Rush Center for Congenital
and Structural Heart Disease

In a normal heart, one without any septal defects, the output from the right and left ventricles are identical.  Whatever leaves the right ventricle will circulate through the lungs, then return to the left heart to be ejected to the body.  In this event, the systemic blood flow (Qs) is equal to the pulmonary blood flow (Qp).  Therefore, the Qp:Qs ratio is 1:1


When there is a septal defect, a communication between the right and left heart is created.  
Blood can travel through this communication.
Blood goes where resistance is least.
Therefore, the direction of blood flow depends upon the resistance of the pulmonary vascular system (pulmonary vascular resistance, or PVR) and that of the systemic circulation (systemic vascular resistance, or SVR).
Normally, the PVR (about 3 Wood units) is much lower than the systemic vascular resistance (about 25 Wood units).
Therefore, with any communication between the left and right sides of the heart, blood will shunt from the high resistance to low resistance sides, i.e. from left to right.  
This will cause increase pulmonary blood flow (PBF).

ASD defect with low PVR results in high PBF.  The pulmonary arteries are engorged and the lungs become edematous.


Qp:Qs ratio is NOT a static phenomenon
Over time, the increase blood flow through the pulmonary vasculature will cause damage to the muscular walls of the pulmonary arterioles, causing them to hypertrophy.  This will eventually cause an increase in the resistance of these blood vessels, leading to elevation of the PVR.
As the PVR elevates, an increase in the resistance to pulmonary blood flow and left to right shunting at the septal defect will result, leading to reduction of PBF.  The patient will actually feel better as congestive heart failure (CHF) is improving.  However, the pulmonary vascular pathology is worsening.

ASD defect with increasing PVR results in reduction of high PBF.  The pulmonary arteries' walls become thicker leading to less PBF and improvement of pulmonary edema.

Cyanosis in ASD indicates high PVR
Eventually, the pulmonary vascular pathology worsens leading to a PVR higher than the systemic vascular resistance (SVR).  Since blood will go were resistance is least, shunting at the atrial level becomes right to left.  This will actually cause a PBF that is less than normal as well as cyanosis.  The pulmonary vascular pathology is now irreversible.

ASD defect with high PVR results in right to left shunting at the ASD manifesting as cyanosis.  The  PBF is actually less than normal and the lungs are no longer edematous.


The animation below summarizes the progress of ASD and PVR over time, the changes in this animation usually occur over 4-5 decades

Is high oxygen saturation a good thing in cyanotic congenital heart disease?
Not really!
An infant with truncus arteriosus awaiting surgical repair is noted to have increasing oxygen saturation.  Is this a good thing?  NO!
An increase in oxygen saturation indicates that the PBF is excessive.  Remember, high PBF results in a large volume of highly oxygenated blood returning to the left atrium, this will in turn dilute the de-oxygenated blood returning from the body, thus making the mixed blood going to the truncus and eventually to the aorta and pulmonary arteries better saturated.  Better oxygen saturated blood may be a good thing, but not when it is at the expense of very high PBF.  High PBF causes CHF and pulmonary edema, which may be so severe that it would compromise respiration.  Significantly edematous lungs poses risks, particularly if surgery and placement of patient on heart-lung by-pass machine.

Small pulmonary artery from the truncus offers resistance to PBF, resulting in small Qp and low oxygen saturation. Mild narrowing of pulmonary artery from the truncus offers mild resistance to PBF, resulting in equal Qp & Qs with acceptable oxygen saturation. Normal size pulmonary artery from the truncus offers no resistance to PBF, resulting in high Qp:Qs and high oxygen saturation.  Pulmonary edema and CHF will be present.