TSRA Content:
Authors: Bryant Fisher, MD, and Mario Castro-Medina, MD
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by Susheel Kumar, MD.
Modified Blalock-Thomas-Taussig Shunt
The Blalock-Thomas-Taussig (BT) shunt originated by Dr. Alfred Blalock in 1944 was an end-to-side anastomosis fashioned between the transected subclavian artery and the branch pulmonary artery. The modified BT shunt was introduced in 1981 and utilized a prosthetic graft to divert flow from the pulmonary artery to the subclavian artery.
Indications for Intervention:
Conditions of reduced pulmonary blood flow (e.g., Tetralogy of Fallot with pulmonary stenosis or atresia, single ventricle physiology with reduced pulmonary blood flow). The modified BT shunt is fundamentally a palliative operation to improve pulmonary blood flow and oxygenation in the neonate with cyanotic defects. It can also be utilized as a bridge in single ventricle physiology to allow time for pulmonary vascular resistance to decrease in the neonatal period.
Technique:
The modified shunt was initially done through a thoracotomy on the side ipsilateral to the aortic arch. In recent years, a median sternotomy has become the approach of choice.
The patient is placed in a supine position and a median sternotomy is performed. After resection of the thymus, the pericardial sac is opened in its upper portion and pericardial stay sutures are placed. The proximal portion of the innominate artery and the right pulmonary artery (RPA) are dissected free. Lymph nodes and tissue between the aorta and the superior vena cava (SVC) may be resected to create a bed for the graft. Care is taken to avoid any injury to the ductus at this stage. The RPA is occluded temporarily to test the feasibility of constructing a modified BT off-pump. If this is not tolerated, the patient is placed on cardiopulmonary bypass (CPB) via aortic and right atrial (RA) cannulation. The ductus is dissected and ligated after instituting CPB. If CPB is not needed, the ductus should not be manipulated until after the shunt is in place. The patient is heparinized at 50-100 units/kg when CPB is not used. The proximal anastomosis is performed first. A suitable site is selected on the distal innominate artery. A Castaneda clamp is applied, and a small incision is placed over the inferior aspect of the artery. An appropriately sized graft is selected and beveled. The anastomosis between the graft and the distal innominate artery is constructed using a running 6-0 or 7-0 Prolene. The clamp is then briefly released to test inflow into the graft. Once satisfied with the anastomosis, the clamp is re-applied, and the graft is flushed with heparinized saline to prevent thrombosis. For the distal anastomosis, the RPA is clamped, and an incision made on its superior aspect. The graft is cut to appropriate length avoiding redundancy and the distal anastomosis to the RPA is constructed with a running 6-0 or 7-0 Prolene. Both clamps are then removed to allow flow through the graft. A decrease in the systolic blood pressure and an increase in the pulse pressure are typically observed. If CPB was used, the patient is gradually weaned off CPB. The patient’s oxygen saturations are monitored with goals of 70-80% for single ventricle physiology while higher saturations can be targeted for two-ventricle physiology. The patient is decannulated and the chest is closed with drains in place. At some centers, a continuous heparin infusion is started to prevent graft occlusion.
Pulmonary Artery (PA) Banding
PA banding is a procedure that reduces pulmonary artery circumference in order to reduce pulmonary over-circulation from large left-to-right-shunts. It was first described by Muller and Dammann in 1951 for palliation of ventricular septal defects.
Indications for Intervention:
The most common indications are:
- Single ventricle physiology to preserve the pulmonary circulation in preparation for a Fontan operation
- Swiss-cheese VSDs, multiple VSDs with coarctation
- Training of the left ventricle before an arterial switch operation (in the setting of d-transposition of great arteries) or the double switch operation (in the setting of corrected transposition of great arteries)
Surgical Technique for Typical PA Banding:
The patient is placed in a supine position and a median sternotomy is performed. The thymus is resected, and the pericardial sac is opened in its upper portion. The aorta and the main PA are dissected to develop the plane between the aorta and pulmonary artery. Trusler's rule may be used to define the length of the PA band (for biventricular repairs: 20 mm + weight in kg, for complex defects: 24 mm + weight in kg). A thin band of material is fashioned and Prolene sutures are used to mark the length as determined by the formula. Various materials including Dacron or PTFE can be used. The band is passed around the aorta and then pulled through the transverse sinus to encircle the main pulmonary artery and prevent injury (subtraction technique). The band is then tightened at the level of the mid portion of the PA to the predetermined length by placement of 5-0 Prolene sutures. Further adjustment of the band is facilitated by placement of medium sized hemoclips. The goal of PA banding is to decrease PA pressure to approximately 50% of systemic pressure with a concomitant rise in systolic blood pressure of 10-20 mmHg. Oxygen saturations of 90-95% are usually targeted in biventricular repairs while a lower saturation of 80-85% is tolerated in those undergoing a single ventricle pathway. Careful attention must be paid to at this point as overtightening can result in bradycardia and ischemia while distal migration may impinge on the branch pulmonary arteries. Following a period of observation for stable hemodynamics and saturations, the band is fixed to the adventitia of the PA using interrupted sutures to prevent migration. Transesophageal echocardiography (TEE) and/or direct PA pressure monitoring may also be used as a guide to position the band appropriately. Pacing wires are placed, hemostasis is confirmed, and the chest is closed over a drain.
When performing a concomitant coarctation repair, banding can be performed through a left thoracotomy. The addition of a BT shunt is necessary in cases of ventricular training and transposition of the great arteries (TGA). A possible complication of PA banding is a drop in pulmonary artery pressure and saturation as the child grows and the band tightens. This can result in heart failure, hypoxemia, and shunt reversal, if not addressed. Band removal is typically performed during the corrective surgery. Residual pulmonary artery stenosis induced by scarring from the band can be addressed by pulmonary arterioplasty.
Glenn Shunt
The bi-directional Glenn Shunt is typically utilized as part of the single ventricle pathway that was described previously but can also be employed in any operation to divert systemic venous return and unload the right ventricle. Specifically, the Glenn shunt is used to divert superior vena cava (SVC) return to the pulmonary circulation. The operation is typically performed after pulmonary vascular resistance has decreased several months after the neonatal period to allow for passive filling of the pulmonary circulation. The Hemi-Fontan is a similar alternative procedure that serves the same purpose. The indications for a Glenn shunt were described previously as part of the single ventricle pathway.
Surgical Technique:
The patient is placed in a supine position and a median sternotomy is performed. The thymus is resected, and the pericardial sac is opened in its upper portion. Cardiopulmonary bypass is instituted with aortic/neoaortic cannulation depending on previous staged procedures and underlying congenital defects. Right atrial appendage cannulation and proximal SVC cannulation for venous drainage are performed. Any previous palliative shunts (e.g., BT shunt) are ligated at this point. The azygous vein is encircled and ligated. The SVC is then circumferentially mobilized and snared. A vascular clamp is placed above the SVC and right atrial junction. The SVC is transected at the level of the right pulmonary artery. The right pulmonary artery is then mobilized, and incisions are made in its superior and inferior aspects. The superior end of the SVC is anastomosed to the superior incision in the pulmonary artery using a running 6-0 Prolene suture. The inferior end is anastomosed in a similar fashion to the inferior incision on the right pulmonary artery. After assessing for hemostasis of the anastomoses, the patient is weaned from bypass and the chest is closed over drains in the usual manner
Fontan Operation
The Fontan was originally described by Fontan and Baudet as an operation to bypass the right ventricle for palliation of tricuspid atresia. Many subsequent modifications have been made to properly tailor the procedure for congenital defects requiring a single ventricle pathway. Indications and general conduct preceding the Fontan procedure are discussed in the chapter on the single ventricle pathway. Briefly, the Fontan procedure is the third stage in the single ventricle pathway that completes diversion of systemic venous return to the pulmonary circulation to offload the systemic ventricle. This is typically performed with either the extra-cardiac Fontan or the lateral tunnel Fontan depending on the operations performed in the previous stages. The extra-cardiac Fontan is described below as it usually follows the bi-directional Glenn shunt described previously.
Surgical Technique:
The patient is placed in a supine position and a redo median sternotomy is performed with the oscillating saw. Arterial cannulation with superior vena cava (SVC) and inferior vena cava (IVC) bicaval venous cannulation are performed and cardiopulmonary bypass initiated. The right atrium, IVC, and right PA are mobilized in preparation for construction of the extra-cardiac Fontan. A clamp is applied above the IVC and right atrial junction. The IVC is divided above the clamp and the atrial defect is oversewn. A PTFE tube graft (18-22 mm) is then anastomosed to the IVC in an end-to-end fashion utilizing a running Prolene suture. The graft is then carefully measured and divided at the level of the right pulmonary artery as to avoid undue tension or kinking. The other end is then anastomosed to the center of the bifurcation of the pulmonary artery in an end-to-side fashion. To avoid complications from high pulmonary vascular resistance in the post-operative period, a fenestration is fashioned between the tube graft and the right atrium to act as a pressure-release valve. At this point, the patient is weaned from bypass and the chest is closed in the usual manner over drains. The post-operative management of this complex physiology is described in the chapters to follow.