What is the hemodynamic explanation of an Atrial Septal Defect (ASD)?

From the Guidelines

An atrial septal defect (ASD) creates abnormal blood flow patterns in the heart due to an opening between the left and right atria, primarily involving left-to-right shunting of blood because left atrial pressure normally exceeds right atrial pressure, as noted in the 2019 AHA/ACC guideline for the management of adults with congenital heart disease 1. The hemodynamics of an ASD involve left-to-right shunting of blood, causing increased blood volume in the right heart chambers, leading to right ventricular volume overload and pulmonary overcirculation.

• The right ventricle and pulmonary arteries must handle this additional blood volume, which can lead to right ventricular dilation and eventual right heart failure, as discussed in the 2018 AHA/ACC guideline for the management of adults with congenital heart disease 1.
• If left untreated for decades, pulmonary hypertension may develop due to chronic high pulmonary blood flow, potentially leading to reversal of the shunt (Eisenmenger syndrome) where deoxygenated blood flows from right to left, causing cyanosis, as mentioned in the 2019 AHA/ACC guideline for the management of adults with congenital heart disease 1.
• The size of the defect significantly impacts hemodynamics - larger defects allow more shunting and cause more significant hemodynamic consequences, while small defects may have minimal hemodynamic effects and can sometimes close spontaneously, as noted in the acc/aha 2008 guidelines for the management of adults with congenital heart disease 1. Key points to consider in the hemodynamic explanation of an ASD include:
• Left-to-right shunting of blood due to the pressure difference between the left and right atria
• Increased blood volume in the right heart chambers, leading to right ventricular volume overload and pulmonary overcirculation
• Potential development of pulmonary hypertension and reversal of the shunt (Eisenmenger syndrome) if left untreated
• Impact of defect size on hemodynamics, with larger defects causing more significant consequences.

From the Research

Hemodynamic Explanation of Atrial Septal Defect (ASD)

The hemodynamic explanation of an Atrial Septal Defect (ASD) can be understood by considering the direction and degree of shunting across the communication, which is mainly determined by the difference in compliance between the right and left ventricle 2.

• The direction of shunting is typically from left to right in children, resulting in dilated right heart structures and normal pulmonary artery pressures (PAP) 2.
• In adults, long-standing volume overload can lead to complications such as pulmonary artery hypertension and right and left ventricular dysfunction 3, 2.
• The degree of shunting can be estimated using the pulmonary to systemic blood flow ratio (Qp/Qs), with a ratio greater than 2.0 indicating significant left-to-right shunting 4.
• The hemodynamic changes in medium-sized ASDs (Qp/Qs 1.5-2.0) are characterized by an increase in Qp/Qs and right ventricle diastolic volume (RVEDV/LVEDV) over time 4.

Factors Influencing Hemodynamics

Several factors can influence the hemodynamics of ASD, including:

• The size and type of the defect 5
• The presence of other congenital heart conditions 2
• The age of the patient, with adults being more likely to experience complications related to long-standing volume overload 3, 2
• The balance between right and left ventricular function, with impaired left ventricular function increasing the risk of pulmonary edema after ASD closure 6