Figure 33-3 Pulsed-wave Doppler echocardiogram of the main pulmonary artery (MPA). At the top of the echocardiogram is a still-frame image of the two-dimensional cross section used to position the Doppler sample volume (the round white sphere). On the bottom two thirds of the echocardiogram is the display in white of the instantaneous blood flow velocities (vertical axis) versus time (horizontal axis) occurring in that sample volume. The electrocardiogram provides timing, and the bold horizontal line is the baseline (zero flow) for the flow velocities. Flow velocities above this line are positive (i.e., toward the transducer) to a maximum of 68 cm/sec. Flow below this line is negative (i.e., away from the transducer) to a maximum of -14 cm/sec. (From Cahalan MK: Intraoperative Transesophageal Echocardiography. An Interactive Text and Atlas. New York, Churchill Livingstone, 1997.)