Norwood → Bidirectional Glenn → Fontan — staged surgical planning for hypoplastic left heart syndrome and other single-ventricle lesions
In hypoplastic left heart syndrome (HLHS) and related lesions, one ventricle is too underdeveloped to support a circulation on its own. A normal heart uses two pumps in series — the right ventricle drives blood through the lungs, the left ventricle drives it through the body. When only one functional pumping chamber exists, it cannot do both jobs simultaneously without help. The entire three-stage palliation pathway exists to solve exactly this problem: reroute the plumbing so a single ventricle can permanently, sustainably support the body while blood reaches the lungs passively, without being pumped a second time.
A normal circulation is two pumps in series: the right ventricle sends deoxygenated blood through the pulmonary circuit at low pressure; the left ventricle sends oxygenated blood through the systemic circuit at high pressure. Total cardiac output is delivered twice — once to the lungs, once to the body.
In HLHS, the left-sided structures (mitral valve, left ventricle, aortic valve, ascending aorta) are severely underdeveloped or atretic. The morphologic right ventricle becomes the only functional chamber. If it were forced to pump through both circuits in series the way a normal heart does, it would fail immediately — it is not built for the sustained high-pressure systemic afterload alone, let alone both circuits.
Instead, in the newborn period the single ventricle drives a parallel, mixed circulation: pulmonary and systemic venous blood mix in the heart, and output splits between the pulmonary and systemic beds in proportion to their relative resistances (the Qp:Qs ratio). This is survivable only transiently, and only with the ductus arteriosus open.
In HLHS, systemic blood flow to the body depends entirely on right-to-left flow across the patent ductus arteriosus (PDA) — the fetal vessel that normally closes within the first days of life. Without it, systemic perfusion collapses.
Pre-operative management centers on:
• Prostaglandin E1 (PGE1) infusion — keeps the ductus open, preserving the only pathway of systemic blood flow • Balancing Qp:Qs — avoiding pulmonary overcirculation (which steals flow from the body and causes shock) via controlled hypoventilation, avoiding supplemental oxygen (a pulmonary vasodilator), and sometimes subambient FiO2 • Monitoring for restrictive atrial septum — if the atrial communication is inadequate, pulmonary venous blood cannot decompress into the systemic ventricle, requiring emergency intervention
This ductal-dependent, mixed-circulation state is inherently unstable and cannot be sustained for more than days to weeks — surgical palliation is not elective, it is the only path to survival.
Cardiac transplantation is limited by donor organ scarcity and is generally reserved for failed palliation or specific anatomic contraindications. For most infants with a single functional ventricle, the pathway forward is staged surgical palliation built on the Fontan principle: a single ventricle can indefinitely support the systemic circulation alone, provided venous blood is allowed to flow passively into the pulmonary arteries without being pumped by a subpulmonary ventricle.
The three operations — Norwood, Glenn, Fontan — are not three attempts at the same fix. Each stage incrementally removes the single ventricle from the job of pushing blood to the lungs, while ensuring it never has to support an unregulated flood of pulmonary flow. By the time the pathway is complete, the ventricle does exactly one job: push blood to the body.
The Norwood operation, performed in the first days to weeks of life, converts the fragile, ductal-dependent mixed circulation into a stable, surgically-controlled circulation. It builds a new, adequately-sized aorta out of the native diminutive aorta and the proximal pulmonary artery, ensures unobstructed mixing at the atrial level, and establishes a fixed, controlled — rather than ductal, unpredictable — source of pulmonary blood flow.
The diminutive native ascending aorta is incorporated into a newly reconstructed "neoaorta" built primarily from the proximal main pulmonary artery, which is transected and anastomosed to the aortic arch. A patch (homograft or synthetic) augments the arch to relieve any coarctation and ensure an unobstructed systemic outflow tract capable of carrying the entire single-ventricle cardiac output. The native small ascending aorta, no longer a primary outflow vessel, is preserved in continuity to perfuse the coronary arteries retrograde.
An adequate, unrestrictive communication between the left and right atria is created (or enlarged) so that pulmonary venous return can freely reach the single ventricle regardless of anatomic side. Any restriction here would cause pulmonary venous hypertension and pulmonary edema, undermining the entire operation.
With the ductus arteriosus ligated, a new, fixed-caliber pathway to the lungs is required:
• Modified Blalock-Taussig-Thomas shunt — a small synthetic tube graft from a systemic artery (subclavian/innominate) to the pulmonary artery; flow is continuous, diastolic run-off from the systemic circulation into the tube • Sano (right ventricle-to-pulmonary artery) conduit — a valveless conduit from the single ventricle directly to the pulmonary artery; avoids the diastolic steal of a BT shunt, at the cost of a ventriculotomy
Both create a fixed, surgeon-controlled Qp:Qs — far more stable than the unpredictable ductal-dependent flow of the pre-operative state, but still a source of vulnerability during the interstage period that follows.
Around 4–6 months of age, once pulmonary vascular resistance has fallen to near-adult levels, the superior vena cava is disconnected from the heart and anastomosed directly to the pulmonary artery. Upper-body venous blood now flows to the lungs passively, without any ventricular contribution — the first concrete step toward the Fontan principle, and a major reduction in the volume the single ventricle must handle.
The superior vena cava is transected near its junction with the right atrium and anastomosed end-to-side to the pulmonary artery. The prior Norwood shunt or Sano conduit is taken down, eliminating the fixed parallel pulmonary pathway entirely. Blood returning from the head and arms now bypasses the heart altogether and flows directly into the pulmonary arteries.
Passive, ventricle-free flow into the pulmonary arteries is only viable once pulmonary vascular resistance (PVR) has dropped from the elevated fetal/neonatal levels toward its normal, low postnatal baseline — typically by 2–6 months of age. At that point, the modest residual pressure in the systemic veins is enough to drive blood through the low-resistance pulmonary bed without a pump. Performed too early, PVR is still too high and flow would stagnate; performed too late, the single ventricle spends longer than necessary carrying the full volume load unassisted.
Before the Glenn, the single ventricle receives and ejects the combined systemic and pulmonary venous return — a substantial chronic volume overload. Removing the SVC contribution (roughly a third to two-fifths of total venous return) meaningfully unloads the ventricle, often improving ventricular geometry, function, and growth trajectory heading into childhood. The inferior vena cava, carrying the larger share of venous return, still empties into the ventricle at this stage — some cyanosis (lower oxygen saturation) persists until the Fontan is completed.
Around 2–4 years of age, the inferior vena cava is connected to the pulmonary arteries as well — via an extracardiac conduit or an intra-atrial lateral tunnel — completing the total cavopulmonary connection (TCPC). From this point forward, all systemic venous blood flows passively into the lungs, and the single ventricle is dedicated exclusively to systemic circulation, exactly as the Fontan principle predicts.
Two accepted techniques route inferior vena caval blood to the pulmonary arteries without passing through the heart:
• Extracardiac conduit — a synthetic tube graft is sewn from the divided IVC directly to the underside of the pulmonary artery, entirely outside the heart. This is now the more commonly used technique, avoiding atrial suture lines that predispose to arrhythmia.
• Lateral tunnel — a baffle is constructed inside the right atrium, channeling IVC flow along the atrial wall to the SVC-PA (Glenn) connection, using part of the native atrium itself as part of the pathway.
Both achieve the same end state: complete separation of systemic venous return from the single ventricle.
A small surgically-created hole (fenestration) between the Fontan pathway and the pulmonary venous atrium is sometimes left in place. It allows a small right-to-left shunt: some venous blood bypasses the lungs, preserving cardiac output (and reducing venous congestion) at the cost of mild desaturation. This trades a small saturation penalty for hemodynamic stability in the early post-operative period, particularly when pulmonary artery pressures or ventricular function are borderline; many fenestrations close spontaneously or are closed later by catheter.
With both cavae now connected directly to the pulmonary arteries, the Fontan circulation is complete: the single ventricle receives only oxygenated pulmonary venous blood and ejects it exclusively into the systemic circulation. There is no subpulmonary ventricle — pulmonary blood flow is entirely passive, driven by the residual pressure gradient between the systemic veins and the pulmonary venous atrium. Systemic arterial oxygen saturation normalizes into the low-to-mid 90s (%), and the chronic ventricular volume overload of the earlier stages is fully resolved.
Two distinct surveillance challenges bookend this pathway: the acutely vulnerable interstage windows between operations — especially between Norwood and Glenn — where seemingly well infants can decompensate rapidly, and the decades-long follow-up Fontan survivors require, because a circulation with no pump behind the lungs produces a predictable, progressive set of complications over time.
The interval between Norwood discharge and the Bidirectional Glenn carries the highest mortality risk of the entire pathway, because the fixed shunt/conduit created at Norwood offers no physiologic buffering — small changes in resistance or hydration can rapidly unbalance the circulation in an infant who otherwise appears well.
Structured home monitoring programs address this by having caregivers track, daily:
• Oxygen saturation (pulse oximetry) — a falling trend can signal shunt narrowing or thrombosis • Weight gain — poor weight gain is often the earliest sign of inadequate systemic output • Feeding tolerance and activity level
Red-flag values trigger immediate contact with the cardiology team, allowing intervention before frank decompensation. These programs have been associated with substantial reductions in interstage mortality since their widespread adoption.
Because the Fontan circulation permanently elevates systemic venous pressure while lacking a subpulmonary pump, a recognizable set of complications accumulates over years to decades:
• Fontan-associated liver disease (FALD) — chronic venous congestion drives progressive hepatic fibrosis, and eventually cirrhosis, in nearly all long-term survivors; surveillance includes periodic liver imaging/elastography • Protein-losing enteropathy (PLE) — elevated central venous pressure and lymphatic dysfunction cause the intestine to leak protein-rich lymph, producing edema, ascites, and immune dysfunction • Plastic bronchitis — lymphatic fluid casts form in the airways, a rare but serious complication of the same lymphatic overflow mechanism • Arrhythmia — atrial suture lines (particularly with lateral tunnel Fontans) and chronic atrial stretch predispose to atrial arrhythmias over time • Thromboembolism — sluggish, non-pulsatile venous flow through the Fontan pathway increases clot risk, often warranting long-term antithrombotic therapy • Exercise intolerance — cardiac output cannot augment normally during exertion because pulmonary flow cannot be actively increased by a ventricle
Because these complications emerge gradually and can be clinically silent early on, Fontan survivors require structured, lifelong surveillance rather than episodic care: periodic echocardiography, cardiac catheterization, liver imaging, and exercise testing at intervals tailored to age and clinical status. As patients age out of pediatric cardiology, transition to dedicated Adult Congenital Heart Disease (ACHD) programs is essential — the physiology, and the complications it produces, do not resemble those of a structurally normal heart, and general adult cardiology is not equipped to manage them alone.