|CONGENITAL HEART DISEASE
|Year : 2003 | Volume
| Issue : 3 | Page : 5
Update on the Norwood Procedure for Hypoplastic Left Heart Syndrome
Richard A Jonas
Children's Hospital, Harvard Medical School, Boston, MA, USA
|Date of Web Publication||22-Jun-2010|
Richard A Jonas
Children's Hospital, Harvard Medical School, 300 Longwood Avenue Boston, MA 02115
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Jonas RA. Update on the Norwood Procedure for Hypoplastic Left Heart Syndrome. Heart Views 2003;4:5
| Introduction|| |
Hypoplastic left heart syndrome (HLHS) is not rare. In the New England Regional Infant Cardiac Program report of 1980  the incidence of hypoplastic left heart syndrome was 7.5% among children with congenital heart disease. Before the advent of prostaglandin El  and reconstructive surgery in the late 1970s, HLHS was responsible for 25% of deaths from congenital heart disease in the first week of life  . It has been estimated by Morris et al  that approximately 600 infants are born each year with HLHS in the United States.
| Anatomy|| |
There is no single generally agreed upon definition of hypoplastic left heart syndrome. However, a reasonable definition is that this is an anomaly in which there is normal segmental anatomy and that the left heart structures are inadequately developed to support the systemic circulation. Because the right heart is usually normally developed, it can be connected surgically to become the single functional systemic ventricle through application of the Norwood procedure.
HLHS involves various degrees of underdevelopment of left heart structures. The mitral valve may be either stenotic or atretic as may the aortic valve. Therefore, HLHS can be subcategorized into four main anatomic subtypes based on the morphology of the left heart valves: aortic and mitral stenosis, aortic and mitral atresia, aortic atresia and mitral stenosis, and aortic stenosis and mitral atresia. The most serious, aortic atresia and mitral atresia, is the most common anatomical subtype of HLHS representing approximately 35% of cases. Aortic stenosis with mitral stenosis generally represents approximately 20% of cases. Aortic atresia tends to be associated with a more severe degree of hypoplasia of the ascending aorta than does aortic stenosis. Patients in the aortic stenosis subgroup are part of a continuum of patients with critical neonatal aortic valve stenosis. Differentiating these two anomalies can be difficult and is discussed below.
Typically the ascending aorta in the neonate with aortic atresia type of HLHS is 2.5 mm in diameter whereas in the neonate with aortic stenosis who has HLHS, the ascending aorta is often 4 to 5 mm in diameter. The arch of the aorta is quite variable in length and is hypoplastic to various degrees. It may be interrupted. A coarctation shelf is present opposite the junction of the ductus with the proximal descending aorta in at least 80% of patients , . The ductus itself is large, often close to 10 mm in diameter. It is a direct extension of the main pulmonary artery which is even larger. The right pulmonary artery arises very proximally from the main pulmonary artery, usually no more than 2 to 3 mm beyond the tops of the commissures of the pulmonary valve.
The left atrium is usually smaller than normal which is exacerbated by leftward displacement of the atrial septum primum which is often heavily muscularized. Occasionally the foramen ovale is severely restrictive. The left atrium may have a thickened and fibrotic endocardium analogous to that seen in endocardial fibroelastosis. Occasionally this process extends into the pulmonary veins resulting in an obliterative generalized stenosis of these veins.
Associated Extracardiac Anomalies
Two reports from Children's Hospital of Philadelphia, one in 1988 and one in 1990 , , have suggested that there is an important incidence of extracardiac anomalies associated with hypoplastic left heart syndrome including diaphragmatic hernia, hypospadias and omphalocele. Brain anomalies included malformations such as agenesis of the corpus collosum and microcephaly. In contrast to these reports, however, the clinical impression at Children's Hospital Boston has been that major associated anomalies are rare. On the other hand developmental studies (see below) suggest that cognitive and motor skills may be below normal though it remains unclear whether this is related to perioperative insults which will decrease as in utero diagnosis becomes more prevalent and perioperative support methods are improved.
| Pathophysiology|| |
Following birth, the child's survival is dependent on continuing ductal patency. In addition there must be a reasonable balance between pulmonary and systemic vascular resistance. As pulmonary resistance falls in the first days and weeks of life, the child's oxygen saturation progressively increases but the child develops congestive heart failure and may acquire metabolic acidosis. On the other hand, if total pulmonary resistance is very high, for example because of a restrictive foramen ovate, which prevents free egress of blood from the left atrium to the right atrium, the child will be severely hypoxic.
Although it is possible to increase pulmonary resistance by adding carbon dioxide or nitrogen to the gas mixture inhaled by the child, if pulmonary resistance is too low, a preferable approach is to proceed with surgery.
| Diagnostic Studies|| |
The diagnosis of HLHS is being made with increasing frequency by prenatal ultrasound. In many cases the diagnosis can be made confidently by 16 to 18 weeks gestation. This has opened the possibility of prenatal intervention by balloon dilation of the stenotic or atretic aortic valve. It is important to remember however that although prenatal echo is sensitive to the diagnosis of HLHS it is not highly specific and can over diagnose the problem. We have seen a number of cases where babies required only coarctation or aortic valve intervention, and on occasion, no intervention at all despite a prenatal diagnosis of HLHS.
The diagnosis of HLHS after birth is made by echocardiography. The physical findings of a slightly cyanotic neonate in respiratory distress with a variable degree of general circulatory collapse are non-specific. Likewise, the appearance of the chest X ray of a slightly enlarged heart with congested lung fields does not help distinguish this anomaly from many others.
| Differentiation of HLHS from Critical Aortic Valve Stenosis|| |
The problem of distinguishing HLHS from critical stenosis of the neonatal aortic valve is one of the major diagnostic challenges in managing this anomaly. The Congenital Heart Surgeons has developed a calculator (available at www.chssdc.org)  , which is the most helpful tool for determining whether an individual child should be managed with a Norwood procedure or if the left heart structures are sufficiently well developed to attempt to achieve a biventricular circulation by performing a balloon aortic valvotomy.
| Current Technique of First Stage Palliation|| |
Our current technique for first stage palliation of hypoplastic left heart syndrome is based on the procedure described by Norwood et al in 1983 but now incorporates the shunt modification described by Sano  . Approach is through a median sternotomy. The thymus is partially excised to allow access to the aortic arch. It is not necessary to dissect the arch vessels at all. It is important to place a 6/0 marking suture on the right side of the tiny ascending aorta to help guide the aortotomy which will be made later.
Following heparinization. An 8 French flexible arterial cannula is inserted in the mid-ductus and an 18 French venous cannula is placed in the right atrium through the atrial appendage. Immediately after beginning bypass, a 5/0 prolene suture ligature is tied around the proximal ductus. We no longer place tourniquets around the branch pulmonary arteries or around the arch vessels as these may cause intimal injury and subsequent stenosis. The child is cooled over about 15 to 20 minutes to a rectal temperature of less than 18C.
During cooling, the proximal main pulmonary artery is divided 2 to 3mm above the tops of the commissures of the pulmonary valve. The distal divided main pulmonary artery is closed obliquely by direct suture. Although many recommend using a patch for closure of the distal pulmonary artery, we believe that long term growth of the central pulmonary arteries is improved if only autologous arterial tissue is used. This also avoids a central bulge of the pulmonary arteries which can result in tortion and kinking of the branch pulmonary artery origins.
The distal anastomosis of the Sano shunt is now constructed as shown in [Figure 1] or to a longitudinal arteriotomy, which is made between the left pulmonary artery takeoff and the main pulmonary artery closure. For neonates between 2.0 and 3.5 kg, a 5 mm stretch Gortex tube graft should be selected.
By that time, an appropriate homograft would have been selected and thawed. It should be shaped for the arch reconstruction according to the length and diameter of the arch. Bypass is then discontinued and cardioplegia is infused through a side arm on the arterial cannula. During perfusion, the head vessels and distal aorta are temporarily occluded with forceps. The arterial cannula is removed and the venous cannula is left open to drain. The ductus is divided at its junction with the descending aorta and redundant ductus tissue is excised. The resulting arteriotomy is extended at least 5 mm distally into the descending aorta.
Proximally, the arch and ascending aorta are filleted open to the level of division of the main pulmonary artery [Figure 1]. An anastomosis is fashioned between the proximal portion of the divided main pulmonary artery and the filleted aorta with a supplementary cuff of arterial wall [Figure 2]  .
Avoiding coronary artery compromise
After division of the main pulmonary artery. A 2 to 3 mm V-shaped incision is made on the proximal pulmonary artery. At least three interrupted 7/0 sutures are placed at the apex of the descending aortotomy.
The atrial septum primum must be completely excised. The septectomy is usually performed through the venous cannulation site in the right atrial appendage.
Many different shunts have been described during the 20 years that the Norwood procedure has been used ,, . Our current preference is a Sano shunt from the right ventricle to the pulmonary bifurcation using a stretch Gortex tube graft. The important advantage of the Sano shunt is that flow occurs only during systole. There is no competition between pulmonary and coronary blood flow during diastole as is the case with the Blalock shunt. This is the most likely explanation for the very much improved stability of neonates which is seen following a Sano shunt.
Avoiding a homograft cuff
Norwood's original technique did not include use of a cuff to supplement the aortic to pulmonary anastomosis. Recently the technique has been repopularized. However, the important disadvantage of this modification is that it can create a bowstring effect of the reconstructed aorta over the left main bronchus, which can result in either bronchial compression or left pulmonary artery stenosis.
Minimizing or avoiding circulatory arrest
Many ingenious technical variations have been described which permit the circulatory arrest time to be reduced or eliminated. Many of these methods involve retrograde perfusion through a Blalock shunt. However, these methods involve a risk that the surgical team will have a false sense of safety and will extend the total repair time to a dangerous degree. It is unclear how homogeneously blood is distributed. In the future, it will be important for proponents of these techniques to demonstrate that they are as safe as a limited period of deep hypothermic circulatory arrest using appropriate neurodevelopmental studies.
Proximal Sano shunt anastomosis
An oblique incision is made in the infundibulum of the right ventricle being careful to avoid injury to pulmonary artery branches and the pulmonary valve [Figure 2]. The muscular edges can be undermined if necessary. The anastomosis of the beveled Goretex tube graft is performed with continuous 5/0 Gortex suture.
If there is any doubt as to the child's stability, the sternum should not be approximated. Since we began using the Sano shunt, we have found that it is usually possible to close the chest unlike the experience with the Blalock shunt where it was frequently necessary to leave the sternum open.
| Intensive Care Management after the Stage-1 Norwood Procedure|| |
Management of the neonate following the Norwood procedure which incorporates a Sano shunt has been very much simplified relative to the management with a Blalock shunt. The child with a Blalock shunt frequently has a tendency to excessive pulmonary circulation. Maneuvers must be applied to maximize systemic blood flow relative to pulmonary blood flow. Monitoring of systemic venous saturation can be a helpful adjunct to assess the success of such maneuvers. However, when a Sano shunt has been performed, management is very much more routine. When bleeding no longer appears to be a problem as can be facilitated by routine use of aprotinin, then the child should be withdrawn from anesthesia and weaning from the ventilator can start. Generally, a spontaneous ieresis is established within 24 to 48 hours by which time extubation can be performed.
| Follow-up after Stage 1 surgery|| |
The principles of follow-up after stage 1 surgery for HLHS are identical to those applied to any child with single ventricle physiology in whom a Fontan procedure is anticipated. Attention should be directed towards optimal pulmonary artery development, maintenance of ventricular function and maintenance of low pulmonary vascular resistance, including absence of restriction at the level of the atrial septal defect. It is important to recognize that the infant is likely to outgrow a Sano shunt earlier than a Blalock shunt because flow is limited to systole. All patients should be catheterized or undergo MRI scan by 4 to 5 months of age irrespective of clinical progress. If there is a suspicion by echocardiography that there is a problem with development, either of the aortic arch or pulmonary artery, then catheterization should be performed sooner. If the catheterization demonstrates a problem such as distortion of the pulmonary arteries, then a bidirectional cavopulmonary (Glenn) shunt should be undertaken including an associated pulmonary arterioplasty. Other indications for early application of a cavopulmonary shunt have included the need for aortic arch reconstruction, the need for atrial septal defect enlargement, early outgrowth of the Sano shunt resulting in unacceptably low oxygen saturation (less than 70 to 75%) and the development of tricuspid regurgitation or right ventricular dysfunction secondary to excessive volume load on the ventricle. Cavopulmonary shunt procedures have been successfully performed in infants as early as 2.5 to 3 months.
| Fontan Procedure Following Bidirectional Glenn shunt|| |
The management of the child with hypoplastic left heart syndrome following a bidirectional Glenn is essentially generic as for the management of any patient traversing a single ventricle pathway. Cardiac catheterization is usually recommended approximately 12 months following the bidirectional Glenn shunt. A fenestrated Fontan procedure should be performed within 6 months of the catheterization procedure.
| Early Results of the Stage 1 Norwood Procedure|| |
The largest and most comprehensive outcome analysis of patients undergoing a stage-1 Norwood procedure was reported by the Congenital Heart Surgeons Society in 2003 (13). 985 neonates with either critical aortic stenosis or atresia were enrolled between 1994 and 2000. Seven hundred and ten of the 985 patients underwent a stage-1 Norwood procedure. Survival was 76% at one month, 60% at one year and 54% at 5 years. Risk factors for death included: patient specific variables such as lower birth weight, smaller ascending aorta and older age at the time of the Norwood procedure; institutional variables including institutions enrolling less than 10 neonates and also two institutions enrolling more than 40 neonates; and procedure variables, including shunt originating from the aorta, longer circulatory arrest time, and the technique of management of the ascending aorta.
Several reports suggest that the results of the Norwood procedure have improved markedly over the last 5 years. For example, Daebritz reviewed 194 patients who underwent a stage-1 Norwood procedure between 1990 and 1998 at Children's Hospital Boston  . The operative mortality decreased from 38.5% between 1990 and 1994 to 21.4% after 1994 (p=0.02). Introduction of the Sano shunt at Children's Hospital Boston in 2002 has been associated with further reduction in stage 1 mortality, which is now less than 10%.
The largest single institutional report is by Mahle and colleagues from Children's Hospital of Philadelphia  . 840 babies underwent the Norwood procedure between 1984 and 1999. The hospital mortality between 1984 and 1988 was 84% while between 1995 and 1998, hospital mortality was 29%. Bove et al  described the outcomes for 253 patients who underwent the Norwood procedure at the University of Michigan between 1990 and 1997. Hospital mortality was 24%. Mortality was strongly influenced by the presence of associated non-cardiac congenital conditions such as severe preoperative obstruction to pulmonary venous return. Survival following the second stage hemi-Fontan procedure with bidirectional Glenn was 97% and survival following the Fontan procedure was 88%.
In 2002, Twedell et al  described 115 patients who underwent the Norwood procedure between 1992 and 2001. Hospital mortality was 47% between 1992 and 1996 but between 1996 and 2001 hospital survival was 93%. Improving results have also been reported by Azakie et al  from Toronto Canada as well as Ishihino  from Birmingham UK.
| Results of the Sano Shunt|| |
In 2003, Malec et al  described 68 children following a Stage 1 procedure. The mortality for the 31 patients who had modified Blalock shunts was 35%. In patients who had the Sano shunt the mortality was 5%. Both Sano et al  and Norwood et al have also described improved results with the Sano modification relative to placement of a modified Blalock shunt.
| Developmental Outcome|| |
Wernovsky et al  reviewed 133 patients who underwent developmental assessment following the Fontan procedure. The mean full scale IQ was 95.7 +/- 17.4. A diagnosis of hypoplastic left heart syndrome as well as other complex anatomical forms of single ventricle were both associated with a worse outcome. This study must be interpreted carefully with the understanding that many of these children underwent their stage 1 Norwood procedure during the 1980s when long periods of circulatory arrest were employed. Even more importantly, the technique of circulatory arrest at that time used a very alkaline pH, severe hemodilution, rapid cooling and a relatively short period of cooling.
Although it is possible that there is an inherent association between suboptimal developmental outcome and hypoplastic left heart syndrome, a more likely explanation is that the technique of cerebral protection employed during the sequence of three operations was suboptimal in the time frame during which these patients were managed.
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[Figure 1], [Figure 2]