Pathologic Anatomy
The left pulmonary artery arises from the right pulmonary artery and passes leftward
between the trachea and esophagus. The ligamentum arteriosum passes posteriorly to the
aorta from the point of origin of the right pulmonary artery from the main pulmonary
artery, effectively creating a vascular ring surrounding the trachea but not the
esophagus.
Associated Tracheal Anomalies
Approximately 50% of patients with a pulmonary artery sling have complete tracheal
rings-that is, the posterior membranous component of the trachea is absent, and the
tracheal cartilages, rather than being U shaped, are O shaped. The presence of complete
rings does not imply that important stenosis necessarily will be present, although the
trachea is often narrower than normal. The complete rings may be localized to the region
where the sling passes around the trachea, although often they extend for the entire
length of the trachea.
In the area where the sling passes around the trachea, there is likely to be tracheal
compression resulting in important functional stenosis, even if there is not an underlying
anatomic stenosis. Bronchus suis, ("pig bronchus"), is also an association of
pulmonary artery sling. The left pulmonary artery is often relatively hypoplastic and
considerably smaller than the right pulmonary artery, which appears larger than normal -
virtually a direct extension of the main pulmonary artery. The small size of the left
pulmonary artery may help to explain the high incidence of anastomotic problems that have
been observed in the past with attempts to reimplant it.
Pathophysiology
Respiratory symptoms predominate because of the direct tracheal compression, with or
without congenital tracheal stenosis, and are essentially the same respiratory symptoms as
those described for vascular rings. Symptoms of esophageal compression are rarely present.
Preoperative Evaluation
Definition of the vascular anatomy may be made in the same fashion as described for
vascular rings. However, unlike vascular rings, which produce a posterior indentation of
the esophagus evident on barium swallow, pulmonary artery slings produce an anterior
esophageal indentation, which can also be demonstrated on barium swallow. Echocardiography
can usually confirm the vascular anatomy.
Assessment of the Trachea
Because of the high incidence of tracheal anomalies other than simple compression by
the sling, it is important to undertake a complete assessment of the trachea. This should
include bronchoscopy and at least one other mode of imaging to delineate the severity and
extent of tracheal stenosis. This may include computed tomographic scanning or magnetic
resonance imaging. Computed tomographic scanning allows accurate quantitation of the
tracheal luminal diameter and area at various levels and demonstrates the presence of
complete rings, which will also have been noted at bronchoscopy. Bronchography can produce
spectacular imaging of tracheal stenoses but is generally reserved for children who are
also undergoing angiography for definition of vascular anatomy, if this is not clear from
echocardiography alone, or for definition of associated cardiac problems.
Medical Management
General supportive respiratory care should be given before proceeding to surgery. As
described for vascular rings, respiratory infection may be difficult to clear completely
before surgery because of the difficulty in adequately clearing secretions.
Surgical Management
History
In 1954 Potts and associates described the approach to a pulmonary artery sling using a
left thoracotomy with division of the left pulmonary artery, followed by translocation
anterior to the trachea and reimplantation. Potts performed reimplantation at the original
site of origin of the left pulmonary artery, but Hiller and Maclean, in 1957, using the
same operation, performed reimplantation into the side of the main pulmonary artery. In
this early report of what was to become the traditional operation for pulmonary artery
sling, these authors also reported for the first time the complication that was to be
commonly seen: namely, occlusion of the left pulmonary artery demonstrated by
postoperative angiography. Mustard and colleagues recognized the importance of the
ligamentum arteriosum in effectively completing a vascular ring as part of this anomaly,
and in 1962 they described simple division of the ligament as management of a pulmonary
artery sling. This operation has not been widely practiced. Repair of a pulmonary artery
sling through a median sternotomy with reimplantation of the left pulmonary artery using
cardiopulmonary bypass was described in 1985 by Kirklin and Barratt-Boyes. None of the
techniques described up to that time had dealt directly with the associated tracheal
stenosis that was often present. These operations were based on the premise that the
problem was primarily one of tracheal compression; although this was true in some cases,
it was certainly not universally true. Although primary repair of the tracheal anomaly had
been suggested previously, it had not been attempted because of fear of early and late
tracheal anastomotic problems in the infant. Improved cardiopulmonary bypass techniques
(which allowed cumbersome airway intubation techniques to be avoided); improved sutures,
such as Maxon; and greater familiarity with microvascular techniques have decreased the
risk of tracheal anastomosis in the infant. Tracheal resection is currently considered an
integral part of the repair of pulmonary artery sling when the sling is associated with
important localized tracheal stenosis.
Indications for Surgery
Because pulmonary artery sling is a rare entity, its natural history remains poorly
defined. Therefore, surgery currently is reserved for children who have respiratory
symptoms referable to the sling. Preoperative studies must indicate whether there is
simple compression stenosis of the carinal region, which may be relieved by translocation
of the left pulmonary artery; localized anatomic stenosis of the trachea (generally
associated with complete tracheal rings in this area), which is best dealt with by
tracheal resection and anterior translocation of the left pulmonary artery; or, finally,
diffuse severe narrowing of the trachea related to complete tracheal rings, which may
necessitate an extensive tracheoplasty procedure in addition to relocation of the left
pulmonary artery.
Technical Considerations
Division and reimplantation of the left pulmonary artery
Although the traditional approach has been through a left thoracotomy, the technique of
median sternotomy and use cardiopulmonary bypass is currently favored. The traditional
approach via thoracotomy can be used if cardiopulmonary bypass is to be avoided. When
using the left thoracotomy approach, it is important to dissect as much length as possible
of the proximal left pulmonary artery so as to minimize tension on the anastomosis. A
side-biting clamp is applied to the side of the main pulmonary artery at a site that will
minimize tension and distortion of the anastomosis. Because the left pulmonary artery has
previously been located posterior to hilar structures and at this point must be brought
anterior to the bronchus, there is a tendency for the anastomosis to be under quite a bit
of tension, no doubt explaining the high rate of occlusion described in a number of
series.
The approach through a median sternotomy has the advantage of allowing complete
mobilization of the main and right pulmonary arteries as well as of the left pulmonary
artery, thereby decreasing tension on the anastomosis. Furthermore, a side-biting clamp
does not have to be applied for the anastomosis, because cardiopulmonary bypass is
employed. The left pulmonary artery can be carefully and completely mobilized where it
passes around the trachea, allowing maximal length for reimplantation. Care must be taken
when dissecting posterior to the trachea, particularly when complete rings are not
present, as there may be an intimate association between the left pulmonary artery and the
membranous component of the trachea. When complete tracheal rings are present, the
dissection is easier, and it is likely that this segment of trachea will be excised. The
anastomosis is constructed with continuous 6-0 or 7-0 Maxon or Prolene sutures.
Cardiopulmonary bypass should be undertaken with an ascending aortic arterial cannula and
a single straight venous cannula in the right atrium. A nearly full-flow perfusion rate is
used, with the heart beating throughout the procedure at a systemic temperature of 32°C
to 34°C.
Tracheal resection and anastomosis with relocation of the left pulmonary artery
The approach is exactly as described for reimplantation of the left pulmonary artery
through a median sternotomy, including the use of cardiopulmonary bypass with a single
venous cannula and with the heart beating at a systemic temperature of 32°C to 34°C. The
aorta is retracted to the left. The main pulmonary artery is traced to the origin of the
left pulmonary artery, which is then dissected free where it passes behind the trachea.
The stenotic segment of trachea, as defined by preoperative studies, is dissected free.
The trachea is divided transversely through the center of the stenotic segment. The left
pulmonary artery is brought forward between the two ends of the divided trachea. Serial
sections are taken of the two ends of the trachea until a satisfactory luminal area is
found; this is usually no farther than three to four tracheal rings. This amount of
resection allows anastomosis with little tension. In fact, it is important not to
compromise the amount of resection, thereby leaving important residual stenosis because of
concern regarding reanastomosis. The tracheal anastomosis is undertaken with a continuous
5-0 or 6-0 Maxon suture. This is a simple anastomosis because of the absence of tubes
through the anastomotic area, the absence of clamps, and the strength of the tracheal
cartilages. In fact, the presence of complete tracheal rings simplifies the anastomosis
because of their strength. If a membranous component is present posteriorly in the
trachea, great care must be taken in this area, with the tension being absorbed by suture
bites through cartilage. When the anastomosis has been completed, the trachea is
pressurized to 40 cm H20 to test for air leaks. The anastomosis is then wrapped
with a flap of pedicled autologous pericardium, generally based on the right side of the
pericardium, anterior to the right phrenic nerve.
The lie of the left pulmonary artery in its new location should be carefully observed.
Because it arises more distally than normal, there is some risk of kinking at its origin.
In two of six patients with pulmonary artery sling who underwent tracheal resection, the
left pulmonary artery is reimplanted more proximally into the main pulmonary artery. In
one case this was related to the fact that the child had absence of the right lung with a
marked mediastinal shift to the right.
Very-long-segment tracheal reconstruction with reimplantation of the left pulmonary
artery
This is an unusual anomaly, usually related to the presence of complete tracheal rings
over the entire length of the trachea, in association with a pulmonary artery sling. It is
important to recognize that the presence of complete tracheal rings per se is not an
indication for surgical intervention in that area of the trachea. Generally, stenosis of
the trachea in a young infant does not become critical until the minimal diameter is
between 1.5 and 2 mm. Although there is a direct relationship between the length of the
stenotic segment and airway resistance, the relationship to luminal diameter is to the
fourth power, so this should be the overriding factor in determining need for surgical
intervention. The natural history for most airway problems, such as tracheomalacia and
minimal compression from a vascular ring, is for improvement with age. For all these
reasons, these authors have not found it necessary to intervene surgically for
very-long-segment tracheal stenosis.
Several surgical methods have been described, including a posteriorly placed
longitudinal incision with direct suturing to the anterior wall of the esophagus, as well
as an anterior longitudinal incision with placement of a longitudinal rib cartilage graft
(as is commonly applied for reconstruction of subglottic stenosis). The largest series has
been described by Backer and associates, who used a longitudinal incision placed
anteriorly along the full length of the trachea utilizing cardiopulmonary bypass. An
autologous pericardial patch is sutured into this anterior defect. The pulmonary artery
sling is managed by division of the origin of the left pulmonary artery, with implantation
into the main pulmonary artery. Ventilatory support is provided for at least 2 weeks, with
the endotracheal tube functioning as a stent. |