|Fig. 36 The short axis plane at aortic valve
level with parasternal transducer position indicated.
Further tilting of the transducer toward the subject's right shoulder,
combined with slight rotation clockwise, brings the scan plane above
the level of the mitral valve annulus, so that it transects the
two atria (Fig.
37). In the center is the aorta, with its three cusps visible.
Below the aorta is the left atrium, with the interatrial septum
running diagonally downward and to the left, dividing it from the
right atrium. The plane of the section passes diagonally across
the tricuspid valve annulus, above the posteroseptal leaflet, part
of which is still visible. Above the tricuspid valve is the crista
supraventricularis and part of its parietal band. The outflow tract
of the right ventricle arches over the top of the aorta toward the
pulmonary valve, the posterior cusp of which is seen.
These features are depicted on the echocardiographic image (Figs.
38 and 39). There is usually some "dropout" in the
atrial septum in the region of the fossa ovalis where it is very
thin. While lack of continuity of the atrial septum on the echo
display is therefore not diagnostic of an atrial septal defect,
this view can be used in conjunction with echocardiographic contrast
studies to demonstrate interatrial shunts. Microbubbles, generated
by rapid injection of fluid into a peripheral vein, enter the right
atrium. Even where the dominant shunt is left to right, there is
usually a small amount of flow from right to left at the onset of
systole, and this is indicated by passage of a few microbubbles
across the interatrial septum into the left atrium.
Tilting the transducer slightly further moves the scan plane above
the level of the aortic valve. In this position, it is possible
to visualize the ostium of the left coronary artery in a substantial
proportion of subjects, and that of the right coronary artery in
rather less. With further refinements in instrument resolution,
it may become possible to detect major stenosis of the proximal
coronary arteries with acceptable reliability.
Further counterclockwise rotation aligns the scan plane along the
main pulmonary artery, seen running posteriorly to the right of
the aorta. It is frequently possible to visualize this vessel as
far as the bifurcation into right and left pulmonary arteries, with
the right seen turning behind the heart and the left in the opposite
40 and 41). Finding the bifurcation is positive identification
of the pulmonary artery, and thus is valuable in congenital heart