Transthoracic echocardiogram with bubble study pre-procedure. Transthoracic echocardiogram with bubble study during the relaxation phase of the Valsalva maneuver. The microbubbles seen in the left ventricle signify a right-to-left shunt and thus a positive test.Beats per minute (BPM), Left atrium (LA), Left ventricle (LV), Right atrium (RA), Right ventricle (RV).