# Single Ventricle Physiology and Staged Palliation

## Overview

Functionally single ventricle (SV) refers to a heterogeneous group of cardiac malformations where biventricular repair is not possible. The single functional ventricle must support both systemic and pulmonary circulations. Staged surgical palliation culminates in the Fontan circulation, in which pulmonary blood flow occurs passively without a subpulmonary ventricle. The spectrum includes hypoplastic left heart syndrome (HLHS), tricuspid atresia, unbalanced atrioventricular septal defect, double-inlet left ventricle, and other complex lesions. These represent some of the most challenging conditions in congenital heart surgery.

## Lesions Comprising the Single Ventricle Spectrum

### Hypoplastic Left Heart Syndrome (HLHS)

HLHS involves underdevelopment of left-sided heart structures including the mitral valve, left ventricle, aortic valve, and ascending aorta. Subtypes include mitral atresia/aortic atresia (MA/AA), mitral stenosis/aortic atresia (MS/AA), and mitral stenosis/aortic stenosis (MS/AS). The systemic circulation is duct-dependent, requiring prostaglandin E1 (PGE1) to maintain patency of the ductus arteriosus. HLHS is the most common indication for Norwood palliation.

### Tricuspid Atresia

Tricuspid atresia involves absence of the tricuspid valve with a hypoplastic right ventricle. The systemic ventricle is the morphologic left ventricle, which confers favorable long-term function. The size of the VSD determines pulmonary blood flow, and the great arteries may be normally related or transposed.

### Double-Inlet Left Ventricle (DILV)

In DILV, both AV valves connect to a dominant left ventricle with a rudimentary right ventricle (outlet chamber). The great arteries are often transposed. Because the single ventricle is a morphologic LV, long-term function is generally favorable.

### Unbalanced Atrioventricular Septal Defect

In unbalanced AVSD, one ventricle is too small to support its respective circulation. This involves a complete AVSD with a dominant right or left ventricle and is common in heterotaxy syndromes.

<image>Anatomic spectrum of single ventricle lesions: HLHS, tricuspid atresia, double-inlet left ventricle, and unbalanced AVSD with key structural features highlighted</image>

## Physiology of Parallel Circulations

In the unrepaired state, systemic and pulmonary circulations operate in parallel rather than in series. The single ventricle ejects to both circulations simultaneously, and the Qp:Qs ratio determines oxygen saturation. When Qp:Qs equals 1, the circulation is balanced with an SpO2 of approximately 75-80%. When Qp:Qs exceeds 1, pulmonary overcirculation produces a higher SpO2 but volume-loads the ventricle and reduces systemic perfusion. When Qp:Qs falls below 1, cyanosis results from inadequate pulmonary blood flow. The goal of initial palliation is to achieve a balanced Qp:Qs of approximately 1:1.

## Stage 1: Norwood Procedure (Neonatal)

### Timing

The Norwood procedure is performed within the first week of life. PGE1 infusion is maintained until surgery to keep the ductus arteriosus open.

### Operative Steps

The operation has three components. First, a wide atrial septectomy creates an unrestrictive communication between the atria, allowing pulmonary venous return to reach the systemic ventricle. Second, neoaortic reconstruction involves transecting the main PA, anastomosing the proximal PA to the augmented native ascending aorta and arch using a homograft patch to create a large neoaorta from the native PA root, and repairing coarctation and arch hypoplasia with patch augmentation. Third, a source of controlled pulmonary blood flow is established using either a modified Blalock-Taussig-Thomas (mBTT) shunt (a 3.0-3.5 mm Gore-Tex graft from the innominate artery to the right PA) or a Sano modification (a 5-6 mm Gore-Tex RV-PA conduit). Both provide controlled pulmonary blood flow with a Qp:Qs of approximately 1:1.

### mBTT Shunt vs. Sano Conduit

The mBTT shunt produces diastolic runoff that may compromise coronary perfusion but provides more stable early hemodynamics. The Sano conduit eliminates diastolic runoff and showed higher early survival in the SVR trial but requires a ventriculotomy that may affect long-term ventricular function. The SVR trial (Single Ventricle Reconstruction) found no significant difference in transplant-free survival at 6 years, and the choice remains surgeon- and institution-dependent.

### Hybrid Approach (Alternative to Norwood)

The hybrid approach involves bilateral PA banding and ductal stenting, performed in the catheterization lab and operating room, thereby avoiding neonatal CPB. An atrial septectomy is performed percutaneously, and a comprehensive Stage 2 combining the Norwood and Glenn is performed at 4-6 months of age. Outcomes are comparable to the Norwood in some series, and this approach is used primarily at centers with high Norwood mortality.

### Norwood Outcomes

Operative mortality ranges from 10-20% (center-dependent, with experienced centers achieving less than 10%). Interstage mortality between Stage 1 and 2 is 5-15%. Interstage home monitoring programs with daily SpO2 and weight measurements have significantly reduced interstage deaths.

<image>Norwood procedure demonstrating neoaortic reconstruction with homograft patch augmentation, atrial septectomy, and comparison of mBTT shunt versus Sano RV-PA conduit</image>

## Stage 2: Superior Cavopulmonary Anastomosis (Glenn/Hemi-Fontan)

### Timing

Stage 2 is performed at 4-6 months of age, once pulmonary vascular resistance has decreased sufficiently to allow passive flow.

### Bidirectional Glenn Procedure

The bidirectional Glenn creates an end-to-side SVC-to-right PA anastomosis with takedown of the prior mBTT shunt or Sano conduit. SVC blood flows passively to both lungs, while IVC blood continues to mix with the systemic circulation, so the patient remains cyanotic with an SpO2 of approximately 80-85%. The key advantage is a significantly lower volume load on the single ventricle compared to the post-Norwood state.

### Hemi-Fontan (Alternative)

The hemi-Fontan connects the SVC to the PAs with a patch dam at the SVC-RA junction. It facilitates later lateral tunnel Fontan completion and is functionally equivalent to the bidirectional Glenn.

### Outcomes

Operative mortality is low at 1-3%, with significant improvement in hemodynamic efficiency due to reduced volume load on the single ventricle.

## Stage 3: Fontan Completion

### Timing

Fontan completion is performed at 2-4 years of age. Prerequisites include adequate PA size (Nakata index greater than 250 mm2/m2), low PVR (less than 3 Wood units), preserved ventricular function, no significant AV valve regurgitation, and normal sinus rhythm.

### Fontan Procedure Types

The lateral tunnel uses an intra-atrial baffle to direct IVC flow to the PAs through a Gore-Tex conduit within the right atrium, allowing growth potential and easy fenestration creation. The extracardiac conduit uses an 18-22 mm Gore-Tex tube graft from the IVC to the PA outside the heart, avoiding atrial suture lines (potentially producing less arrhythmia) but lacking growth potential, and is currently the most common technique. A fenestrated Fontan includes a 4 mm fenestration (punch hole) between the Fontan pathway and the atrium that acts as a pop-off valve, allowing right-to-left shunting if Fontan pressures are high. This maintains cardiac output at the cost of mild cyanosis (SpO2 85-92%), reduces early postoperative effusions, ascites, and low cardiac output, and can be closed percutaneously later.

### Fontan Physiology

In the Fontan circulation, pulmonary blood flow is entirely passive and driven by the transpulmonary gradient (CVP minus LA pressure). There is no subpulmonary ventricle to pump blood through the lungs. CVP is chronically elevated to 12-15 mmHg. Cardiac output is preload-dependent and fixed at a relatively low level, and exercise capacity is limited (typically 60-70% of normal VO2max).

### Fontan Outcomes

Operative mortality in the current era is 1-3%. Twenty-year survival is approximately 80-85%, and 30-year survival is approximately 70%.

<image>Three stages of single ventricle palliation: Norwood (neonatal), bidirectional Glenn (4-6 months), and Fontan completion (2-4 years) showing progressive separation of circulations</image>

## Staged Palliation Summary

| Stage | Procedure | Age | Goal | Operative Mortality |
|-------|-----------|-----|------|-------------------|
| 1 | Norwood (or Hybrid) | First week of life | Unobstructed systemic outflow; controlled pulmonary blood flow (Qp:Qs ~1:1) | 10-20% (< 10% at experienced centers) |
| 2 | Bidirectional Glenn (or Hemi-Fontan) | 4-6 months | SVC flow to PAs; reduce ventricular volume load | 1-3% |
| 3 | Fontan completion (lateral tunnel or extracardiac conduit) | 2-4 years | Total cavopulmonary connection; near-normal SpO2 | 1-3% |

## Long-Term Fontan Complications

### Fontan Failure

Progressive single ventricle dysfunction develops over decades, manifesting as exercise intolerance, fluid retention, and edema. Management includes diuretics, ACE inhibitors, and PDE5 inhibitors, with heart transplantation as the ultimate treatment.

### Protein-Losing Enteropathy (PLE)

PLE results from loss of serum proteins into the GI tract due to chronically elevated venous pressure. It affects 3-15% of Fontan patients and presents with edema, ascites, diarrhea, and hypoalbuminemia. Treatment is difficult and includes heparin, steroids, sildenafil, and fenestration creation. Mortality reaches up to 50% at 5 years, and transplantation may be the only definitive treatment.

### Plastic Bronchitis

Plastic bronchitis involves cast formation in the airways from lymphatic congestion, presenting with chronic cough, dyspnea, and respiratory distress. Emerging treatments include lymphatic imaging and intervention such as thoracic duct embolization and lymphovenous anastomosis.

### Fontan-Associated Liver Disease (FALD)

FALD is a universal finding in long-term Fontan patients. Chronic hepatic venous congestion leads to fibrosis, cirrhosis, and hepatocellular carcinoma. Surveillance includes liver ultrasound, AFP, liver function tests, and MRI for characterization. HCC screening is recommended after 10 years of Fontan circulation.

### Arrhythmias

Intra-atrial reentrant tachycardia (IART) is the most common arrhythmia, along with atrial fibrillation and sinus node dysfunction. These are poorly tolerated because loss of AV synchrony significantly reduces cardiac output. Management includes antiarrhythmics, catheter ablation, and Fontan conversion with arrhythmia surgery.

### Thromboembolic Events

Elevated Fontan pathway pressures and sluggish flow predispose to thrombosis. There is no consensus on the optimal anticoagulation strategy, with both warfarin and aspirin being used. Stroke and pulmonary embolism are significant risks.

### Fontan Conversion

For patients with failing atriopulmonary (older-type) Fontan circulation with arrhythmia, conversion to an extracardiac Fontan with arrhythmia surgery (Maze-type atrial procedure) and pacemaker placement is performed. This is a high-risk operation but can improve symptoms and arrhythmia control.

## Clinical Pearls

HLHS is a duct-dependent lesion, and any neonate with ductal closure presents in cardiogenic shock -- PGE1 is life-saving. The Norwood procedure is one of the highest-risk operations in all of surgery, with outcomes directly tied to center and surgeon volume. Interstage home monitoring with daily SpO2 and weight measurements has been one of the most impactful quality improvement initiatives in congenital heart surgery. Fontan physiology is a "ticking clock," requiring lifelong surveillance for liver disease, protein-losing enteropathy, arrhythmia, and ventricular dysfunction. Every Fontan patient should be screened for liver disease after 10 years, as hepatocellular carcinoma is a real risk. Fenestration at the time of Fontan reduces early morbidity and should be strongly considered in higher-risk patients. The growing population of adults with Fontan circulation requires specialized multidisciplinary care at adult congenital heart disease centers.

## References
- Ohye RG et al. "Comparison of shunt types in the Norwood procedure for single-ventricle lesions (SVR trial)." *N Engl J Med*. 2010.
- d'Udekem Y et al. "The Fontan procedure: contemporary techniques have improved long-term outcomes." *Circulation*. 2007.
- Rychik J et al. "Evaluation and Management of the Child and Adult With Fontan Circulation: AHA Scientific Statement." *Circulation*. 2019.
- Goldberg DJ et al. "Fontan-associated liver disease." *J Am Coll Cardiol*. 2017.
- Baumgartner H et al. "2020 ESC Guidelines for the management of adult congenital heart disease." *Eur Heart J*. 2021.