Page 55 - Clinical Anatomy

ECA1 7/18/06 6:31 PM Page 40

40 The Thorax

of this arterial shunt and, in the course of the next 2–3 months, its complete
obliteration. Similarly, ligature of the umbilical cord is followed by throm-
bosis and obliteration of the umbilical vessels.

Congenital abnormalities of the heart and great vessels
The complex development of the heart and major arteries accounts for the
multitude of congenital abnormalities which may affect these structures,
either alone or in combination.
Dextro-rotation of the heart means that this organ and its emerging vessels
lie as a mirror-image to the normal anatomy. It may be associated with
reversal of all the intra-abdominal organs; I have seen a student correctly
diagnose acute appendicitis as the cause of a patient’s severe left iliac fossa
pain because he found that the apex beat of the heart was on the right side!

Septal defects
At birth, the septum primum and septum secundum are forced together,
closing the flap valve of the foramen ovale. Fusion usually takes place
about 3 months after birth. In about 10% of subjects, this fusion may be
incomplete. However, the two septa overlap and this patency of the
foramen ovale is of no functional significance. If the septum secundum is
too short to cover the foramen secundum in the septum primum, an atrial
septal defect persists after the septum primum and septum secundum are
pressed together at birth. This results in an ostium secundum defect, which
allows shunting of blood from the left to the right atrium. This defect lies
high up in the atrial wall and is relatively easy to close surgically. A more
serious atrial septal defect results if the septum primum fails to fuse with
the endocardial cushions. This ostium primum defect lies immediately above
the atrioventricular boundary and may be associated with a defect of the
pars membranacea septi of the ventricular septum. In such a case, the child
is born with both an atrial and ventricular septal defect.
Occasionally the ventricular septal defect is so huge that the ventricles
form a single cavity, giving a trilocular heart.
Congenital pulmonary stenosis may affect the trunk of the pulmonary
artery, its valve or the infundibulum of the right ventricle. If stenosis occurs
in conjunction with a septal defect, the compensatory hypertrophy of the
right ventricle (developed to force blood through the pulmonary obstruc-
tion) develops a sufficiently high pressure to shunt blood through the
defect into the left heart; this mixing of the deoxygenated right heart blood
with the oxygenated left-sided blood results in the child being cyanosed at
birth.
The commonest combination of congenital abnormalities causing
cyanosis is Fallot’s tetralogy (Fig. 33). This results from unequal division of
the truncus arteriosus by the spinal septum, resulting in a stenosed pul-
monary trunk and a wide aorta which overrides the orifices of both the ven-
tricles. The displaced septum is unable to close the interventricular septum,
which results in a ventricular septal defect. Right ventricular hypertrophy