Single Ventricle Palliation #2: Stage I Norwood Operation

The Stage I Norwood operation is the first major reconstructive step for neonates with hypoplastic left heart syndrome (HLHS) and related single-ventricle lesions in which the native systemic outflow tract is inadequate. The operation transforms a ductal-dependent circulation into a surgically reconstructed single-ventricle circulation, in which the systemic ventricle supports both systemic and pulmonary blood flow.

The central goals of the Norwood operation are threefold:

1. Unobstructed systemic outflow
2. Secured and controlled pulmonary blood flow
3. Nonrestrictive atrial communication

These principles are also emphasized in the original educational slide, which highlights systemic outflow reconstruction, RV–PA conduit physiology, and the persistent importance of Qp/Qs balance after Stage I palliation.

1. Surgical Concept

The Norwood operation does not restore a normal biventricular circulation. Instead, it creates a stable parallel circulation that allows survival until Stage II palliation.

In this circulation, the single ventricle ejects into a reconstructed neo-aorta. From this common ventricular output, blood is distributed to both the systemic and pulmonary vascular beds.

Therefore:

Cardiac output = Qs + Qp

This equation is the central physiological concept of the post-Norwood circulation.

• Qs: systemic blood flow
• Qp: pulmonary blood flow
• Qp/Qs balance: the relationship between pulmonary and systemic perfusion

The surgical reconstruction must therefore achieve not only anatomical patency, but also physiological balance.

2. Reconstruction of Systemic Outflow

The first major objective is to create an unobstructed systemic outflow tract.

In HLHS and related lesions, the ascending aorta and aortic arch are often hypoplastic, and systemic perfusion before surgery depends on ductal flow. During the Norwood operation, the main pulmonary artery is incorporated into the systemic outflow pathway, and the aortic arch is reconstructed to create a neo-aorta.

The reconstructed systemic outflow should provide:

• unobstructed ventricular ejection into the systemic circulation
• reliable coronary and cerebral perfusion
• elimination of ductal-dependent systemic blood flow
• relief of native arch obstruction or coarctation
• low afterload for the systemic ventricle

Residual obstruction of the reconstructed arch is poorly tolerated. Even mild narrowing may increase ventricular afterload, reduce systemic output, and impair coronary and end-organ perfusion.

3. Nonrestrictive Atrial Communication

The second essential component is creation of a nonrestrictive atrial communication.

Because pulmonary venous return enters the left atrium, it must cross the atrial septum freely to reach the systemic ventricle. A restrictive atrial septum causes pulmonary venous hypertension, impaired pulmonary drainage, pulmonary edema, low systemic output, and rapid postoperative instability.

A successful atrial septectomy should therefore allow:

• free egress of pulmonary venous return
• effective atrial-level mixing
• prevention of pulmonary venous hypertension
• stable preload to the systemic ventricle

This component is sometimes less visually dramatic than the arch reconstruction, but physiologically it is equally important.

4. Securing Pulmonary Blood Flow

After the main pulmonary artery is used to construct the neo-aorta, pulmonary blood flow must be supplied through a separate controlled source.

The two dominant strategies are:

1. Right ventricle–to–pulmonary artery shunt/conduit (RV–PA shunt, Sano modification)
2. Modified Blalock–Taussig–Thomas shunt (mBTT shunt)

Both strategies can achieve the basic surgical goal of providing pulmonary blood flow, but they differ substantially in hemodynamics, coronary perfusion, interstage risk, and reintervention profile.

5. Modified Blalock–Taussig–Thomas Shunt Physiology

The modified BTT shunt supplies pulmonary blood flow from the systemic arterial circulation, usually from the innominate or subclavian artery, to the pulmonary artery.

Its major physiological limitation is diastolic runoff.

Because the shunt connects the systemic arterial tree to the pulmonary arteries, blood can continue to flow into the pulmonary circulation during diastole. This may reduce diastolic pressure in the neo-aorta and potentially compromise coronary perfusion, particularly when pulmonary vascular resistance is low.

Potential advantages include:

• no ventriculotomy
• long surgical experience
• technically familiar construction
• avoidance of direct incision into the systemic ventricle

Potential disadvantages include:

• diastolic runoff into the pulmonary circulation
• lower systemic diastolic pressure
• possible vulnerability of coronary perfusion
• risk of excessive pulmonary blood flow
• shunt thrombosis or distortion of the pulmonary artery

In this configuration, pulmonary blood flow can become a significant “steal” from systemic output when Qp is excessive.

6. RV–PA Conduit Physiology

The RV–PA conduit supplies pulmonary blood flow directly from the systemic ventricle to the pulmonary arteries.

This modification was introduced to reduce the disadvantages of systemic-to-pulmonary arterial shunts, particularly diastolic runoff and unstable coronary perfusion. Early studies suggested that the RV–PA conduit may provide more favorable postoperative hemodynamics, including higher diastolic blood pressure and fewer ventilatory manipulations to control Qp/Qs balance [1, 2].

Potential advantages include:

• reduced diastolic runoff from the neo-aorta
• higher systemic diastolic pressure
• potentially improved coronary perfusion
• more stable early postoperative hemodynamics
• reduced interstage mortality in several analyses

Potential disadvantages include:

• ventriculotomy in the systemic ventricle
• conduit stenosis
• proximal or distal conduit obstruction
• branch pulmonary artery distortion
• higher frequency of catheter-based or surgical reinterventions

Thus, the RV–PA conduit is not simply a superior shunt in all circumstances. It provides important early hemodynamic advantages, but these advantages must be balanced against the need for more frequent reintervention and potential long-term concerns regarding ventricular function.

7. Evidence Comparing RV–PA Conduit and mBTT Shunt

Early mortality

Early observational studies showed improved or favorable hemodynamics with the RV–PA conduit, but did not uniformly demonstrate a clear survival advantage over systemic-to-pulmonary shunts [1, 2]. Later single-center comparisons also found no significant difference in surgical mortality, time to extubation, hospital length of stay, or overall mortality between RV–PA conduit and mBTT shunt strategies [3].

The multicenter randomized Single Ventricle Reconstruction trial provided the most influential comparison. In that trial, transplant-free survival at 12 months was better in patients assigned to the RV–PA shunt compared with those assigned to the mBTT shunt [4].

Interstage mortality

The interstage period after Norwood remains one of the most vulnerable phases in single-ventricle palliation. In the SVR trial analysis, interstage mortality remained substantial, and the risk was higher in patients with an mBTT shunt than in those with an RV–PA shunt, particularly when postoperative atrioventricular valve regurgitation was absent or mild [5].

This finding supports the concept that the early physiological stability of the RV–PA conduit may be clinically important during the period between Stage I and Stage II palliation.

Timing of Stage II palliation

Patients with an RV–PA conduit may require earlier Stage II palliation because conduit stenosis, somatic growth, and changing pulmonary vascular physiology can narrow the margin of safety during the interstage period [6].

This does not mean that all RV–PA conduit patients require early Glenn timing. Rather, it highlights the need for close surveillance of oxygen saturation, conduit patency, pulmonary artery growth, ventricular function, atrioventricular valve regurgitation, and overall clinical trajectory.

Reintervention burden

A consistent finding across several studies is that the RV–PA conduit is associated with more frequent catheter-based or surgical reintervention [3, 7, 8]. These interventions may involve the conduit itself, the pulmonary arteries, or other components of the reconstructed circulation.

This trade-off is central:

• RV–PA conduit: better early/interstage physiology in many studies, but more reinterventions
• mBTT shunt: avoids ventriculotomy and may require fewer conduit-related interventions, but carries the risk of diastolic runoff and less stable early physiology

8. Qp/Qs Balance After Norwood

Regardless of shunt type, the post-Norwood circulation remains a parallel circulation. The systemic ventricle must provide both Qs and Qp.

The essential clinical problem is that pulmonary blood flow is necessary but potentially dangerous if excessive.

Excessive Qp

When pulmonary blood flow is excessive, a larger proportion of ventricular output is directed to the lungs. This may produce:

• pulmonary overcirculation
• systemic hypoperfusion
• low diastolic pressure
• reduced coronary perfusion
• metabolic acidosis
• rising lactate
• ventricular volume overload

In this setting, high oxygen saturation can be misleading. A saturation that appears “good” may actually reflect excessive pulmonary blood flow at the expense of systemic perfusion.

Insufficient Qp

When pulmonary blood flow is inadequate, the patient develops:

• systemic desaturation
• low pulmonary venous return
• inadequate oxygen delivery
• risk of shunt or conduit thrombosis
• progressive hypoxemia

Therefore, the postoperative goal is not maximal oxygen saturation. The goal is balanced oxygen delivery, with adequate systemic perfusion and controlled pulmonary blood flow.

9. Postoperative Management Principles

Postoperative care after Norwood requires continuous adjustment of the relationship between pulmonary and systemic vascular resistance.

Important determinants include:

• pulmonary vascular resistance
• systemic vascular resistance
• shunt or conduit size and resistance
• systemic ventricular function
• atrioventricular valve competence
• reconstructed arch patency
• atrial-level restriction
• hematocrit and oxygen-carrying capacity
• ventilation, oxygen concentration, and carbon dioxide level

Clinical monitoring should focus on systemic perfusion rather than oxygen saturation alone.

Important parameters include:

1. Serum lactate and acid-base status

Rising lactate suggests inadequate systemic oxygen delivery.

2. Urine output and renal perfusion

Reduced urine output may indicate systemic hypoperfusion.

3. Systemic diastolic pressure

Diastolic pressure is particularly important for coronary perfusion.

4. Cerebral and somatic oxygenation

Near-infrared spectroscopy can help assess regional perfusion balance.

5. Echocardiographic assessment

Evaluation should include ventricular function, atrioventricular valve regurgitation, arch obstruction, atrial communication, and shunt or conduit patency.

10. Long-Term Perspective

The early advantage of the RV–PA conduit does not necessarily translate into a sustained long-term survival advantage.

In follow-up analyses of the SVR trial, the early benefit of the RV–PA shunt diminished over time. At 3 years, transplant-free survival was no longer significantly different between the RV–PA and mBTT groups, and RV–PA patients had more catheter interventions and slightly worse right ventricular ejection fraction before Fontan [9]. At 6 years, transplant-free survival remained similar between the two groups, although the RV–PA shunt continued to show an early pre-Stage II advantage [10].

This long-term evidence reinforces a balanced interpretation: the RV–PA conduit provides meaningful early physiological benefits, particularly during the interstage period, but it is associated with a higher reintervention burden and does not clearly confer superior long-term transplant-free survival.

11. Educational Summary

The Stage I Norwood operation is both an anatomical reconstruction and a physiological balancing procedure.

Anatomically, the operation must achieve:

• unobstructed systemic outflow
• nonrestrictive atrial communication
• reliable pulmonary blood flow

Physiologically, the operation creates a parallel circulation in which the systemic ventricle supports both Qs and Qp. The key postoperative challenge is not simply to provide pulmonary blood flow, but to provide the correct amount of pulmonary blood flow.

The RV–PA conduit and mBTT shunt represent two different solutions to the same problem. The RV–PA conduit reduces diastolic runoff and may improve early hemodynamic stability and interstage survival, but at the cost of more reinterventions. The mBTT shunt avoids ventriculotomy and has long historical experience, but may be associated with diastolic runoff, lower diastolic pressure, and greater vulnerability to pulmonary overcirculation.

Therefore, the choice of pulmonary blood flow source should be individualized according to anatomy, ventricular function, pulmonary artery size, coronary perfusion concerns, institutional experience, and anticipated interstage management.

The central principle remains constant:

After the Norwood operation, survival depends not only on surgical patency, but on maintaining a stable balance between pulmonary and systemic blood flow.

References

[1] Pizarro C, Malec E, Maher KO, Januszewska K, Gidding SS, Murdison KA, Baffa JM, Norwood WI. Right ventricle to pulmonary artery conduit improves outcome after stage I Norwood for hypoplastic left heart syndrome. Circulation. 2003;108 Suppl 1:II155-II160.

[2] Azakie A, Martinez D, Sapru A, Fineman J, Teitel D, Karl TR. Impact of right ventricle to pulmonary artery conduit on outcome of the modified Norwood procedure. Ann Thorac Surg. 2004;77(5):1727-1733.

[3] Tabbutt S, Dominguez TE, Ravishankar C, Marino BS, Gruber PJ, Wernovsky G, Gaynor JW, Nicolson SC, Spray TL. Outcomes after the stage I reconstruction comparing the right ventricular to pulmonary artery conduit with the modified Blalock Taussig shunt. Ann Thorac Surg. 2005;80(5):1582-1590.

[4] Ohye RG, Sleeper LA, Mahony L, Newburger JW, Pearson GD, Lu M, Goldberg CS, Tabbutt S, Frommelt PC, Ghanayem NS, Laussen PC, Rhodes JF, Lewis AB, Mital S, Ravishankar C, Williams IA, Dunbar-Masterson C, Atz AM, Colan SD, Minich LL, Pizarro C, Kanter KR, Jaggers J, Jacobs JP, Krawczeski CD, Pike N, McCrindle BW, Virzi L, Gaynor JW; Pediatric Heart Network Investigators. Comparison of shunt types in the Norwood procedure for single-ventricle lesions. N Engl J Med. 2010;362(21):1980-1992.

[5] Ghanayem NS, Allen KR, Tabbutt S, Atz AM, Clabby ML, Cooper DS, Eghtesady P, Frommelt PC, Gruber PJ, Hill KD, Kaltman JR, Laussen PC, Lewis AB, Lurito KJ, Minich LL, Ohye RG, Schonbeck JV, Schwartz SM, Singh RK, Goldberg CS; Pediatric Heart Network Investigators. Interstage mortality after the Norwood procedure: results of the multicenter Single Ventricle Reconstruction trial. J Thorac Cardiovasc Surg. 2012;144(4):896-906.

[6] Rüffer A, Arndt F, Potapov S, Mir TS, Weil J, Cesnjevar R. Early stage 2 palliation is crucial in patients with a right-ventricle-to-pulmonary-artery conduit after the Norwood operation. Ann Thorac Surg. 2011;91(3):873-879.

[7] Loomba RS, Shah PH, Chandrasekar S. Short-term outcome comparison of Norwood procedures with right ventricle to pulmonary artery conduit versus modified Blalock-Taussig shunt: a meta-analysis. Ann Pediatr Cardiol. 2011;4(2):145-149.

[8] Sharma V, Deo SV, Huebner M, Dearani JA, Burkhart HM. In search of the ideal pulmonary blood source for the Norwood procedure: a meta-analysis and systematic review. Ann Thorac Surg. 2014;98(1):142-150.

[9] Newburger JW, Sleeper LA, Frommelt PC, Pearson GD, Mahle WT, Chen S, Dunbar-Masterson C, Mital S, Williams IA, Ghanayem NS, Goldberg CS, Jacobs JP, Krawczeski CD, Lewis AB, Pasquali SK, Pizarro C, Gruber PJ, Atz AM, Khaikin S, Gaynor JW, Ohye RG; Pediatric Heart Network Investigators. Transplantation-free survival and interventions at 3 years in the Single Ventricle Reconstruction trial. Circulation. 2014;129(20):2013-2020.

[10] Newburger JW, Sleeper LA, Gaynor JW, Hollenbeck-Pringle D, Frommelt PC, Li JS, Mahle WT, Williams IA, Atz AM, Burns KM, Cnota JF, Dunbar-Masterson C, Ghanayem NS, Goldberg CS, Hill KD, Jacobs JP, Krawczeski CD, Lewis AB, Minich LL, Pizarro C, Schwartz SM, Tabbutt S, Ohye RG; Pediatric Heart Network Investigators. Transplant-free survival and interventions at 6 years in the SVR trial. Circulation. 2018;137(21):2246-2253.