| While it is a technically demanding procedure, Norwood's operation consists of a number of steps which in sequence can routinely be accomplished in less than an hour (sometimes as little as 35 - 40 minutes) of myocardial and cerebral ischemia. With respect to the fragility of the preoperative physiology, little manipulation of the heart is undertaken prior to initiation of cardiopulmonary bypass. The simplest means of cannulation, as described by Norwood, involves essentially no preliminary dissection nor any manipulation of the ductus arteriosus. Arterial inflow is accomplished by cannulation of the main pulmonary artery just distal to the sino-tubular junction. A single venous cannula is inserted through the right atrial appendage . Application of suture tourniquets to the right and left branch pulmonary arteries immediately after the initiation of bypass ensures satisfactory perfusion through the ductus arteriosus. Isolation of the left branch pulmonary artery for tourniquet occlusion is facilitated by reduction of the pump flow to a very low level for a few seconds. Others have recommended direct cannulation of the ductus arteriosus with tourniquet occlusion proximal to the cannula, but this may be complicated by the friability of the ductal tissue. When cooling has been accomplished, the branch vessels of the aortic arch are occluded with suture tourniquets. The primary purpose for occlusion of the arch vessels at the time of hypothermic circulatory arrest is to prevent air embolism at the time of reperfusion. While the tourniquets must be occlusive, applying them too tightly may fracture the delicate tissue of the arterial walls, which can be particularly problematic if the innominate artery is injured at the site where the arterial anastomosis to the shunt would later be reconstructed.
While it is not clear that cardioplegic myocardial protection is absolutely necessary during hypothermic circulatory arrest, the most straightforward means of achieving cardioplegic arrest is by delivering cardioplegic solution directly into the cannulation site in the proximal main pulmonary artery while the descending thoracic aorta is occluded with a forceps or appropriate vascular clamp.
Atrial septectomy can usually be accomplished through the right atrial cannulation site after removal of the cannula. To facilitate this, a generous purse string is utilized at the site of venous cannulation. Through the cannulation site, septum primum is visualized, grasped with a forceps and excised. The surgeon must be aware of the frequent variant of atrial septal anatomy, that is, extreme leftward deviation of the superior posterior attachment of septum primum. In cases where this arrangement makes adequate visualization of septum primum through the cannulation site difficult, or when there is a very restrictive intra-atrial communication (usually in association with a thick, spongy muscular intra-atrial septum) visualization and excision of the intra-atrial septum may be best accomplished through a separate right atriotomy.
Transection of the main pulmonary artery is begun anteriorly and is accomplished at a level immediately proximal to the origin of the right branch pulmonary artery (which often takes off slightly more proximally than the left branch pulmonary artery). Once the main pulmonary artery is opened anteriorly by means of a transverse incision, inspection inside confirms the level of the commissures of the pulmonic valve and the points of origin internally of the right and left branch pulmonary arteries. Often there is very little distance between the tops of the commissures of the pulmonic valve and the origin of the branch pulmonary arteries, and it is of utmost importance to avoid injury to the valve. Closure of the distal main pulmonary artery with a small patch helps to avoid any narrowing in the pulmonary artery confluence. Since the arch reconstruction is later accomplished with a portion of a cryopreserved pulmonary artery homograft, there is virtually always enough homograft tissue to use for this patch. Others however have advocated the use of autologous pericardium. The ductus arteriosus is then isolated, ligated, and transected a t its entrance into the thoracic aorta. The opening of the ductus into the thoracic aortic must be extended distally down the medial aspect of the descending thoracic aorta for a distance of at least 1 cm, and often more than 1.5 cm. Inspection through the opening will reveal the features of coarctation if present. There is considerable controversy as to the true incidence of aortic coarctation in association with hypoplastic left heart syndrome (references 11, 13, 16, 19). A large surgical experience reveals that coarctation of the aorta is by no means a constant feature of hypoplastic left heart syndrome, but rather because the isthmus of the aorta and the aortic arch actually function as a branch of the main pulmonary artery - ductus - thoracic aorta continuum, the junction of the isthmus and the thoracic aorta has internally an intimal ridge as is usually present at major arterial branch points.
Beginning at the opening of the ductus into the thoracic aorta an incision is t hen carried proximally along the underside of the transverse arch and down the medial aspect of the diminutive ascending aorta to a level corresponding to that of the transsected proximal main pulmonary artery. Beginning distally on the descending thoracic aorta, the entire aortic arch complex is augmented with a gusset of cryopreserved pulmonary artery homograft sewn in place with continuous monofilament suture. After experience with a variety of materials, pulmonary artery homograft is now routinely used because of its advantageous elasticity and hemostatic properties as well as availability. The fact that it is thinner than aortic homograft material has prompted some surgeons to raise concerns about its durability, but in experience with hundreds of cases we have observed no aneurysm formation. Once the homograft patch has been applied to the descending thoracic aorta and transverse arch, an open pair of forceps is directed down into the descending thoracic aorta to expand the lumen of the aorta. This makes it easy to appreciate the intimal ridge at the juncture of the isthmus and thoracic aorta, whether or not coarctation is present. Using an #11 blade, this ridge protruding into the lumen of the aorta is incised, and t hen with forceps the intimal tissue is peeled off the wall of the aorta. While this can result in a very focal discontinuity of the intima, in hundreds of cases it has not been associated with either hemorrhage or subsequent aneurysm formation, and the inclusion of this step has led to a marked reduction in incidence of late distal arch obstruction.
Proximally, the amalgamation of the main pulmonary artery with the ascending aorta is best accomplished with a few interrupted sutures in instances where the ascending aorta is tiny (most cases of aortic atresia) but may well be accomplished with continuous suture when the aorta is 3 or 4 mm or more in diameter. Growth of the homograft portion of the augmented aorta is not anticipated, but as long as a portion of the aorta is composed of viable autologous tissue at all levels, satisfactory growth of the aorta is anticipated and has been confirmed by over a decade of experience.
The choice of shunt is a subject of some controversy. Dr. Norwood's experience in Philadelphia included large series of patients who received 3 and 4 mm central shunts (interposed between the underside of the reconstructed arch and the superior aspect of the pulmonary artery confluence) as well as large groups of patients who received so called right modified Blalock Taussig shunts (interposition grafts between the innominate artery and the right branch pulmonary artery) of 3.5 and 4 mm caliber. At one point in time it appeared that the central shunt may be associated with more even distribution of blood flow to the right and left branch pulmonary arteries, but certainly pulmonary artery distortion was not completely eliminated by the central shunt. By necessity, the central shunt is a relatively short length, and with the 4 mm central shunt pulmonary blood flow may sometimes be excessive. In my current practice a 4 mm right modified Blalock Taussig shunt (innominate artery to right branch pulmonary artery) is used in the majority of instances, and a 3.5 mm right-modified Blalock Taussig shunt is used in particularly tiny babies, as for example 2.8 kg or less. Bove has stressed the importance of locating the anastomosis of the shunt to the pulmonary artery on the most proximal (central) portion of the right pulmonary artery. The modified Blalock Taussig shunt has the advantage that it can be partially or entirely constructed during either the cooling or re-warming phase of cardiopulmonary bypass, thus shortening the duration of hypothermic circulatory arrest. In addition it is very easily exposed at the time of subsequent staged reconstruction. The central shunt however has not been abandoned completely, and is utilized selectively (for example in instances where there is aberrant origin of the right subclavian artery from the descending thoracic aorta and the first branch of the aortic arch is a right carotid artery).
Separation from cardiopulmonary bypass can usually be accomplished with minimal or n o inotropic support. When there is severe hypoxemia (Pa02 < 27) on oxygen and with hyperventilation, technical problems related to the shunt must be ruled out. In the absence of technical problems, catecholamine infusions may in this instance be useful to elevate the systemic resistance relative to the pulmonary vascular resistance (which must be assumed already to be quite high). The utility and effect of catecholamine infusions has been studied in experimental models by Austin and others and has been reviewed in clinical experience (reference 27). It must be remembered that any vasopressor drug will influence the resistance in both the systemic and pulmonary vascular beds. In the majority of cases separation from cardiopulmonary bypass can be accomplished with low Fi02 (21 - 40%) with the expectation of arterial P02 in the 30 - 40 range. When the P02 early after completion of the repair is higher than this, or when there is persistent metabolic acidosis suggestive of inadequate systemic perfusion, there must be a high level of suspicion of narrowing in the distal aortic reconstruction.
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| The two major treatable problems that must be ruled out by careful surveillance after initial palliation are the occurrence of residual or recurrent distal arch obstruction and the development of restriction at the level of the intra-atrial communication and thus pulmonary venous obstruction. Distal arch obstruction may be present because of suboptimal initial reconstruction, but also occurs in up to 10% of cases where there was no evidence of residual coarctation early after stage I. It results in combined pressure and volume overload of the single ventricle, directing more and more of the ventricle's output through the shunt (which of course has its origin proximal to the arch obstruction) resulting in an increasing volume load and in the extreme, inadequate systemic perfusion. Distal arch obstruction must be considered in any infant with a suboptimal clinical course after initial palliation, particularly those with progressive cardiomegaly and signs of congestive heart failure o r failure to thrive. Pulse examination and measurement of four extremity blood pressures are important, but can be misleading since the take-off of the systemic-to-pulmonary artery shunt from the inominate artery, and the origin of the left subclavian artery in the region of aortic reconstruction occasionally result in diminution of the measured blood pressure in one arm or the other. Echocardiography is of course important in the diagnosis of distal arch obstruction, but one must be aware that using Doppler techniques it is quite usual to measure a peak velocity in the range of 2.5 meters per second, and occasionally as high as 3.5 meters per second in the distal arch after reconstruction, even if there is not a hemodynamically significant obstruction. If however echocardiography reveals narrowing, elevated velocity and turbulence in the distal arch, it is suggestive of recurrent coarctation and cardiac catheterization should be undertaken. In the majority of instances recurrent distal arch obstruction can be managed satisfactorily in the cath lab by balloon angioplasty, and in hypoplastic left heart syndrome this can be accomplished with venous access and antegrade catheterization through the neoaortic valve into the arch, avoiding femoral artery trauma.
The development of a hemodynamically important restrictive intra-atrial communication after first stage palliation is rare if care is exercised in the excision of septum primum. Nonetheless it does occur on occasion, most often when the intra-atrial septum has been thick and muscular in the first instance. Obstruction at the level of the intra-atrial septum of course translates into pulmonary venous obstruction and thus diminished pulmonary blood flow. This becomes evident as progressive cyanosis. Early occurrences of restriction at the level of the intra-atrial communication can alter the normal maturation of the pulmonary vasculature, preventing the normal maturational process of the diminution of the pulmonary vascular resistance. As such, in the small number of babies who have presented with restriction at the level of the intra-atrial communication in the first few months after initial palliation, it has proved best to address this problem in isolation (i.e. accomplish a repeat atrial septectomy, rather than combining that with a hemi-Fontan or bi-directional Glenn procedure). The normal expectation of a satisfactory outcome from cavopulmonary anastomosis at several months of age would be altered by persistent elevation of the pulmonary vascular resistance.
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