
An opening in the septa between the atria which persists after birth
~67 in 100 000 live births
Usually due to failure of the foramen ovale to close
However because LA pressure>RA pressure, blood flows left to right so very little/no deoxygenated blood ends up in atrial circulations
Describe patent foramen ovale
Acyanotic
Not true ASD
Generally clinically silent due to pressure LA>RA
Can be a route for venous emboli to migrate into systemic circulation (aka paradoxical embolism) if transient RA pressure increase
Describe ventricular septal defects
Acyanotic
Hole/defect in the ventricular septum
Most commonly occurs in membranous section
Pressure in LA>RA so blood moves from left to right
Describe patent ductus arteriosus
Acyanotic
Failure of ductus arteriosus (foetal shunt between aorta and pulmonary trunk) to close after birth
Blood moves from aorta to pulmonary trunk
A constant mechanical murmur may be heard
If untreated may result in remodelling of pulmonary circulation and increased pulmonary resistance
This may cause blood flow in the ductus arteriosus in the opposite direction (Eisenmenger Syndrome)
What happens to the foetal shunts after birth?
Foramen ovale: closes when RA pressure<LA pressure. Becomes fossa ovale
Ductus arteriosus: contracts & becomes ligamentum arteriosus
Ductus venosus: no blood coming in via umbilical vein so closes & becomes ligamentum venosus
Umbilical vein: becomes ligamentum teres
Describe coarctation of the aorta
Acyanotic
Narrowing of the aortic lumen near the ligamentum arteriosum
This increases the after load on the left ventricle
May cause left ventricular hypertrophy
Delayed femoral pulse and reduced blood flow to lower body
In severe cases may present as heart failure in babies
In mild cases, may only be detected in adult life
Give some examples of cyan optic heart defects
Tetralogy of fallot
Tricuspid atresia
Transposition of the great arteries
Hypoplastic left heart
Describe tetralogy of fallot
Cyanotic VSD Overriding aorta Pulmonary stenosis Right ventricular hypertrophy Occurs as a result of outflow tract of interventricular septum too far in the anterior and cephalad direction
Describe tricuspid atresia
Cyanotic Lack of development of tricuspid valve Must be: -Complete right to left shunting of blood returning to RA (by ASD or PFO) -PDA or VSD to allow blood flow to lungs
Describe transposition of the great arteries
Cyanotic
Results in two unconnected parallel circulations instead of two in series
LV connected to pulmonary trunk and RV connected to aorta
Incompatible with life after birth unless shunt exists eg ductus arteriosus or ASD
Describe hypoplastic left heart
Cyanotic
LV and aorta fail to develop properly
PFO or ASD also present with systemic circulation via PDA
Incompatible with life after birth without surgical correction
