Systemic Pulmonary Shunting

Because most congenital heart defects are managed by total correction, shunting procedures are now performed in select patient populations. Systemic to pulmonary artery shunts offer excellent palliation in patients with anatomically complex cardiac anomalies, in whom definitive repair is best delayed. They are also indicated as a source of controlled pulmonary blood flow in the initial management of neonates with single-ventricle anatomy.

A common application of the systemic to pulmonary artery shunt is in the neonate with a ductal-dependent pulmonary circulation. The ability to keep the ductus arteriosus patent with an infusion of prostaglandin E₁ allows these patients to be stabilized and undergo surgery on a semiurgent basis in an unhurried manner.

TYPES OF SHUNTS

The Blalock-Taussig shunt was introduced in 1945. Classically, it consists of anastomosing the subclavian artery to the pulmonary artery on the side opposite the aortic arch. However, with some technical modifications, the subclavian artery can be anastomosed to the pulmonary artery on the same side as the aortic arch.

Other shunting procedures were subsequently introduced. They include the Potts shunt (descending aorta to the left pulmonary artery), Waterston shunt (ascending aorta to the right pulmonary artery), central shunt (interposing a graft between the ascending aorta and the main pulmonary artery), and the modified Blalock-Taussig shunt (interposing a Gore-Tex tube graft between the subclavian or innominate artery and the right or left pulmonary artery).

The Potts shunt was abandoned because it was cumbersome to perform, difficult to close, and could cause high flow and the early development of pulmonary vascular disease. The Waterston shunt lost favor because of the high incidence of injury to the pulmonary artery and the difficulty in controlling the amount of flow through the shunt. The classical Blalock-Taussig shunt is rarely used. Currently, some surgeons perform a central shunt or modified Blalock-Taussig shunt through a median sternotomy with the belief that the relative disadvantage of this approach requiring a redo sternotomy and dissection of adhesions for the next procedure is outweighed by the superior exposure and ability to place the patient on cardiopulmonary bypass should hemodynamic instability occur. Others prefer performing the operation off bypass through a lateral thoracotomy, rendering the subsequent completion operation one that is performed through a primary median sternotomy.

MODIFIED BLALOCK-TAUSSIG SHUNT WITH GORE-TEX TUBE GRAFT INTERPOSITION

Interposition of a Gore-Tex tube graft between the subclavian or innominate artery and the right or left pulmonary artery is the most commonly performed shunt procedure. With either sternotomy or thoracotomy, it should be remembered that the lumen of the subclavian or innominate artery is the limiting factor to the volume of flow. In neonates, a 3.5- or 4-mm graft is used; for older infants, a 5-mm graft is usually selected.

Median Sternotomy Approach

This approach has several advantages. The pulmonary end of the shunt can be placed more centrally, potentially allowing better and more uniform growth of both pulmonary arteries. The ductus arteriosus can be occluded at the conclusion of the procedure, preventing excessive pulmonary circulation in the early postoperative period. The ductus arteriosus can be ligated when a left thoracotomy approach is used but can rarely be accessed through a right thoracotomy. Finally, if the patient becomes unstable, cardiopulmonary bypass can be quickly initiated through a median sternotomy.

Incision

A standard median sternotomy with resection of the thymus is used.

Technique

After opening the pericardium, traction sutures are placed on the pericardial edges. The aorta and pulmonary arteries are dissected free using scissors or electrocautery on a low setting. Downward traction on the main pulmonary artery allows the ductus arteriosus to be identified and encircled with a tie or cleaned free of surrounding tissues in preparation for later metal clip closure. The innominate artery is dissected to allow application of a C-clamp. The right pulmonary artery is then dissected away from the posterior aspects of the ascending aorta and superior vena cava. It is mobilized circumferentially and the right upper lobe branch is identified.

Use of Heparin

If the shunt is being performed without cardiopulmonary bypass, light systemic heparinization (50 units/kg) is administered just before the clamp is applied to the innominate artery.

The Gore-Tex graft is trimmed obliquely. A fine vascular C-clamp is applied to the innominate artery so that the inferior aspect of the artery is centered in the excluded portion (Fig. 18.1). The handle of the clamp is then raised to position the inferior edge of the innominate artery anteriorly. A longitudinal incision is made in the artery, and a fine adventitial suture is placed on the superior edge of the arteriotomy to keep the lumen open. The anastomosis is completed with Prolene suture (Fig 18.2).

FIG. 18.1 Modified Blalock-Taussig shunt through a sternotomy: Placing a side-biting clamp on the innominate artery and rotating it to expose the inferior aspect of the artery. A vein retractor under the innominate vein improves exposure.

With the other end of the graft occluded, the vascular clamp on the innominate artery is carefully removed and the anastomosis is checked for leaks. The length of the Gore-Tex graft is measured to just reach the superior aspect of the proximal right pulmonary artery. The graft is divided transversely at this site after placing a fine straight vascular clamp on the graft just below the innominate anastomosis. The right pulmonary artery is grasped with a fine C-clamp so that the cranial aspect is in the middle of the clamp. The clamp is then rotated so that a longitudinal incision can be made on the superior edge of the pulmonary artery. The arterial opening should be approximately two-third of the diameter of the graft lumen as the pulmonary artery stretches. The anastomosis is completed with a 7-0 Prolene (Fig. 18.3). The clamps are removed and hemostasis confirmed.

Centrally Located Shunt

The median sternotomy approach allows the pulmonary artery end of the shunt to be placed more centrally. The aorta must be mobilized and retracted leftward with a traction suture on the right side of the aorta, a vein retractor, or the back of the C-clamp itself (Fig. 18.3).

Coronary Ischemia

Care must be taken when applying traction to the aorta to prevent compression or kinking of the coronary arteries. If any electrocardiographic changes are noted or hemodynamic instability occurs, the traction suture, retractor, or clamp must be repositioned immediately.

Pulmonary Flooding

When the shunt is opened and flow through it confirmed, the ductus arteriosus, if present, should be occluded to prevent pulmonary overcirculation. Too much pulmonary blood flow may lead to systemic hypoperfusion and an inadequate diastolic blood pressure, resulting in coronary ischemia.

Hemodynamic Instability with Right Pulmonary Artery Clamping

Before incising the pulmonary artery, hemodynamic stability and systemic oxygenation with the C-clamp in place should be assessed. The clamp may interfere with ductal flow, and reapplying it more distally on the right pulmonary artery may rectify the problem. However, if desaturation or hemodynamic compromise persists after repositioning the clamp, the patient should be placed on cardiopulmonary bypass for support during this anastomosis.

Incorrect Length of the Tube Graft

Tension on the anastomosis owing to too short a tube graft may cause suture line bleeding and an upward pull on the
pulmonary artery, which can lead to distortion or stenosis of the proximal right pulmonary artery. A graft that is too long may kink, thereby compromising flow through the graft.

FIG. 18.2 Modified Blalock-Taussig shunt through a sternotomy: The end-to-side anastomosis of a Gore-Tex tube graft to the innominate artery. The inferior suture line is completed first.

FIG. 18.3 Modified Blalock-Taussig shunt through a sternotomy: Completing the pulmonary artery anastomosis. The side-biting clamp has been placed so that the cranial edge of the right pulmonary artery is exposed.

If pericardial reapproximation is desired, a Gore-Tex pericardial membrane should be used in lieu of direct pericardial approximation since minor changes in mediastinal structures can cause compression and thrombosis of the shunt. A small chest tube is placed in the anterior mediastinum before a standard sternotomy closure is performed.

Modified Right Blalock-Taussig Shunt

It may be preferable to place an interposition Gore-Tex tube graft between the innominate/subclavian and pulmonary arteries through a thoracotomy incision. Some surgeons prefer a thoracotomy approach for the initial shunt. In this case, a right-sided shunt may be used because it is easier to take down. The technique is essentially the same for both sides. The following description pertains specifically to the right side.

Incision

A right thoracotomy through the fourth intercostal space provides satisfactory exposure.

Technique

The right lung is retracted inferiorly and posteriorly, and the local anatomy is evaluated. The right pulmonary artery is identified. The parietal pleura overlying it is incised, and the artery is mobilized medially toward the pericardium and distally to the hilum of the lung and proximally toward the mediastinum. Often the use of a blunt peanut dissector allows for mobilization without disruption of the pericardial edge. It is often helpful to ligate and divide the azygous vein, and also resect the lymphatic tissue that lies posterior and lateral to the superior vena cava, as this tissue will be in most direct path from the innominate artery to the right pulmonary artery.

FIG. 18.4 Right-modified Blalock-Taussig shunt: Operative view of the hilum of the right lung and right subclavian artery with loose snares around the right pulmonary artery and its lobar branches.

Identification of the Right Pulmonary Artery

Sometimes the exact identity of the vessels within the hilum of the lung may not be clear. If there is any doubt as to the exact location of the pulmonary artery, it can be traced from within the pericardium through a short longitudinal incision on the pericardium, just anterior and parallel to the left phrenic nerve.

The right pulmonary artery, with its lobar branches having been clearly identified, is prepared for clamping or snaring with fine vascular elastic bands. The parietal pleura over the innominate or subclavian artery is incised, and the artery is mobilized and dissected free of its parietal sheath (Fig. 18.4).

A 3.5- or 4-mm Gore-Tex tube graft is used for neonates, and a 5-mm graft is rarely used for older patients. Because the size of the lumen of the innominate or subclavian artery is the limiting factor to the flow of blood, grafts larger than the subclavian artery do not necessarily increase the flow to the lungs and therefore are not responsible for pulmonary flooding, if it occurs.

The distal end of the graft is trimmed obliquely. An appropriate segment of the subclavian artery is excluded within a delicate vascular clamp. A longitudinal incision is then made in the artery. A fine adventitial traction suture on the anterior edge of the arteriotomy will keep the lumen of the artery open.

The anastomosis is started near the toe with a 7-0 Prolene suture and completed as a continuous anastomosis (Fig. 18.5).

With the other end of the graft temporarily occluded with fine, atraumatic forceps, the vascular clamp on the subclavian artery is loosened to detect a gross anastomotic leak that may require additional suture reinforcement. A cross-clamp is then reapplied to the graft. The length of the Gore-Tex tube graft is meticulously evaluated; it is divided
transversely at the appropriate site so that when its divided end is in close apposition to the right pulmonary artery, it is under no tension and is not kinked.