What does the Qp represent in an atrial septal defect shunt ratio measurement (Qp/Qs)?

In the calculation of the shunt ratio Qp/Qs, Qp represents pulmonary blood flow, which is calculated as the stroke volume of the right ventricular outflow tract (RVOT). Stroke volume is obtained by measuring the RVOT cross-sectional area and the RVOT time velocity integral (VTI).

Qp (pulmonary flow) divided by Qs (systemic flow) quantifies the magnitude of left-to-right shunting in atrial septal defects and other congenital heart diseases.

This method is described in the "Textbook of Clinical Echocardiography, 6e", Chapter on Shunt Quantification and Flow Calculations【20:360-365†Textbook of Clinical Echocardiography】.