vsd and asd systemic PaO2 effects

[sanj238]

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How come systemic O2 does not decrease in a VSD or ASD? If more blood is going into the RA or RV- that means less blood and therefore less O2 in the system...?

However, there is an increase in O2 in the right side into the pulmonary artery- this makes sense- but why the reverse is not happening in the systemic I dont understand.

 

[Pons Asinorum]

All of the blood returning to the LA is oxygenated. All blood leaving the LV is oxygenated. It's just that the CO has to increase to meet systemic demand while simultaneously feeding the ASD/VSD. As long as the increase in CO can be compensated by the LV, all is good. But regardless, as long as the LV is able to compensate for the increased CO demands, everything leaving will be fully oxygenated.

 

[pvtbacon]

How come systemic O2 does not decrease in a VSD or ASD? If more blood is going into the RA or RV- that means less blood and therefore less O2 in the system...?

However, there is an increase in O2 in the right side into the pulmonary artery- this makes sense- but why the reverse is not happening in the systemic I dont understand.

In VSD and ASD, you are right to think that the left to right shunt will move oxygenated blood into deoxygenated blood territory. However, the quality of the blood in the left ventricle going into systemic circulation has not decreased at all.

 

[Pons Asinorum]

This is all predicated on this being a left-to-right shunting, which is normally the case (Eisenmerger being the most common exception.)

 

[Charles_Carmichael]

It all depends on time and severity. Initially, since the shunt is left-to-right, there's no mixed venous blood being pumped out into the systemic circulation. So, you won't see a drop on PaO2. Remember, PaO2 is just the partial pressure of O2 in arterial blood (ie. oxygen dissolved in blood) -- it's determined by ventilation (PAO2) and the perfusion of ventilated areas of the lung.

Eventually in the disease process, if it's severe enough, high pressures in the pulmonary circuit result in Eisenmenger syndrome -- now, the shunt reverses to become a right-to-left shunt. Now, you have mixed venous blood (that hasn't seen the lungs) also entering the systemic circulation. This is when you'll have a drop in PaO2, because you have a portion of the blood not perfusing ventilated lung and directly entering the system circulation.

It all depends on time-course and the severity of the defect (small defects will probably just be asymptomatic). Make sense?