Univentricular Heart

Univentricular heart is a group of congenital heart diseases in which one ventricle is too small, absent or poorly connected to support a separate pulmonary or systemic circulation. The dominant ventricle receives both atrioventricular connections or most of the venous return, and the circulation is managed through staged palliation to a Fontan circulation.

Examples are hypoplastic left heart syndrome, tricuspid atresia, double inlet left ventricle, pulmonary atresia with intact septum and a hypoplastic right ventricle, unbalanced atrioventricular septal defect, and some forms of heterotaxy.

Combined, these lesions account for about 2 to 4% of congenital heart disease. Long term survival has improved, and about 85 to 90% of those who complete Fontan reach adulthood. Fontan failure, arrhythmia, liver disease and protein losing enteropathy are the main late problems.

Morphology of the dominant ventricle (left, right or indeterminate) affects long term function. A systemic right ventricle fails earlier than a systemic left ventricle.

Pathophysiology.

Systemic and pulmonary venous blood mix in a single ventricle (or one pumps both circulations through a shunt). Saturation depends on the ratio of pulmonary to systemic flow, which is set by pulmonary stenosis or atresia, or by pulmonary vascular resistance.

Overcirculation causes heart failure and pulmonary vascular disease. Underperfusion of the lungs causes cyanosis. The ventricle is under chronic volume load until staged surgery reduces it.

In the Fontan circulation, systemic venous blood flows passively to the lungs without a pumping chamber. Cardiac output depends on low pulmonary vascular resistance, good ventricular function, sinus rhythm and a normal atrioventricular valve. Central venous pressure is chronically raised (10 to 15 mmHg), which loads the liver and gut.

Candidacy for Fontan completion depends on: mean pulmonary artery pressure of 15 mmHg or less, pulmonary vascular resistance (indexed) under 3 Wood units x m2, adequate pulmonary artery size (Nakata index over 250 mm2/m2), ventricular ejection fraction over 50 to 60%, normal end diastolic pressure (under 12 mmHg), no more than mild atrioventricular valve regurgitation, and no obstruction to systemic outflow.

Neonates present with cyanosis or heart failure, and duct dependent lesions with shock at duct closure. Older children and adults have exertional intolerance, cyanosis (Glenn circulation), oedema, ascites, arrhythmia, protein losing enteropathy, plastic bronchitis, thromboembolism and liver disease.

Signs include a single second heart sound, murmurs from outflow obstruction or atrioventricular valve regurgitation, hepatomegaly, ascites and peripheral oedema in the failing Fontan.

• Foetal and postnatal echocardiography: ventricular morphology and size, atrioventricular connections and valve function, great artery relationships, outflow obstruction, arch obstruction, pulmonary flow, atrial septum.
• Saturations, arterial blood gas and lactate in neonates, with target saturation of 75 to 85% in balanced circulations.
• MRI or CT for anatomy, ventricular volumes, pulmonary arteries, and collaterals. Cardiac catheterisation before the Glenn and Fontan stages: pressures, resistance, pulmonary artery size, ventricular end diastolic pressure, collaterals and venovenous connections.
• Fontan surveillance: ECG and ambulatory ECG, echocardiography yearly, cardiopulmonary exercise test every 1 to 3 years, liver ultrasound or elastography and platelet count from age 10, and annual albumin, creatinine, full blood count, ferritin and urine protein.
• Genetic testing for syndromic associations.

Differential diagnosis.

• Biventricular lesions that can be repaired (borderline ventricle, complex transposition, unbalanced atrioventricular septal defect).
• Cardiomyopathy and pulmonary hypertension.
• In older Fontan patients, other causes of liver disease, ascites and low albumin, and constrictive pericarditis.

Neonatal stage.

• Alprostadil intravenously for duct dependent lesions: 0.05 to 0.1 microgram/kg/min to reopen the duct, then 0.01 to 0.03 microgram/kg/min. Watch for apnoea (intubate for transport or doses over 0.05 microgram/kg/min), fever, hypotension and flushing. Long courses cause gastric outlet obstruction.
• Avoid excess oxygen and hyperventilation when pulmonary flow is high; keep saturation at 75 to 85%.
• Restrictive atrial septum: balloon atrial septostomy or stenting.
• Surgical first stage by physiology: Norwood or Damus-Kaye-Stansel operation (systemic outflow obstruction), Blalock-Taussig-Thomas shunt (low pulmonary flow), or pulmonary artery banding (excess pulmonary flow). Hospital survival is about 85 to 95% for the Norwood procedure in experienced centres.

Glenn stage (3 to 6 months). Bidirectional cavopulmonary connection, then Fontan completion at 2 to 4 years.

Interstage and long term medication.

• Furosemide 1 mg/kg orally every 12 hours (maximum 6 mg/kg daily) for congestion; electrolytes and creatinine at 1 week and with changes. Enalapril 0.05 to 0.1 mg/kg once daily, increased gradually to 0.5 mg/kg daily, for ventricular dysfunction or valve regurgitation (potassium and creatinine at 1 to 2 weeks). Carvedilol 0.05 mg/kg twice daily, doubled every 2 weeks to 0.4 to 0.5 mg/kg twice daily (maximum 25 mg twice daily) when stable. Evidence for heart failure drugs in the Fontan circulation is limited.
• Aspirin 3 to 5 mg/kg orally once daily (adults 75 to 100 mg) after shunt, Glenn or Fontan. Warfarin to INR 2.0 to 3.0 is used for thrombus, atrial arrhythmia, fenestration with right-to-left shunt, or low flow, by specialist decision.
• RSV prevention in infants under 2 years: palivizumab 15 mg/kg intramuscularly monthly in the season (up to 5 doses), or nirsevimab in a single dose (50 mg under 5 kg, 100 mg for 5 kg or more) where available.
• Endocarditis prophylaxis (prosthetic material, previous endocarditis, cyanosis): amoxicillin 50 mg/kg (maximum 2 g) orally 30 to 60 minutes before dental procedures.
• Adults with atrial tachyarrhythmia: avoid class I drugs; use metoprolol succinate 25 to 100 mg daily, amiodarone 200 mg daily after loading (thyroid and liver tests every 6 months), early catheter ablation and anticoagulation to INR 2.0 to 3.0.
• Pulmonary vasodilators: sildenafil (adult dose 20 mg three times daily) or bosentan (62.5 mg twice daily for 4 weeks, then 125 mg twice daily; monthly liver tests) are used for raised pulmonary resistance in selected patients, with limited evidence and no use in pregnancy.
• Protein losing enteropathy: check albumin and stool alpha-1 antitrypsin, search for obstruction at catheterisation, give oral budesonide 9 mg daily, subcutaneous heparin by specialist direction, and a high protein, medium chain triglyceride diet.

Failing Fontan. Optimise rhythm and valve function, close venovenous collaterals, treat ascites with spironolactone 25 to 100 mg daily (potassium under 5.0 mmol/L and eGFR over 30; check at 1 and 4 weeks), and refer for heart transplantation assessment, with combined liver assessment when cirrhosis is present.

Monitoring. Saturation, weight and growth at each visit. Echocardiography every 3 to 6 months in infancy, then yearly. Cardiac catheterisation before each surgical stage. Lifelong care in an adult congenital heart disease centre.

Referral and escalation. Refer every suspected single ventricle lesion to a paediatric cardiac centre at once. Urgent review for fall in saturation, poor feeding, arrhythmia, ascites, low albumin, thrombosis or deterioration in exercise capacity.

Special groups.

• Pregnancy: possible in selected women with a Fontan circulation, with higher risk of miscarriage, preterm birth, arrhythmia and thrombosis; plan before conception, avoid ACE inhibitors and endothelin receptor antagonists, and replace warfarin with low molecular weight heparin at 6 to 12 weeks.
• Children: development and feeding support, with neurodevelopmental screening.
• Older adults: liver disease and arrhythmia dominate.
• Region specific: where staged surgery is unavailable, balanced circulations may survive into adulthood and need care of cyanosis, iron deficiency, hydration and endocarditis prevention.