Pulmonary Artery Sling

Anatomy and Physiology

Embryologically, an aberrant left pulmonary artery arises from a lack of development of the sixth aortic arch.[11] In normal embryologic development, the left pulmonary artery connects to the main pulmonary artery anterior to the trachea.[3] However, in pulmonary artery sling (PAS), the left pulmonary artery arises from the right pulmonary artery resulting in a longer course and runs between the esophagus and trachea, causing compression of these structures. This subtype of a vascular ring is also known as an "anomalous left pulmonary artery."[1] With advanced imaging, this diagnosis can also be made prenatally with fetal ultrasound and follow-up CT/MRI post-natally, with an association of intrauterine growth retardation (IUGR).[12] If diagnosed post-natally, patients are usually symptomatic by one-year-old.[3][11]

Respiratory symptoms can range from mild tachypnea, wheezing, to severe distress requiring mechanical ventilation.[10] The portion of the trachea, which is compressed, is the right bronchus and lower trachea. The right mainstem bronchus and esophagus are compressed anteriorly.[4] This bronchial compression can cause atelectasis and pneumonia.[4] Symptoms range in presentation, but PAS patients can become symptomatic early in life - within the 1st year of life.[2][11] There is a range of ages for patients undergoing surgical repair for PAS, with a median age of 7-9 months (from 6 days to 27 months).[4][13] The majority are neonatal and infants with congenital tracheal stenosis and PAS.[13]

All PAS patients need to be thoroughly evaluated with advanced imaging such as computerized tomography (CT) or magnetic resonance imaging (MRI). A bronchoscopy is necessary to evaluate for complete tracheal rings as a cause of tracheal stenosis. About 50%-79% of patients with PAS also have congenital tracheal stenosis with complete cartilaginous rings.[1][9][10][14]

Patients with congenital tracheal stenosis from complete cartilaginous rings, absent membranous trachea, also have a higher association with cardiovascular anomalies, with PAS being the most common, around 50%.[14][4] Tracheal cartilages are circular instead of U-shaped, giving rise to the term "stovepipe."[4] Bronchoscopy can evaluate mucosal detail, dynamic airway changes, and location of extrinsic pulsation.[15] In 1984, this association was coined with the phrase "ring-sling complex" by Berdon and colleagues.[9] These respiratory symptoms can be more severe and include stridor, retractions, and dyspnea.[1][3] Other associated airway anomalies include tracheomalacia or bronchus suis.[14]

About 30% of patients can also have concomitant cardiac lesions (atrial septal defect, tetralogy of Fallot, and ventricular septal defect) that can be repaired simultaneously during PAS repair, making performing a pre-operative echocardiogram also necessary.[9][11][4] The most common associated cardiac lesion is tetralogy of Fallot.[14] Other cardiovascular anomalies include left superior vena cava, aberrant right subclavian, coarctation, atrial septal defect, and ventricular septal defect.[15] Gastrointestinal anomalies include duodenal atresia, biliary atresia, Meckel diverticulum, and Hirschsprung disease.[13][15]

Right lung agenesis can also exist with the failure of the development of the right main-stem bronchus. One more associated complexity is associated with VACTERL (vertebral defects, anal, cardiac, tracheal, esophageal, renal, and limb) syndrome.[4] Other genetic syndromes can also be associated, including Down syndrome, Holt-Oram, or Kartagener syndrome.[4][13]

Classification for PAS can be qualified with CTA/MRI according to the tracheobronchial anatomy. Type 1A described normal tracheobronchial anatomy, with sling just above the carina. Type 1B is where the right main bronchus and distal trachea leading to tracheomalacia or air-trapping. Type 2 is the most common with the presence of tracheal stenosis secondary to cartilaginous rings, and more caudally at T5-6 or low T-shaped carina.[11][15][16] MR imaging protocols are available for ECG-gated sequences for vascular rings and PAS.[16]

Indications

All patients diagnosed with pulmonary artery sling (PAS) should undergo work-up for surgical repair.[3] This includes computerized tomography (CT), bronchoscopy to evaluate for complete tracheal rings and extent of tracheal stenosis, and echocardiogram to assess for cardiac lesions.[9] The diagnosis is typically made from echocardiography.[4] It is sometimes difficult to pass the bronchoscope beyond the area of stenosis, so CT can help evaluate the distal airway.[15]

Congenital tracheal stenosis with complete cartilage tracheal rings will also require concomitant tracheal repair.[9] Indications for tracheal repair include severe tracheal stenosis, narrow trachea with complete rings, or inability to wean from ventilation after initial PAS repair.[10] Some groups report a tracheal diameter of <3 mm and length ratio >60% as a cut-off for surgical indication, and another group has suggested the diameter/length ratio <5.9 for tracheal stenosis.[13] There are no absolute indicators for concomitant tracheal stenosis and PAS due to a lack of sufficient evidence.    

Patients with mild tracheal narrowing and absence of complete tracheal cartilaginous rings can have only pulmonary artery sling reimplantation without tracheal reconstruction.[4] Patients with mild symptoms of congenital tracheal stenosis need careful pre-operative assessment due to an increased risk of mortality with surgical intervention.[13] Other imaging modalities can be considered but will have limited information. Esophagram has limited pre-operative work-up since the findings can be confusing or non-specific from PAS.[15] Nuclear medicine imaging can show differential pulmonary perfusion and can help differentiate between airway or vascular abnormality.[15]

Contraindications

One case series from Indiana University of 14 patients demonstrates that right lung agenesis (two patients who underwent repair) is not an absolute contraindication to pulmonary artery sling (PAS) repair.[4]

Equipment

Equipment for the pulmonary artery sling (PAS) repair includes but is not limited to the following:

• Operating room
• Sterile drapes
• Sterile gowns
• Sterile supplies 
• Sternotomy saw
• Cardiopulmonary bypass machine
• Surgical knives
• Surgical clamps
• Sutures

Equipment for the pulmonary artery sling repair procedure should be available in a cardiac surgery operating room with appropriate instruments and staff.

Personnel

Personnel needed to perform pulmonary artery sling (PAS) repair includes but is not limited to the following:

• Trained cardiac surgeon
• Cardiac anesthesiologist
• First surgical assistant
• Surgical technician
• Surgical nurse
• Perfusionist
• Cardiologist +/- echo sonographer

Post-operative care should ideally be in a specialized cardiac intensive care unit where intensivists and nurses are familiar with these patients' care needs.

Preparation

Once a patient is diagnosed with pulmonary artery sling (PAS) and undergoing preparation for surgery needs a complete series of tests to prepare thoroughly. These examinations include chest radiograph, CT/CTA, MR/MRA, electrocardiography, transthoracic echocardiogram, and bronchoscopy.  

The anesthesia team should perform endotracheal intubation for patients with proper preparation for a "difficult airway." Flexible bronchoscopy should also be performed before incision. For a gentle neck extension, a shoulder roll is useful.[5]

Technique or Treatment

The goal for surgical repair of a pulmonary artery sling (PAS) is to adjust the left pulmonary artery so that it no longer travels between the trachea and esophagus - to relieve the compression.  It is also essential to ensure long-term patency of the vascular supply to the left lung. The congenital tracheal stenosis or intracardiac lesions can be addressed simultaneously during the same operation.

There are two types of surgical repair techniques for an anomalous origin of the left pulmonary artery from the right pulmonary artery: 1) reimplantation of the left pulmonary artery into the main pulmonary artery, 2) tracheal transaction, and left pulmonary artery translocation. PAS can be performed from a left thoracotomy or median sternotomy.[4] The preferred surgical incision is from a median sternotomy and using the cardiopulmonary bypass machine.[1][4] In 1999, Backer and associates described left pulmonary artery implantation for PAS.[4]

The procedure is initiated similarly to other open cardiac surgery procedures. A median sternotomy is done, and cannulation is performed for cardiopulmonary bypass. Mild hypothermia for temperature target is about 32 degrees Celsius. Bicaval venous cannulation and cardioplegic arrest are recommended if concomitant intra-cardiac lesions need to be repaired as well. The ligamentum arteriosum or patent ductus arteriosus should also be ligated. The left pulmonary artery is dissected and usually arises from the posterior/right pulmonary artery. It is ligated from the anomalous origin on the right pulmonary artery, brought anterior to the trachea, and reimplanted to the main pulmonary artery.[4]

There is some controversy surrounding the optimal tracheal repair for long-segment congenital tracheal stenosis.[4] Tracheal stenosis repair techniques can include resection with end-to-end anastomosis, patch tracheoplasty, tracheal autograft, tracheal resection, and slide tracheoplasty. Short segment tracheal stenosis (less than 30%) can be repaired with resection and primary end-to-end anastomosis.[7] Patch tracheoplasty can be formed using different materials, including tracheal autograft, rib cartilage graft, or autologous pericardium.[4] Patch tracheoplasty patients postoperatively are required to be sedated and paralyzed to allow adherence to surrounding structures.[4] Slide tracheoplasty is the preferred approach in the current era.[10][9][14] 

Grillo and associates describe this surgical technique for long-segment stenosis.[17][18] Slide tracheoplasty offers an advantage by using autologous tracheal tissue, shortens the trachea by half of the involved stenotic segment instead of the full length, earlier extubation, and a longer anastomotic length to distribute tension. This technique shortens the trachea but doubles the diameter of the lumen.[15] One alternative option can be to perform tracheal resection and anastomosis should only be considered if less than four rings are involved. It is important to preserve the lateral attachments to the trachea to preserve the vascular supply.[14] It is important to use an absorbable suture for the tracheal anastomosis.[17] Vicryl or polydioxanone suture (PDS) are appropriate suture options, often 5-0. Running or interrupted suturing techniques can be performed.[5]  

In planning complex procedures and surgical repair of multiple abnormalities, the order can vary depending on preferred techniques. If a tracheal repair needs to be performed, this can be done after left pulmonary artery reimplantation. Bronchoscopy can also be performed at this part of the procedure. Sengupta and Murthy present a detailed operative approach for concomitant tracheal stenosis and PAS repair, preferring to complete the tracheal surgery before cardiac defect repair.  Some tracheal release maneuvers may need to be performed, including a "Montgomery-type supra-hyoid release" to allow a tension-free anastomosis. An endotracheal tube (ETT) should be secured at the portion of the tracheal anastomosis. Bronchoscopy should be performed to clear the airway and identify residual stenosis. A leak test should also be performed with saline and increasing airway pressure with a Valsalva maneuver. A "Grillo" stitch can also be placed after the chest is closed, which anchors the chin down to the chest wall to prevent extension and disruption of the anastomosis postoperatively.[5]

When right-sided lung hypoplasia is present, additionally, an aortopexy can be beneficial to move the aorta away from the airway.[15]

Complications

Patients not requiring tracheal repair at the time of pulmonary artery sling (PAS) repair have an excellent prognosis and long-term results.[10][15][6] Pre-operative risk factors for decreased survival after the combined tracheal and cardiac repair include ECMO, tracheomalacia, or bronchial stenosis.[19]

Complications after PAS can include:[20][4][13][5]

• Mortality
• Arrhythmias
• Low cardiac output syndrome
• Renal failure
• Abdominal sepsis
• Mediastinitis
• Re-exploration for bleeding
• Diaphragm paralysis from phrenic nerve injury
• Technical errors - left pulmonary artery kinking from too much length
• Tracheitis
• Tracheostomy
• Residual tracheal/bronchial stenosis
• Recurrent tracheal stenosis from granulation tissue
• Pneumonia
• Wound infection
• Pulmonary hypertension

Surgical technical complications in left pulmonary artery reimplantation include tension, kinking, narrowing of the left pulmonary artery, and compression of the trachea anteriorly.[15] Any complications related to intra-cardiac lesion repair can also occur, including pacemaker insertion.[4] Re-operation can include re-operation for tracheal reconstruction, recurrent tracheal stenosis, and diaphragm plication.[20][4]

Postoperative complications from slide tracheoplasty include injury to the recurrent laryngeal nerve, anastomotic dehiscence, or recurrent stenosis.[14] Two cases describe early mortality from PAS and tracheal repair; however, it is notable that the technique performed for tracheal repair was autologous pericardial patch tracheoplasty.[20] Patients undergoing pericardial patch tracheoplasty can be at risk for patch collapse. After tracheal repair, bronchoscopy may be necessary with laser resection of granulation tissue.[4]

Patients can die from mediastinitis due to anastomotic leak.[13] Some groups advocate using inhaled ciprofloxacin to minimize the granuloma formation of post-tracheal repair.[5] The risk of leak and restenosis increases with a longer segment and excessive tension.[7]

Patients undergoing simultaneous PAS repair and tracheal repair are prone to early mortality (within 30 days of surgery or before hospital discharge) compared to PAS repair only patients.[10] In patients with long-segment tracheal stenosis, mortality rates can be as high as 22%.[15]  Extensive long-segment tracheal stenosis, especially involving the carina, may put patients at risk for postoperative tracheostomy after tracheoplasty. Pre-operative ventilatory support seemed to put patients at risk for requiring tracheostomy.[7]

In one case series of 14 patients with PAS and tracheal stenosis, there was a mortality rate of 14% (2 out of 14). All patients on long-term follow-up for about three years show patent left pulmonary artery anastomosis.[4] Chen et al., in a follow-up of 116 patients, in-hospital mortality was about 6%, cardiovascular anomalies were significantly associated with postop tracheomalacia with intermediate outcomes.[21]

Clinical Significance

Overall, vascular ring anomalies are a broad category of embryologic vascular anatomic variants that can be associated with respiratory and feeding problems. Symptomatic children with pulmonary artery sling (PAS) and associated tracheal stenosis have increased mortality if no surgery is performed.[4] This is due to tracheomalacia and compression from the PAS.[13] Surgical treatment can be useful for the relief of tracheoesophageal compression.[2]

The variety of tracheal surgical repair techniques and PAS repair techniques has led to improved overall mortality after surgery, especially over the past 20 years. Left pulmonary artery transaction with reimplantation, slide tracheoplasty, median sternotomy, and cardiopulmonary bypass is recommended for the repair of both defects.[9] Regardless of surgical techniques, children benefit from a multidisciplinary, team-based approach with surveillance perioperatively will include outcomes due to the complexity of cardiopulmonary complications.[5]

The 15-year survival for patients who had PAS repair alone was 100% in a retrospective study across two institutions of 33 patients. However, PAS and tracheal repair survival were decreased to 76% (+/- 10%). Late survival is good, with the majority remaining asymptomatic.[10] Left pulmonary artery reimplantation using cardiopulmonary bypass is the preferred surgical approach with good long-term results with left pulmonary arteries patent seen on perfusion scan in 34 patients from another series.[9]

Outcomes for PAS appear to be associated with the complexity of commonly associated anomalies, including complex cardiac and tracheal lesions.[6][7] Advances have been made to improve outcomes, but further research needs to be performed to optimize outcomes for particular at-risk groups.[6] Although patients who undergo surgical intervention for PAS can have successful surgical outcomes, patients with tracheal stenosis requiring surgical intervention are at increased risk.[10][9] Thus, patients with mild symptoms of congenital tracheal stenosis need careful pre-operative assessment due to an increased risk of mortality with surgical intervention.[10]

The exact indicators for surgical intervention of congenital tracheal stenosis are still being explored since the range of tracheal stenosis, and symptoms are variable.[13] Patients who have PAS and other associated genetic syndromes are a particularly high-risk subgroup. When approaching intra-cardiac anomalies, more complex lesions should be palliated or observed after PAS and tracheal reconstruction.[4]

Enhancing Healthcare Team Outcomes

Pulmonary artery sling (PAS) symptoms can be non-specific, but effective treatment requires proper pre-operative workup and surgical intervention.[3] Since the differential diagnosis can be broad, a well-prepared healthcare team can improve recognition, diagnosis, and treatment. [Level 5]

A thorough multimodal pre-operative workup is essential for precise surgical planning since there is associated tracheal stenosis with PAS: CT, echo, and bronchoscopy.[9] [Level 3]

Patients with mild symptoms of congenital tracheal stenosis need careful pre-operative assessment due to an increased risk of mortality with surgical intervention.[13] Some patients with mild tracheal disease can undergo only PAS repair.[4] [Level 3]

A crucial component to ensuring successful surgical outcomes in a thorough, multidisciplinary approach for perioperative management.[5] [Level 3]

Nursing, Allied Health, and Interprofessional Team Interventions

Failure to recognize severe symptoms of respiratory failure caused by congenital anomalies can lead to sudden death in infants. This emphasizes the importance of recognition and appropriate workup to diagnose this complex disease.[4][22] [Level 3]

Pediatric patients with pulmonary artery sling (PAS) and tracheal stenosis can be critically ill. Some patients may need to be intubated pre-operatively.  There are many associated congenital anomalies, including cardiac and airway. Pre-operative workup and post-operative care require multiple disciplines to ensure optimal surgical outcomes.[4] [Level 3]

Patients who have PAS and other associated genetic syndromes are a particularly high-risk subgroup for increased morbidity and mortality. Multidisciplinary care is essential in discussing possible staged procedures for complex cardiac lesions.[4] [Level 3]

Nursing, Allied Health, and Interprofessional Team Monitoring

Knowledge of post-operative complications is required due to the complexity and range of cardiopulmonary symptoms. Post-operative imaging includes chest X-rays and MRI/CT during early post-operative care. Long-term, MR phase-contrast imaging and nuclear lung perfusion can demonstrate differences in lung flow. Bronchoscopy will need to be employed to evaluate granulation tissue development and possibly balloon dilatation.[15] [Level 3]

With a variety of surgical strategies, children benefit from a multidisciplinary, team-based approach, and surveillance for improved surgical outcomes.[4] [Level 5]