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 2, 3
• 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 2

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, 4
• Modern techniques include:
  • Extracardiac conduit: A synthetic tube connecting the IVC directly to the pulmonary arteries outside the heart 4
  • Lateral tunnel: A Gore-Tex baffle within the right atrium directing IVC blood to the pulmonary arteries 4
  • Fenestration: A small opening between the Fontan pathway and atrium, used selectively to decompress the venous system 4, 5
• Performed at 2-5 years of age to complete the staged palliation 2, 3

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. 6, 4, 7 This creates:

• Elevated central venous pressure as the driving force for pulmonary blood flow 4, 7
• Decreased preload to the systemic ventricle 4
• Impaired cardiac output augmentation during exercise or physiologic stress 4
• Series connection of systemic and pulmonary circulations downstream of the single ventricle 6, 2

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. 4 Specific diagnoses include:

• Hypoplastic left heart syndrome (most common indication) 2, 8
• Tricuspid atresia 2, 8, 9
• Pulmonary atresia with intact ventricular septum 8, 9
• Other complex univentricular heart defects 8, 9

Critical Preoperative Requirements

Cardiac catheterization must be performed before both Glenn and Fontan procedures to assess hemodynamic suitability 4, 10. Essential criteria include:

• Pulmonary vascular resistance < 3 Wood units (resistance >3 Wood units significantly increases mortality risk) 4, 8, 9, 10
• Mean pulmonary artery pressure < 15 mmHg (elevated PA pressure is an independent predictor of poor outcomes) 9, 10
• Preserved ventricular function assessed by echocardiography or cardiac MRI 4
• Absence of significant atrioventricular valve regurgitation (>moderate regurgitation increases risk) 9
• No pulmonary artery distortion or obstruction 8, 9, 10

Surgical Outcomes

Glenn Procedure

• Hospital mortality: 8% in large series 8
• Actuarial survival: 92% at 1 month and beyond 8
• Risk factors for mortality: Pulmonary vascular resistance >3 Wood units and pulmonary artery distortion 8
• Provides excellent palliation in high-risk patients as staging before Fontan 8, 9

Fontan Procedure

• Ten-year survival: approximately 90% depending on preoperative risk factors 4
• Long-term prognosis: Nearly one-third of patients will either die or require transplantation within 35 years post-Fontan 4
• Many patients experience a major adverse event by 20 years following the operation 4

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 4, 7
• Ventricular dysfunction requiring advanced heart failure consultation when severely depressed 4, 7
• Fontan pathway obstruction (less common with modern TCPC techniques) 4

Systemic Complications

• Protein-losing enteropathy from chronic venous hypertension 11, 7, 5
• Fontan-associated liver disease (FALD) from hepatic congestion 4, 7
• Plastic bronchitis 7
• Exercise intolerance with subnormal VO2max and cardiac output responses 11, 7
• Thromboembolic complications (stroke incidence 9% in some series) 12

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. 4 Important considerations:

• Stroke can occur despite therapeutic anticoagulation (documented in patients on warfarin at therapeutic INR) 12
• Intracardiac thrombus detected by echocardiography in a subset of stroke patients 12
• Lower stroke risk (2.4/1000 patient-years) with antithrombotic treatment versus no treatment (13.4/1000 patient-years) 12

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). 4 Required annual surveillance includes:

• Echocardiography or cardiac MRI to assess ventricular function, Fontan pathway, and valve function 6, 4
• ECG and Holter monitoring for arrhythmia detection 4
• Exercise testing to monitor functional capacity 4
• Biochemical and hematological testing 4
• Liver imaging to assess for Fontan-associated liver disease 4

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 6, 2. CMR provides:

• Detailed visualization of Fontan connections (lateral tunnel or extracardiac conduit) using contiguous transaxial cines 6
• Assessment of pulmonary artery anatomy and potential stenosis (peak velocity >1 m/s suggests significant stenosis) 6
• Quantification of ventricular function and volumes 6, 2
• Detection of aortopulmonary collaterals using flow quantification 6, 2
• Identification of thrombus in Fontan pathway or atrium 6
• Myocardial fibrosis assessment using late gadolinium enhancement in patients with impaired function 6

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 4, 10
• Do not overlook aortopulmonary collaterals after Glenn procedure, as they impose volume load and may require coil embolization 2, 10
• Do not delay advanced heart failure referral when ventricular dysfunction becomes severe—early consultation improves outcomes 4
• Endocarditis prophylaxis is required in all Fontan patients regardless of Fontan type or clinical status 4
• Physical activity should be limited to recreational sports only on a symptom-limited basis with routine exercise testing 4