Morphology & Embryology
Early during fetal development, the vascular plexus within the lung buds connects with
systemic segmental arteries originating from the dorsal aorta. By the 40th day
of gestation, the vascular plexus has differentiated into pulmonary segmental arteries,
supplying the terminal bronchopulmonary units. For a short time, the pulmonary parenchyma
receives a dual blood supply (from the right ventricle and the pulmonary arteries that
originate from the sixth branchial arches and from the previously described systemic
segmental arteries). However, by the 50th day of gestation, the systemic
arterial supply normally involutes, and during subsequent normal fetal development, flow
to the developing lungs is delivered exclusively by the pulmonary arteries. In the more
complex forms of tetralogy with pulmonary atresia, this normal development is affected,
whereby some bronchopulmonary segments are supplied by true pulmonary arteries, and others
by aorto-pulmonary collaterals. It is important to note that before entering the lung
parenchyma, these systemic collaterals retain their histologic characteristics of muscular
arteries, whereas after penetrating the pulmonary parenchyma, the median muscular layer
gradually changes into an elastic lamina, structurally resembling true pulmonary arteries.
Unobstructed flow through aorto-pulmonary collaterals can lead to pulmonary vascular
obstructive disease, while stenosis within aorto-pulmonary collaterals protects against
the development of pulmonary vascular obstructive disease.
Tetralogy of Fallot with pulmonary atresia is at times referred to as ventricular
septal defect with pulmonary atresia. However, the outlet septum in this lesion in
deviated in a manner reminiscent to that of classic tetralogy of Fallot, in fact so
severely so as to cause complete right ventricular outflow tract obstruction. Hence, the
central feature in the tetralogy of Fallot as related to antero-cephalad deviation of the
outlet septum and the abnormal relationships to the ventriculoinfundibular fold and
septomarginal trabeculation are also present in this lesion. The proper terminology for
this lesion should therefore be that of tetralogy of Fallot with pulmonary atresia.
The pulmonary atresia may be found at the level of the subpulmonary infundibulum, in
which case it is often an acquired lesion, or more commonly, at the level of the muscular
septum or pulmonary annulus, in which case the cause is likely to be congenital in origin.
The ventricular septal defect is usually perimembranous, but can also have a muscular
postero-inferior rim. Both the subpulmonary infundibulum and the outlet septum may be
completely missing, in which case both the ventricular septal defect and outflow tract is
reminiscent of that of truncus arteriosus.
When tetralogy is accompanied by pulmonary atresia, the determinant of clinical
presentation and prognosis is the source of the pulmonary blood flow, which under these
circumstances can be derived from either a persistent arterial duct or from
aorto-pulmonary collaterals. When pulmonary blood flow is derived from a persistent
arterial duct, then the branch pulmonary arteries are usually confluent and the duct is
left-sided, irrespective of which side the aortic arch is located. Rarely in the presence
of a patent arterial duct, the branch pulmonary arteries may be nonconfluent, in which
case each branch pulmonary artery is supplied by a one of a bilateral pair of arterial
ducts. In the presence of duct-dependent pulmonary blood flow, aorto-pulmonary collaterals
are usually clinically insignificant irrespective of their presence or number.
When pulmonary blood flow is derived from aorto-pulmonary collaterals, the anatomy is
much more complex. Aorto-pulmonary collaterals most frequently arise from the descending
aorta and vary in number from two to six. They may also arise from the brachiocephalic
arteries, or rarely, from the coronary arteries. Almost always, aorto-pulmonary
collaterals coexist with intrapericardial pulmonary arteries, in which case they
anastomose within the parenchyma of the lungs. It is important in planning a course of
ultimate unifocalization to determine the source of arterial blood supply for each segment
of lung, namely whether it is derived from an intrapericardial pulmonary artery or whether
it is derived from an aorto-pulmonary collateral. In some cases of nonconfluent pulmonary
arteries, one lung may be supplied by aorto-pulmonary collaterals, while the other lung is
supplied by a single branch pulmonary artery derived from either the arterial duct, or
directly from a systemic blood source.
Preoperative Diagnosis
The diagnostic challenge in tetralogy of Fallot with pulmonary atresia is to identify
preoperatively the presence, size, and continuity of native pulmonary arteries and then to
detail the origin, the course, and the distribution of all aorto-pulmonary collaterals. It
is virtually always necessary to make selective injections into all direct and indirect
aorto-pulmonary collaterals to obtain a complete and detailed map of the entire pulmonary
blood supply.
Accurate measurement of the size of a pulmonary artery preoperatively presents a number
of problems. First, with diminished pulmonary blood flow, the maximal capacity or
compliance of the non-distended pulmonary arteries cannot be accurately assessed.
Consequently, the potential postoperative size of a pulmonary artery carrying a normal
volume of blood is difficult to predict. Nevertheless, several methods to quantify
pulmonary artery size and its effect on the post-repair outcome have been used. One
popular formula is known as the McGoon ratio[1924], which is based on the diameter of the
right and left pulmonary arteries, normalizing these by relating them to the diameter of
the descending thoracic aorta at the level of the diaphragm. Right and left pulmonary
arteries are considered to be nonrestrictive when the combined diameter is about 2 or
greater, while a combined diameter of less then 0.8 is supposed to indicate severely
restrictive central pulmonary arteries. One drawback of the McGoon ratio is that the
descending aorta at the diaphragm tends to be more narrow in patients with tetralogy of
Fallot than in normal individuals, making the McGoon ratio falsely more favorable.
Nakata and colleagues[466] measured the diameter of the right and left pulmonary
arteries immediately proximal to their first branching. Magnification errors are corrected
either by using previously determined values from the catheterization laboratory or by
relating vessel size to the known size of an appropriate catheter. Pulmonary artery size
is reported as the sum of the cross-sectional areas of the right and left pulmonary
arteries, indexed to body surface area. The normal cross-sectional index is 330 + 30 mm2/m2,
and are considered diminutive when the Nakata index is less than 150 mm2/m2.
Using statistical techniques, Blackstone and colleagues[471] predict the postoperative
PRV:LV based on the dimensions of the right ventricular
outflow tract and the size of the central branch pulmonary arteries (and not the
peripheral pulmonary artery branches). If the pulmonary valve annulus is hypoplastic, it
tends to remain so throughout life. Therefore, Blackstone and Kirklin expressed the
annular size relative to the child’s size as a Z value which represents the number of
standard deviations that the patient’s pulmonary valve annulus deviates from a mean
normal value for age and size.
All of these angiographic assessments are, however, limited in that the size of the
pulmonary arteries may enlarge significantly after establishing right ventricular to
pulmonary arterial continuity, given the increased volume and distending pressure. On the
other hand, there clearly is a subset of patients in whom the central branch pulmonary
arteries are too diminutive in size, generally less than 3 mm in diameter, that they
cannot carry right ventricular output, thereby contraindicating ventricular septal defect
closure.
Indications for Operation
Most patients with tetralogy of Fallot with pulmonary atresia and a duct-dependent
pulmonary circulation have sufficiently large pulmonary arteries, (generally with a Nakata
index greater than 150 mm2/m2) that they can be successfully
repaired at a low operative risk with good late hemodynamic and electrophysiological
results.
A therapeutic challenge is presented in the subset of patients with diminutive
pulmonary arteries, generally with a Nakata index less than 100 mm2/m2,
and large aorto-pulmonary collaterals that supply a variable number of bronchopulmonary
segments. The ultimate therapeutic goal in this subset of patients is to establish right
ventricular-dependent pulmonary circulation, which would ideally include all 20
bronchopulmonary segments. Hemodynamically, the aim is to achieve a postoperative PRV/LV
ratio of less than 0.6 with no residual left-to-right shunt at any level. Until relatively
recently, this ideal result has been achieved only in isolated cases, primarily due to
limitations of surgical technique in dealing with these complex anatomic features.
Recently it has become evident that many of these complex lesions, including the presence
of dual-supply segments and stenoses can be managed with by invasive interventional
techniques, and by aggressively treating these lesions during early infancy.
In the past, these patients were managed somewhat haphazardly by a variety of medical
or surgical approaches, including primary repair or staged operations such as preliminary
systemic-to-pulmonary artery shunts or establishment of right ventricular-to-pulmonary
artery continuity. These procedures were primarily aimed at providing relief of cyanosis
and stimulating enlargement of the hypoplastic central pulmonary arteries, with the
expectation that later the ventricular septal defect would close and eliminate any
remaining functionally important aorto-pulmonary collaterals. However, most of these
attempts proved unsuccessful. By 1984, it had been shown that hypoplastic or stenotic
pulmonary arteries could be enlarged by trans-catheter balloon dilatation and also that
aorto-pulmonary collaterals could be successfully interrupted by trans-catheter placement
of coils. Consequently, a new staged approach to the management of patients with tetralogy
of Fallot with pulmonary atresia and diminutive pulmonary arteries began to evolve. This
approach consists of early surgical relief of right ventricular outflow tract obstruction,
leaving the ventricular septal defect open, followed by interventional catheterization to
dilate stenotic peripheral pulmonary arteries and occlude redundant aorto-pulmonary
collaterals with coils. Whenever indicated, unifocalization procedures are added,
connecting as many aorto-pulmonary collaterals to the true pulmonary arteries as possible.
These preliminary procedures are then followed by surgical relief of any residual right
ventricular outflow tract obstruction and closure of the ventricular septal defect.
Interposition of a valved homograft between the right ventricle and the pulmonary artery
during early infancy not only stimulates enlargement of the pulmonary arteries, but also
provides an avenue for balloon dilatation of stenotic peripheral pulmonary arteries, which
are common in this entity. It also facilitates angiographic delineation of the blood
supply in peripheral pulmonary arterial segments. |