Pathophysiology of Congenital Heart Disease Flashcards
Qp/Qs ratio
the ratio of flow through the pulmonary circuit compared with the flow through the systemic circuit
if greater than 2, accepted as an indication for repair
>1 means left to right
<1 means right to left
Fick principle
in the systemic circuit, O2 consumption = change in O2 sat x CO
systemic blood flow = cardiac output
O2 delivery = constant x (arterial O2 sat - venous O2 sat)
therefore CO = Qs = (O2 consumption)/(systemic arterial sat - systemic venous sat)
for the pulmonary circuit - Qp = O2 uptake/(SATpv - SATpa)
Even though the pressures in the LA are not typically that different than the pressures in the RA, why is there an R -> L shunt?
the determinant of shunting is actually the “capacitance” of the two circuits - measures of resistance
atrioventricular septal defect or complete AV canal (CAVC)
a combination of ASD and VSD
involves the malformation of the AV valves
creates a “common AV valve”
partial anomalous pulmonary venous connection (PAPVR)
some (but not all) pulmonary veins drain into the RA instead of the LA
physiology is similar to that of an ASD - increased volume load
aortopulmonary window
uncommon
incomplate separation of the pulmonary artery from the aorta
the right heart only sees a normal cardiac output
the flow crosses over outside of the heart
as a result, the chamber that has to do the extra work is the LV, because it pumps all of systemic flow, as well as the flow that will get shunted across the AP window and into the pulmonary circuit
complications of L -> R shunting
pulmonary overcirculation - initial congestion with SOB, long term pulmonary vascular disease and hypertension
ventricular volume overload - premature failure of systolic function, atrial and ventricular arrhythmias
physiologic effect of pulmonary overcirculation
lung congestion
stiff lungs increases work of breathing - tachnypnea and retractions
inftants have feeding difficulty due to heavy breathing
increased WOB (work of breathing) leads to higher calory utilization
failure to thrive because of falling off of growth curve
frequent respiratory infections
When should VSDs be closed?
if RV and PA pressure is significantly elevated, but not if PVR is too high
progression to Eisenmenger’s Syndrome
period of preserved oxygen saturation until pulmonary hypertension and gradual reversal of flow, leading to gradual desaturation
most rapid clinical progression is in patients with Down syndrome and also CAVC defects - irreversible PHT can be seen in these patients as soon as 6 months after birth
problems with closing an Eisenmenger’s Shunt
results in severe pulmonary hypertension
morphology of the RV is not suited to generating flow under high pressure
RV may not be able to maintain pulmonary flow against a high resistance
inadequate CO - syncope
RV failure, right heart congestion
How do we assess when it’s dangerous to close a shunt?
at cath, once can measure the change in pressure across the pulmonary bed (mean PA pressure - LA pressure, estimated by the PCWP)
flow is estimated by estimating the O2 consumption of thermodilution
if the resistance is < 2 Wood units, it is normal
if resistance is > 6-8 Wood units, it can cause significant problems at the time of surgery, and may contraindicate some procedures
Which L->R shunts lead to RV volume overload?
ASD/PAPVR
Which L->R shunts lead to LV volume overload?
VSD/PDA/AP window
transposition of the great arteries
rather than having two circuits feeding into each other, you have two circuits in parallel
key to survival - communication of the two circuits through the PDA or a sufficient hole thorugh the atrial septum
tricuspid atresia
systemic venous return into the RA crosses the PFO into the LA
complete mixing of systemic venous and pulmonary venous blood
resulting saturation is the “weighted average” of the 2 saturations
depending on the relative flow to the 2 circuits
the chamber downstream from the absent connection typically is hypoplastic, as it never received the blood flow that is a strong stimulus for development
RV communicates with LV through a VSD
2 outflows, one to pulmonary and one to systemic
ideal if some stenosis in the pulmonary outflow tract to keep the circulation balanced
left heart atresia - hypoplastic left heart syndrom
severe hypoplasia/atreasia of the mitral and aortic valves, as well as LV
pulmonary venous return crosses from LA to RA, and is ejected from the RV to the PA
systemic output to the aorta is almost completely provided through the PDA - ductual dependent lesion
ductal closure results in shock
total anomalous pulmonary venous return (TAPVR)
lack of connection of the pulmonary veins to the LA
pulmonary venous blood returns to a confluence, then drains through systemic veins to the RA
saturated pulmonary venous blood mixes completely with the desaturated systemic venous blood, causing cyanosis
