8th March 2024

Investigations

On an ECG, both primum and secundum ASDs display first degree AV block and RSR’ pattern in V1 but can be differentiated by right axis deviation in secundum ASD and left axis deviation in primum ASD. 

A chest x-ray may show cardiomegaly and prominent vascular markings.

The first line diagnostic modality is transthoracic echocardiography (TTE), where intravenous contrast studies can be performed to elicit the shunt and colour-flow Doppler can identify intra-arterial flow. The degree of shunting can be assessed using the shunt fraction (Qp/Qs), which is the ratio of pulmonary flow (Qp) to systemic flow (Qs). A left-to-right shunt is considered significant when the ratio of pulmonary to systemic blood flow (Qp/Qs) is >1.5 or when the RV dilates. There may also be functional tricuspid regurgitation, RA and RV enlargement, as well as abnormal septal motion. TOE is optional for further anatomical definition. 

Cardiac magnetic resonance (CMR) imaging provides high quality assessment of RV size and function and clear views of the pulmonary veins. It is useful for quantification of pulmonary to systemic flow ratio (Qp:Qs) where either a ratio > 1.5 or evidence of RV dilation confirms a significant shunt. 

Cardiac catheterisation is not routinely required for diagnostic purposes except to assess for pulmonary pressures or coronary artery disease. Additional investigations involve oximetry sampling that can demonstrate an oxygen step-up from the vena cava to the right atrium.