Overview
Definition:
Single ventricle physiology describes a spectrum of congenital heart defects characterized by the presence of a single functional ventricle that pumps oxygenated and deoxygenated blood into a common systemic circulation
This necessitates palliative surgical stages (Norwood, Glenn, Fontan) to separate pulmonary and systemic circulations and improve oxygenation.
Epidemiology:
Single ventricle defects represent approximately 1-2% of all congenital heart defects, affecting around 1 in 10,000 live births
Common conditions include hypoplastic left heart syndrome (HLHS), tricuspid atresia, pulmonary atresia with intact ventricular septum, and unbalanced atrioventricular canal defects
Incidence varies slightly by geographic region and diagnostic capabilities.
Clinical Significance:
Understanding single ventricle physiology is paramount for pediatricians, cardiologists, and cardiac surgeons
It forms the basis for the management of complex cyanotic heart diseases and requires lifelong follow-up
Mastery of the Norwood–Glenn–Fontan sequence is crucial for exam preparation and clinical decision-making, directly impacting patient outcomes and quality of life.
Clinical Presentation
Symptoms:
Infants typically present within days to weeks of birth with cyanosis
Progressive dyspnea and tachypnea are common
Poor feeding, failure to thrive, and lethargy are also observed
Older children may exhibit exercise intolerance, recurrent syncope, and signs of heart failure
Severe cases can present with shock.
Signs:
Cyanosis (central and peripheral) is a hallmark finding
A single, loud second heart sound (S2) is typical
A systolic ejection murmur may be present depending on the degree of pulmonary stenosis or shunt
Gallop rhythm and hepatomegaly indicate right ventricular dysfunction
Cool extremities and weak pulses suggest inadequate systemic perfusion.
Diagnostic Criteria:
Diagnosis is primarily based on echocardiography, which reveals the anatomy of the single ventricle, great arteries, and associated anomalies
Cardiac catheterization may be used to assess hemodynamics and pulmonary vascular resistance
Genetic evaluation is sometimes indicated for associated syndromes
There are no formal "diagnostic criteria" in the same sense as for acquired diseases
diagnosis is anatomical and physiological.
Diagnostic Approach
History Taking:
Detailed birth history (e.g., prolonged labor, maternal illness, prenatal imaging)
Family history of congenital heart disease
Onset and progression of cyanosis and dyspnea
Feeding history and weight gain
Episodes of syncope or near-syncope
History of prior cardiac interventions
Red flags: profound cyanosis, tachypnea, poor feeding, lethargy.
Physical Examination:
Systematic assessment focusing on color (mucous membranes, nail beds), respiratory rate and effort, heart sounds (timing, presence of murmurs), peripheral pulses (strength, symmetry), liver size, and presence of edema
Assessment of perfusion (capillary refill time).
Investigations:
Echocardiography: The primary imaging modality to define ventricular morphology, atrioventricular valve, ventriculoarterial connections, presence of collateral circulations, and pulmonary artery size
Electrocardiography (ECG): May show axis deviation or ventricular hypertrophy depending on dominance
Chest X-ray: Can show cardiomegaly and pulmonary vascularity, though often non-specific
Cardiac MRI/CT: Useful for detailed anatomical assessment, especially in complex cases or prior to surgical planning
Cardiac Catheterization: For precise hemodynamic measurements, pulmonary vascular resistance assessment, and visualization of collateral flow
Arterial blood gas: To assess the degree of hypoxemia (PaO2, SaO2).
Differential Diagnosis:
Other causes of cyanosis in neonates and infants: sepsis, severe pneumonia, persistent pulmonary hypertension of the newborn, choanal atresia, metabolic disorders, methemoglobinemia
Other forms of congenital heart disease with significant shunting or outflow tract obstruction.
Management
Initial Management:
For neonates with ductal-dependent pulmonary blood flow (e.g., HLHS), immediate prostaglandin E1 infusion is critical to maintain ductal patency
Oxygen therapy is used cautiously to avoid worsening pulmonary vasodilation and shunting
Management of acidosis and hypoperfusion with appropriate fluids and inotropes if needed
Respiratory support if indicated.
Medical Management:
Primarily supportive and aimed at optimizing hemodynamics
Diuretics may be used for fluid overload in Fontan circulation but require careful monitoring due to potential for decreased cardiac output
Propranolol or other beta-blockers might be used to manage supraventricular tachycardias
Management of pulmonary hypertension is crucial.
Surgical Management:
The Norwood–Glenn–Fontan sequence is a staged surgical palliation: 1
Norwood Procedure: Creates a new aorta from the right ventricle and a pulmonary artery band, with systemic-pulmonary shunting (usually modified Blalock-Taussig shunt)
Used in the neonatal period
2
Bidirectional Glenn Shunt: Connects the superior vena cava to the pulmonary artery, diverting caval blood directly to the lungs
Performed at 3-6 months
3
Fontan Procedure: Connects the inferior vena cava to the pulmonary artery, completing the separation of systemic and pulmonary circulations
Performed at 18-36 months
Variations include the lateral tunnel Fontan and extracardiac conduit Fontan.
Supportive Care:
Nutritional support is vital for growth and development
Immunizations are important but may require modified schedules
Careful monitoring for signs of heart failure, infection, and thromboembolic events
Psychosocial support for the child and family is essential
Lifelong cardiology follow-up is mandatory.
Complications
Early Complications:
Early complications post-Norwood: shunt thrombosis/stenosis, myocardial dysfunction, phrenic nerve injury, chylothorax, recurrent laryngeal nerve injury, stroke
Early complications post-Glenn/Fontan: pleural effusions, ascites, pulmonary hypertension, obstruction of venous pathways, bleeding.
Late Complications:
Late complications include protein-losing enteropathy (PLE), plastic bronchitis, liver dysfunction/fibrosis, venous thromboembolism, arrhythmias (atrial flutter/fibrillation), progressive ventricular dysfunction, protein-losing enteropathy, and impaired neurodevelopmental outcomes
Dilatation of systemic venous pathways and development of collateral vessels can occur.
Prevention Strategies:
Careful surgical technique, optimal pulmonary artery banding, adequate anticoagulation (especially post-Fontan), judicious fluid management, early recognition and management of pleural effusions, nutritional optimization to prevent PLE, and proactive management of arrhythmias and liver disease are key prevention strategies.
Prognosis
Factors Affecting Prognosis:
The degree of ventricular dysfunction, presence of associated anomalies (e.g., arch hypoplasia, atrioventricular valve regurgitation), pulmonary vascular resistance, surgical technique, development of complications (PLE, plastic bronchitis, liver disease), and adherence to medical follow-up significantly impact prognosis
Overall survival has improved significantly with advances in surgical techniques and perioperative care.
Outcomes:
With successful staged palliation, many children with single ventricle physiology can achieve reasonable exercise tolerance and quality of life
However, they remain at risk for long-term complications
Survival rates have dramatically improved, with >80% survival into adulthood for patients undergoing the Fontan procedure
Risk of reoperation or cardiac transplantation remains.
Follow Up:
Lifelong, multidisciplinary follow-up is essential
This includes regular cardiology assessments, echocardiography, Holter monitoring for arrhythmias, and monitoring for signs of PLE, hepatic dysfunction, and thromboembolic events
Specific protocols for anticoagulation, fluid management, and exercise prescription are guided by the Fontan physiology team.
Key Points
Exam Focus:
Understand the sequential haemodynamic changes at each stage: Norwood (single ventricle pumps to both circulations, systemic-pulmonary shunt), Glenn (SVC to PA, IVC to PA via lower body/hepatic circulation), Fontan (IVC to PA, systemic circulation from single ventricle)
Key physiology: absence of a sub-pulmonary ventricle, reliance on passive pulmonary flow, increased systemic venous pressure, potential for AV valve regurgitation.
Clinical Pearls:
In post-Fontan patients, a "well" Fontan circulation has balanced systemic venous pressure with adequate pulmonary flow
Signs of a failing Fontan include ascites, edema, hepatomegaly, protein-losing enteropathy, and pleural effusions
Monitor for Fontan-associated liver disease and neoplastic transformation
Anticoagulation strategies are crucial but complex and individualized.
Common Mistakes:
Over-oxygenation in ductal-dependent lesions
delayed prostaglandin initiation
Inadequate assessment of pulmonary vascular resistance pre-Glenn/Fontan
Misinterpretation of signs of heart failure in post-Fontan patients (may mimic liver failure)
Underestimating the risk of thromboembolism and PLE
Failure to adhere to lifelong follow-up protocols
Treating ascites solely with diuretics without addressing the underlying venous hypertension.