|Fig. 51 The four-chamber plane with the subcostal
transducer position indicated.
The anatomy of the four-chamber plane seen from the subcostal views
is identical to that already described in the apical view, but with
orientation as shown in Fig.
52. In the body, the inferior surface of the right ventricle,
seen at the top (Fig.
52), rests on the diaphragm, and the ultrasound beam approaches
the heart through a corner of the liver and across the diaphragm.
The echocardiographic image of the subcostal four-chamber plane
is shown in Fig.
53 and 54. Although the apical region is not seen as well as
from the apex, more of the atria are visualized. In addition, clearer
echoes are generated by the interatrial and interventricular septa
because the ultrasound beam approaches them more nearly at right
angles. Thus, while dropout of the echo signals from the septa in
the apical view is most likely to be artifactual, consistent absence
of echoes from a region of either the interatrial or interventricular
septum in the subcostal view suggests the presence of a defect.
Two-dimensional echocardiography is helpful for diagnosing these
lesions and, in the case of the interatrial septum, it is usually
possible to differentiate a defect in the area of the fossa ovalis
(secundum type) from a low defect extending down to the mitral annulus
(primum) or a sinus venous defect high in the roof of the atria.
The subcostal four-chamber view is the only satisfactory way to
visualize the right atrium, and it also affords the best view of
the right ventricle, since these chambers like nearest to the transducer.
Right atrial morphology can be confirmed by tilting the scan plane
inferiorly so that the entrance of the inferior vena cava is seen.
In addition to the identification of the atrioventricular valves
by their relative levels of attachment to the septum, it is frequently
possible to determine ventricular morphology by locating the moderator
band within the right ventricle. To a certain extent it is also
possible to assess the thickness of the right ventricular walls,
and in some patients to diagnose right ventricular hypertrophy.
If a pericardial effusion is present, its extent and the degree
of separation of the heart from the pericardium should be determined
in the subcostal view if pericardiocentesis is contemplated.