A “bubble study” is often utilized to confirm the presence of shunting across the interatrial septum (IAS) via patent foremen ovale (PFO) or atrial septal defects (ASD). This study is used in combination with color flow doppler and simple 2-dimensional inspection of the IAS anatomy to determine if there indeed is a defect.
A Valsalva maneuver increases intrathoracic pressure and compresses the compliant, low-pressure superior and inferior vena cavas. This, in turn, transiently drops blood return to the right atrium creating low right atrial pressure. Once the Valsalva is relieved, blood rushes back into the right atrium generating a higher right atrial pressure than at baseline. This should cause the interatrial septum to bow into the left atrium. Identification and closure of any PFO/ASDs is imperative before surgeries like a left ventricular assist device (LVAD) implantation to prevent right-to-left shunting and hypoxemia.
Here’s a great video showing the process!