 |
| Fig. 10 |
In a normal subject of average build, the position of the heart
can be defined from the rib cage (Fig.
10). The cardiac outline extends from the junction of the second
left costal cartilage with the sternum at the sternal angle to the
apex beat, usually in the fifth intercostal space on the midclavicular
line. From the apex, the inferior surface of the heart runs across
to a point in the right fifth intercostal space about 1 cm from
the sternal edge, then it parallels the sternum to a point 1 cm
to the right of its junction with the third right costal cartilage.
The base of the heart is marked by a line joining the right third
costal cartilage and the left second costal cartilage.
 |
| Fig. 11 |
Most of the heart is covered anteriorly by bony structures, the
sternum and ribs, or by the lungs within their pleural membranes,
and these tissues are virtually impenetrable to ultrasound. However,
as shown in Fig.
11, the left lung does not cover the heart completely and in
most individuals beneath the third, fourth and fifth intercostal
spaces, for 2 or 3 cm to the left of the sternal border, the pericardium
lies directly beneath the chest wall and pleural membranes. This
region, termed the left parasternal area, provides the best access
for echocardiography. Moreover, it lies over the center of the heart,
and the distance from the chest wall to the furthest part of the
normal heart is only about 12 cm.
 |
| Fig. 12 |
Additional access (Fig.
12), important particularly for two-dimensional echocardiography,
can usually be obtained from the cardiac apex and by a subcostal
route, with the transducer placed near the xiphisternum.
The great arteries and the base of the heart can also be visualized
from the suprasternal notch.
The approaches described above are available, to a greater or lesser
degree, in most adults. In young children, the ribs and lungs do
not attenuate the ultrasound beam so severely, and in neonates the
transducer can be placed almost anywhere on the precordium. By contrast,
in adults with "barrel" chests, or who have hyper inflated lungs,
for example as a result of chronic emphysema, it can be almost impossible
to obtain any echocardiographic images. Lack of adequate access
to the heart is the greatest limitation to echocardiography and
a large proportion of the technical skill required to perform the
examination lies in being able to find a transducer site from which
clear images can be obtained.
2004