What are the considerations and recommendations for a pediatric patient with a single ventricle heart defect undergoing the Fontan or Glenn procedure?

Fontan and Glenn Procedures: Surgical Anatomy and Clinical Overview

Understanding the Procedures

The Fontan procedure is a palliative cardiac operation that directs systemic venous blood directly to the pulmonary arteries without passing through a ventricle, fundamentally separating systemic and pulmonary circulations in series for patients with single-ventricle heart defects. 1 The Glenn procedure (bidirectional Glenn or BDG) is a direct anastomosis of the superior vena cava to a branch pulmonary artery, serving as an intermediate staging procedure before Fontan completion. 1

Anatomical Configurations

Glenn Procedure (Stage II)

• Superior vena cava is directly connected to the right pulmonary artery, creating a bidirectional cavopulmonary connection that directs blood from the head and upper extremities directly into the pulmonary circulation 1
• Performed at 3-6 months of age after pulmonary vascular resistance has adequately decreased 1, 2
• The single ventricle no longer pumps directly to the pulmonary circulation, reducing volume overload, though systemic-to-pulmonary collaterals can develop and impose additional volume load 1

Fontan Procedure (Stage III)

• Inferior vena cava blood is directed to the pulmonary arteries via either a lateral tunnel (intra-atrial baffle) or extracardiac conduit, completing total cavopulmonary connection 1, 3
• Modern techniques include:
  • Extracardiac conduit: A synthetic tube connecting the IVC directly to the pulmonary arteries outside the heart 3
  • Lateral tunnel: A Gore-Tex baffle within the right atrium directing IVC blood to the pulmonary arteries 3
  • Fenestration: A small opening between the Fontan pathway and atrium, used selectively to decompress the venous system 3, 4
• Performed at 2-5 years of age to complete the staged palliation 1, 2

Physiologic Mechanism

The Fontan circulation eliminates the subpulmonary ventricle, resulting in chronically elevated systemic venous pressure (typically 12-18 mmHg) that passively drives non-pulsatile blood flow through the pulmonary vascular bed. 1, 3, 5 This creates:

• Elevated central venous pressure as the driving force for pulmonary blood flow 3, 5
• Decreased preload to the systemic ventricle 3
• Impaired cardiac output augmentation during exercise or physiologic stress 3
• Series connection of systemic and pulmonary circulations downstream of the single ventricle 1

Indications and Patient Selection

Any child with a functional single ventricle where the rudimentary pulmonary ventricle is less than 30% of normal volume, making biventricular repair impossible, is a candidate for staged Fontan palliation. 3 Specific diagnoses include:

• Hypoplastic left heart syndrome (most common indication) 1, 6
• Tricuspid atresia 1, 6, 7
• Pulmonary atresia with intact ventricular septum 6, 7
• Other complex univentricular heart defects 6, 7

Critical Preoperative Requirements

Cardiac catheterization must be performed before both Glenn and Fontan procedures to assess hemodynamic suitability 3, 8. Essential criteria include:

• Pulmonary vascular resistance < 3 Wood units (resistance >3 Wood units significantly increases mortality risk) 3, 6, 7, 8
• Mean pulmonary artery pressure < 15 mmHg (elevated PA pressure is an independent predictor of poor outcomes) 7, 8
• Preserved ventricular function assessed by echocardiography or cardiac MRI 3
• Absence of significant atrioventricular valve regurgitation (>moderate regurgitation increases risk) 7
• No pulmonary artery distortion or obstruction 6, 7, 8

Surgical Outcomes

Glenn Procedure

• Hospital mortality: 8% in large series 6
• Actuarial survival: 92% at 1 month and beyond 6
• Risk factors for mortality: Pulmonary vascular resistance >3 Wood units and pulmonary artery distortion 6
• Provides excellent palliation in high-risk patients as staging before Fontan 6, 7

Fontan Procedure

• Ten-year survival: approximately 90% depending on preoperative risk factors 3
• Long-term prognosis: Nearly one-third of patients will either die or require transplantation within 35 years post-Fontan 3
• Many patients experience a major adverse event by 20 years following the operation 3

Long-Term Complications

Common late complications requiring specific management strategies include:

Cardiac Complications

• Atrial arrhythmias (intra-atrial reentrant tachycardia, focal atrial tachycardia) requiring prompt thromboembolic prevention and electrophysiology consultation 3, 5
• Ventricular dysfunction requiring advanced heart failure consultation when severely depressed 3, 5
• Fontan pathway obstruction (less common with modern TCPC techniques) 3

Systemic Complications

• Protein-losing enteropathy from chronic venous hypertension 9, 5, 4
• Fontan-associated liver disease (FALD) from hepatic congestion 3, 5
• Plastic bronchitis 5
• Exercise intolerance with subnormal VO2max and cardiac output responses 9, 5
• Thromboembolic complications (stroke incidence 9% in some series) 1

Thrombosis Management

For primary thrombosis prophylaxis in the first 2 years post-Fontan, both aspirin (5 mg/kg/day) and warfarin (INR 2-3) show similar efficacy, though both have suboptimal thrombosis rates. 3 Important considerations:

• Stroke can occur despite therapeutic anticoagulation (documented in patients on warfarin at therapeutic INR) 1
• Intracardiac thrombus detected by echocardiography in a subset of stroke patients 1
• Lower stroke risk (2.4/1000 patient-years) with antithrombotic treatment versus no treatment (13.4/1000 patient-years) 1

Mandatory Lifelong Surveillance

All patients after Fontan palliation require lifelong follow-up with at least yearly evaluation by a cardiologist with expertise in adult congenital heart disease (ACHD). 3 Required annual surveillance includes:

• Echocardiography or cardiac MRI to assess ventricular function, Fontan pathway, and valve function 1, 3
• ECG and Holter monitoring for arrhythmia detection 3
• Exercise testing to monitor functional capacity 3
• Biochemical and hematological testing 3
• Liver imaging to assess for Fontan-associated liver disease 3

Role of Advanced Imaging

Cardiac MRI is the preferred modality for comprehensive anatomic and functional assessment when echocardiography is inadequate due to poor acoustic windows 1. CMR provides:

• Detailed visualization of Fontan connections (lateral tunnel or extracardiac conduit) using contiguous transaxial cines 1
• Assessment of pulmonary artery anatomy and potential stenosis (peak velocity >1 m/s suggests significant stenosis) 1
• Quantification of ventricular function and volumes 1
• Detection of aortopulmonary collaterals using flow quantification 1
• Identification of thrombus in Fontan pathway or atrium 1
• Myocardial fibrosis assessment using late gadolinium enhancement in patients with impaired function 1

Critical Clinical Pitfalls

Avoid these common errors in Fontan/Glenn management:

• Do not rely solely on echocardiography for preoperative assessment—cardiac catheterization is mandatory to measure pulmonary vascular resistance and pressures 3, 8
• Do not overlook aortopulmonary collaterals after Glenn procedure, as they impose volume load and may require coil embolization 1, 8
• Do not delay advanced heart failure referral when ventricular dysfunction becomes severe—early consultation improves outcomes 3
• Endocarditis prophylaxis is required in all Fontan patients regardless of Fontan type or clinical status 3
• Physical activity should be limited to recreational sports only on a symptom-limited basis with routine exercise testing 3