Pathophysiology
In unaffected individuals, the chambers of the left side of the heart are under higher pressure than the chambers of the right side of the heart. This is because the left ventricle has to produce enough pressure to pump blood throughout the entire body, while the right ventricle needs only to produce enough pressure to pump blood to the lungs.
In the case of a large ASD (>9mm), which may result in a clinically remarkable left-to-right shunt, blood will shunt from the left atrium to the right atrium. This extra blood from the left atrium may cause a volume overload of both the right atrium and the right ventricle. If untreated, this condition can result in enlargement of the right side of the heart and ultimately heart failure.
Any process that increases the pressure in the left ventricle can cause worsening of the left-to-right shunt. This includes hypertension, which increases the pressure that the left ventricle has to generate in order to open the aortic valve during ventricular systole, and coronary artery disease which increases the stiffness of the left ventricle, thereby increasing the filling pressure of the left ventricle during ventricular diastole. The left-to-right shunt increases the filling pressure of the right heart (preload) and forces the the right ventricle to pump out more blood than the left ventricle. This constant overloading of the right side of the heart will cause an overload of the entire pulmonary vasculature. Eventually, pulmonary hypertension may develop.
The pulmonary hypertension will cause the right ventricle to face increased afterload. The right ventricle will be forced to generate higher pressures to try to overcome the pulmonary hypertension. This may lead to right ventricular failure (dilatation and decreased systolic function of the right ventricle).
When the pressure in the right atrium rises to equal the pressure in the left atrium, there is no longer a pressure gradient between these heart chambers, and the left-to-right shunt will diminish or cease. In other words, there is no longer a net flow of blood across the ASD.
If the ASD is left uncorrected, the pulmonary hypertension progresses and the pressure in the right side of the heart will become greater than the left side of the heart. This reversal of the pressure gradient across the ASD causes the shunt to reverse; a right-to-left shunt will exist. This phenomenon is known as Eisenmenger's syndrome. Once right-to-left shunting occurs, a portion of the oxygen-poor blood will get shunted to the left side of the heart and ejected to the peripheral vascular system. This will cause signs of cyanosis.
Read more about this topic: Atrial Septal Defect