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Functionally univentricular hearts and Fontan circulation 

Functionally univentricular hearts and Fontan circulation
Chapter:
Functionally univentricular hearts and Fontan circulation
Author(s):

Sara Thorne

and Sarah Bowater

DOI:
10.1093/med/9780198759959.003.0015
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The functionally univentricular heart

Introduction

Functionally univentricular hearts describes a variety of rare and complex congenital cardiac defects in which:

  • There is functionally a single ventricular cavity.

  • Biventricular repair is not anatomically or surgically achievable.

The ventricle may be of right or left ventricular morphology and, in the majority of cases, there is a second rudimentary nonfunctional ventricle.

Associated abnormalities are often present including:

  • Abnormal atrioventricular and ventriculoarterial connections.

  • Atrial isomerism.

  • Dextrocardia.

  • Outflow tract abnormalities.

Common types of functionally univentricular heart

  • Tricuspid atresia (rudimentary RV with dominant LV) (Fig. 15.1).


Fig. 15.1 Schematic representation of tricuspid atresia. Systemic venous blood leaves the RA via an atrial septal defect and mixes with pulmonary venous blood in the LA. The LV thus supports both the systemic and pulmonary circulations and the patient is cyanosed. The rudimentary RV does not play a functional role. Ao aorta; LA left atrium; LV left ventricle; PA pulmonary artery; RA right atrium; RV right ventricle.

Fig. 15.1 Schematic representation of tricuspid atresia. Systemic venous blood leaves the RA via an atrial septal defect and mixes with pulmonary venous blood in the LA. The LV thus supports both the systemic and pulmonary circulations and the patient is cyanosed. The rudimentary RV does not play a functional role. Ao aorta; LA left atrium; LV left ventricle; PA pulmonary artery; RA right atrium; RV right ventricle.

  • Double inlet LV (Fig. 15.2)—often with VA discordance ± PS.


Fig. 15.2 Schematic representation of double inlet LV with VA discordance. Both atria connect to the LV via the tricuspid and mitral valves, so that systemic and pulmonary venous blood mix in the LV and the patient is cyanosed. The LV supports both the systemic and pulmonary circulations. The aorta arises from the rudimentary RV via the VSD. If the VSD is restrictive, it creates obstruction to systemic blood flow. Ao aorta; LA left atrium; LV left ventricle; PA pulmonary artery; RA right atrium; RV right ventricle; VA ventriculoarterial; VSD ventricular septal defect.

Fig. 15.2 Schematic representation of double inlet LV with VA discordance. Both atria connect to the LV via the tricuspid and mitral valves, so that systemic and pulmonary venous blood mix in the LV and the patient is cyanosed. The LV supports both the systemic and pulmonary circulations. The aorta arises from the rudimentary RV via the VSD. If the VSD is restrictive, it creates obstruction to systemic blood flow. Ao aorta; LA left atrium; LV left ventricle; PA pulmonary artery; RA right atrium; RV right ventricle; VA ventriculoarterial; VSD ventricular septal defect.

  • Unbalanced atrioventricular septal defect—often associated with atrial isomerism.

  • Pulmonary atresia with intact septum and hypoplastic RV (see p. [link]).

  • Hypoplastic left heart syndrome (Fig. 174)

Presentation

  • May be diagnosed prenatally with fetal echo.

  • Presentation in neonates depends on the pulmonary blood flow:

    • Too little pulmonary blood flow leads to profound hypoxaemia and circulatory collapse, requiring emergency palliation.

    • Too much pulmonary blood flow and the hypoxaemia will be less severe but child may develop congestive heart failure. If left untreated, pulmonary vascular remodelling will occur with PAH developing in early life.

Principles of management

  • Physiological and anatomical considerations:

    • Correction to a biventricular circulation is not feasible.

    • All therapeutic strategies are palliative with the functionally single ventricle supporting the systemic circulation.

    • Low pulmonary vascular resistance is needed for good outcome.

    • SVC flow accounts for 70% of venous return in infant.

    • The aim of treatment is to improve cyanosis, effort tolerance, and survival.

Left untreated the natural history of univentricular hearts is very poor and few survive early childhood.

Staged approach to achieve definitive palliation

  • The end result of this approach is a Fontan-type circulation (Fig. 15.3).


Fig. 15.3 Evolution of Fontan and total cavopulmonary connection.

Fig. 15.3 Evolution of Fontan and total cavopulmonary connection.

  • Current practice is for patients to undergo a staged approach as outlined in the following sections. The number of stages will depend on the initial anatomy.

  • There are many variations of the Fontan operation and this will mainly depend on timing.

Initial stage

  • Initial management is to regulate the pulmonary blood flow:

    • If pulmonary blood flow is unrestricted, flow is restricted by banding the PA.

    • If pulmonary blood flow is too little (i.e. significant RVOTO), flow is augmented with a systemic-pulmonary shunt such as a modified Blalock-Taussig shunt between subclavian artery and pulmonary artery.

    • Rarely, the circulation is well balanced, and no early intervention is needed.

Cavopulmonary shunt (Glenn operation)

  • Systemic venous shunt.

    • SVC is disconnected from the heart and connected directly to the PAs—a cavopulmonary anastomosis.

  • This is done after 3–4 months of age when the PVR is low.

  • Today, the bidirectional Glenn is used (Fig. 15.3).

    • SVC disconnected from RA; SVC connected to RPA end-to-side anastomosis; RPA left in continuity with main PA.

  • Historically, a classical Glenn was performed—older patients may still have this shunt.

    • RPA disconnected from main PA; SVC disconnected from RA; SVC to RPA connected end-to-end.

Fontan operation

The final stage is the completion of the Fontan operation and is usually done around five years of age. It separates the pulmonary and systemic circulations and should abolish cyanosis. It is discussed in detail in the following sections.

Fontan circulation

General principles

  • A Fontan circulation uses the functionally single ventricle to support the systemic circulation.

  • The systemic venous return is directed straight into the PAs, i.e. there is no subpulmonary ventricle.

  • Flow in the pulmonary circulation is therefore:

    • Passive (not pulsatile)—relies on high systemic venous pressures to provide a head of pressure to drive flow through the pulmonary vasculature.

    • Dependent on low pulmonary vascular resistance.

Types of Fontan operation

There are two main surgical approaches:

  • Atriopulmonary connection—the original Fontan procedure; many variations, e.g. RA appendage connected directly to the PA.

    • Many older patients will have this type of Fontan circulation.

  • Total cavopulmonary connection (TCPC):

    • Both SVC and IVC are connected separately to the PAs using a Glenn for the SVC and routing the IVC with either a lateral tunnel within the RA or with an extracardiac conduit.

    • TCPC has been the procedure of choice since 71990.

    • The conduit is frequently fenestrated (i.e. small communication created between the Fontan circuit and the pulmonary venous atrium) to act as an escape valve for high systemic venous pressures. The result is a (small) R–L shunt that causes a degree of desaturation; however, the benefit is to offset high venous pressures and improve systemic cardiac output.

Post–Fontan surgery

See Figs 15.415.6.


Fig. 15.4 Double inlet LV with VA discordance, post–Fontan operation. MRI scan of a 24-year-old man with double inlet LV and ventriculoarterial discordance. This sagittal section shows the anterior aorta arising from a rudimentary anterior RV. Blood passes from the LV through a large VSD. There is mild AR (arrow). The patient has undergone a Fontan procedure (not shown on this image). Ao aorta; LV left ventricle; RV right ventricle; VSD ventricular septal defect.

Fig. 15.4 Double inlet LV with VA discordance, post–Fontan operation. MRI scan of a 24-year-old man with double inlet LV and ventriculoarterial discordance. This sagittal section shows the anterior aorta arising from a rudimentary anterior RV. Blood passes from the LV through a large VSD. There is mild AR (arrow). The patient has undergone a Fontan procedure (not shown on this image). Ao aorta; LV left ventricle; RV right ventricle; VSD ventricular septal defect.


Fig. 15.5 TGA post–Fontan operation. MRI scan (transaxial view) of a 22-year-old woman with TGA, PS, straddling MV, and inlet perimembranous and muscular VSDs. The straddling MV (straddle not seen in this view) rendered biventricular repair impossible, so she underwent palliation with an atriopulmonary Fontan operation at age six. LA left atrium; LV left ventricle; mVSD muscular ventricular septal defect; pVSD perimembranous ventricular septal defect; RA right atrium; RV right ventricle.

Fig. 15.5 TGA post–Fontan operation. MRI scan (transaxial view) of a 22-year-old woman with TGA, PS, straddling MV, and inlet perimembranous and muscular VSDs. The straddling MV (straddle not seen in this view) rendered biventricular repair impossible, so she underwent palliation with an atriopulmonary Fontan operation at age six. LA left atrium; LV left ventricle; mVSD muscular ventricular septal defect; pVSD perimembranous ventricular septal defect; RA right atrium; RV right ventricle.


Fig. 15.6 Tricuspid atresia post–Fontan operation. Transaxial section MRI scan of a 41-year-old man with tricuspid atresia who underwent a bidirectional Glenn operation aged 10, and an atriopulmonary Fontan operation aged 21 years. There is a single atrioventricular (mitral) valve that connects to the LV. The rudimentary RV lies anteriorly. The atrial septum bows to the L (arrow) as a result of elevated pressures in the hugely dilated RA. LA left atrium; LV left ventricle; VSD ventricular septal defect; RA right atrium; RV right ventricle.

Fig. 15.6 Tricuspid atresia post–Fontan operation. Transaxial section MRI scan of a 41-year-old man with tricuspid atresia who underwent a bidirectional Glenn operation aged 10, and an atriopulmonary Fontan operation aged 21 years. There is a single atrioventricular (mitral) valve that connects to the LV. The rudimentary RV lies anteriorly. The atrial septum bows to the L (arrow) as a result of elevated pressures in the hugely dilated RA. LA left atrium; LV left ventricle; VSD ventricular septal defect; RA right atrium; RV right ventricle.

Insert Fig. about hereThe physiological consequence of a Fontan-type operation is a circulation with high systemic venous pressures and passive pulmonary blood flow. This leads to:

  • RA dilatation.

  • Poor flow from atrium to PA with energy dissipation.

  • Loss of effort tolerance.

  • Atrial arrhythmias, often with life-threatening consequences.

Whilst the TCPC may mitigate against some of these complications, by bypassing the RA, all types of Fontan surgery rely on passive flow into PAs and produce a chronic low cardiac output state.

Physical examination

  • Cyanosis and clubbing may be present if fenestration or collateral vessels.

  • Pulse should be regular (check ECG).

    • Radial pulse may be absent if previous shunt.

  • JVP usually is elevated ≥2 cm due to high Fontan pressures.

  • Auscultation will not reveal murmur of underlying congenital cardiac malformation, but PSM may indicate atrioventricular valve regurgitation (AVVR).

  • Often single second sound.

  • May have parasternal heave if single RV.

  • Normal to feel liver edge.

  • Ascites or pulmonary effusion should be investigated as may be a sign of protein-losing enteropathy (PLE).

  • Chest should be clear, but restrictive lung defects are common owing to previous thoracotomies.

Investigations

  • ECG:

    • Check in SR.

    • May have axis deviation dependent on ventricular morphology.

    • May show atrial hypertrophy if atriopulmonary Fontan.

    • Intra-atrial reentry tachycardia may be mistaken for SR and requires prompt cardioversion (Functionally univentricular hearts and Fontan circulation see Chapter 16, p. [link]).

  • CXR:

    • Previous surgical procedures.

    • Kyphoscoliosis—perform lung function tests if present.

    • Indication of situs and isomerism—gas bubble, symmetry of bronchi.

  • Transthoracic echocardiography:

    • Anatomy, situs and ventricular morphology.

    • Assess ventricular function.

    • Assess degree of AVVR.

    • Ventricular outflow tract obstruction.

    • Aortic incompetence.

    • Turbulent pulmonary venous return.

  • Transoesophageal echocardiography:

    • Usually only performed as part of investigation of failing Fontan.

    • Evaluation of AVVR and Fontan pathway.

    • Exclude pulmonary venous obstruction.

  • Cardiac MRI:

    • Assess flow in Fontan pathway.

    • Assess ventricular function and anatomy.

    • Exclude pulmonary venous obstruction.

  • Metabolic exercise testing (Functionally univentricular hearts and Fontan circulation see Chapter 3, p. [link]).

    • Useful in defining the cause of exercise limitation, e.g. pulmonary or cardiac.

    • A ‘good Fontan’ generally has 770% predicted MVO2.

    • Not been evaluated in terms of prognosis in congenital heart disease.

  • Blood tests:

    • FBC—Hb may be raised and platelets low reflecting cyanosis.

    • U&Es—impaired renal function is a cause for concern.

    • LFTs are often mildly deranged due to hepatic congestion. A low albumin should raise the possibility of PLE.

  • Annual liver surveillance with USS and alpha fetoprotein to assess for evidence of cirrhosis and hepatic carcinoma.

Long-term complications following Fontan surgery

The risk of both cardiac and non-cardiac complications increase with age; they include:

  • Atrial arrhythmias.

  • SA node dysfunction.

  • Systemic AV valve regurgitation.

  • Ventricular dysfunction.

  • Fontan pathway obstruction.

  • Pulmonary venous pathway obstruction.

  • Cyanosis (due to opening up of venous collaterals from the Fontan circuit to the pulmonary veins and/or flow through the fenestration).

  • Development of subaortic stenosis.

  • Thromboembolism.

  • PLE due to high mesenteric venous pressures.

  • Hepatic dysfunction and cirrhosis.

  • Hepatocellular carcinoma.

Management of the post-Fontan patient

General management points

  • Adult patients with an AP type Fontan should be anticoagulated with warfarin lifelong. The flow in the Fontan circuit is passive, not pulsatile, and therefore spontaneous thrombus formation is possible.

    • Protein C and S deficiency is common, further increasing the risk of thrombosis.

    • Micro thrombi in the distal pulmonary arterioles will lead to elevated pulmonary vascular resistance, detrimental to the Fontan circuit.

    • Anticoagulation in a TCPC Fontan circulation is less clear, and will vary between centres.

  • Avoid dehydration:

    • Dehydration reduces the filling pressure in the Fontan circuit and reduces pre-load in the single ventricle, compromising cardiac output and systemic BP.

    • Rehydrate during intercurrent illness.

    • If nil by mouth (NBM), give IV fluids, 1 L normal saline over 12 hours.

  • GA:

    • Senior advice should be sought early on.

    • Hydrate with IV fluids when NBM.

    • All anaesthetic agents cause systemic vasodilatation.

    • Positive pressure ventilation further reduces venous return and therefore cardiac output.

    • Lack of pre-load recruitment makes low systemic vascular resistance (SVR) difficult to overcome.

    • Have metaraminol available to maintain SVR and systemic BP during anaesthesia.

  • Atrial flutter/tachycardia (Functionally univentricular hearts and Fontan circulation see also Chapter 16, p. [link], and Fig. 15.7):


Fig. 15.7 Atrial tachyarrhythmia post–Fontan operation for tricuspid atresia.

Fig. 15.7 Atrial tachyarrhythmia post–Fontan operation for tricuspid atresia.

A 24-year-old man who underwent Fontan palliation for tricuspid atresia in childhood had episodes of interatrial tachyarrhythmia. 12-lead ECG (a) shows him in sinus rhythm. He had been advised to seek urgent medical assistance if he developed palpitation. He began to feel unwell and breathless with palpitation, but delayed for two days before presenting to a local emergency department. His ECG (b) showed an IART but was misdiagnosed as sinus tachycardia and no treatment was given. The following day he had a cardiac arrest and could not be resuscitated. At-risk patients should be advised to seek help rapidly if palpitation occurs, and given copies of their ECGs to carry, along with a letter detailing their diagnosis and instructions for emergency cardioversion.

    • A common and life-threatening event, especially with advancing age.

    • Mechanism is a scar-related intra-atrial reentrant tachycardia (IART) or atypical atrial flutter.

    • Rhythm is usually regular and often not rapid (HR 90–120).

    • ECG may be mistaken for SR—compare with ECG in normal sinus rhythm (ensure patients carry a copy of their normal ECG at all times).

    • Do not attempt chemical cardioversion as may trigger rapid conduction and circulatory collapse.

    • Arrange prompt cardioversion.

    • All Fontan patients who have had an episode of atrial flutter should be discussed with an electrophysiologist with expertise in this patient group. An EP study and ablation may be appropriate to reduce the risk of further episodes—check that there is access to the heart.

    • Long-term prevention with amiodarone may be necessary (check thyroid function every three months: thyrotoxicosis is a common late side effect, which may cause permanent deterioration in functional status).

The failing Fontan

Definition

  • Without Fontan-type surgery, the majority of single-ventricle patients would not survive into adulthood.

  • A Fontan circuit is by its nature palliative.

  • The Fontan circulation is a chronically low cardiac output state.

  • Effort tolerance is limited even in ‘well’ patients.

  • Signs of ‘failure’ include:

    • Ventricular dysfunction.

    • Reduced effort tolerance.

    • Arrhythmias.

    • PLE.

Why it happens

Ventricular dysfunction

  • Increased afterload leads to hypertrophy of ventricle.

  • Limited pre-load recruitment leads to diastolic dysfunction.

  • Ventricular hypertrophy leads to systolic dysfunction.

  • Management:

    • No data to support conventional therapy, e.g. ACE inhibitors, beta-blockade, spironolactone.

    • Evidence emerging from small studies that selective pulmonary vasodilator drugs, such as sildenafil and bosentan, may improve ventricular dysfunction and functional class (see Chapter 19, Heart failure).

    • O2 therapy may help.

    • Diuretics should be used with caution due to risk of hypovolaemia.

    • Exclude underlying causes, e.g. outflow tract obstruction, AVVR, Fontan pathway obstruction, pulmonary venous obstruction, paroxysmal arrhythmias.

Reduced effort tolerance

  • Impaired ventricular function.

  • AVVR, often due to annular dilatation; leads to increased atrial pressure.

  • This further reduces the effective gradient down which blood flows and thus further reduces the cardiac output.

  • Management:

    • Ensure no respiratory contributing factors.

    • A prescribed exercise program may improve symptoms of breathlessness and improve exercise capacity.

Pulmonary vascular remodelling

  • Distal muscularization of pulmonary arterioles occurs leading to:

    • ↑ pulmonary vascular resistance.

    • ↓ flow through lungs.

    • Failure to pre-load recruit, which in turn leads to lower cardiac output.

  • Management:

    • Impossible to prove without lung biopsy (not recommended).

    • Empiric therapy with targeted pulmonary vasodilator therapy may help, but to date, evidence is available only for small, single-centre studies and case reports.

Atrial arrhythmias

  • Multiple scars, related to suture lines and bypass cannulation, provide circuits for macro-reentrant tachycardias.

  • Increasing incidence with age.

  • Associated with decline in ventricular function.

  • Management:

    • See previous discussion.

PLE

  • Particular problem for Fontan patients.

  • High mesenteric venous pressure leads to protein loss into the gut.

  • Assessed by demonstrating low serum albumin and immunoglobulins and raised faecal alpha-1 antitrypsin levels from a fresh stool sample (contact local biochemistry department for details of sampling).

  • Low albumin, total protein and immunoglobulin levels leads to effusions, ascites, dependent oedema, malnutrition, recurrent cellulitis, and septicaemia.

  • No reliably effective treatment; may benefit from:

    • Daily unfractionated SC heparin.

    • Oral budesonide.

    • Targeted pulmonary vasodilator therapy.

    • Fenestration of Fontan pathway.

  • 50% five-year mortality—seek expert help early if suspected.

Long-term outcome of Fontan surgery

  • The long-term outcome is not known.

  • Current adult patients are the pioneers of this operation.

  • Complications, as already discussed, will inevitably cause significant morbidity and mortality.

  • Fontan patients form a small proportion of the total number of patients with congenital heart disease, but account for 50% of emergency admissions.

  • Prompt management of their medical emergencies will reduce the risk of premature death.

Hypoplastic left heart syndrome

Hypoplastic left heart syndrome (HLHS) includes a range of cardiac conditions in which there is stenosis, hypoplasia, or atresia at different levels of the left heart. HLHS includes severe aortic stenosis/atresia, mitral atresia, and unbalanced AVSD. This results in a small left ventricle that is unable to support the systemic circulation and a diminutive aorta which gives rise to the coronary arteries.

HLHS accounts for 2% of all congenital cardiac lesions.

Presentation

  • Detected in utero or shortly after birth when the duct closes, leading to cardiovascular collapse.

  • Not compatible with survival unless operated.

Surgical management

In infancy, patients undergo multistage repair, termed a Norwood repair, first described in 1983.

Stage I

  • Performed in first few days of life.

  • The systemic outflow tract and aorta are reconstructed using the right ventricle and main pulmonary artery (Damus-Kaye-Stansel procedure).

  • Pulmonary blood flow is provided through a systemic-pulmonary shunt (Blalock-Taussig) or, in recent modifications of the operation, an RV–PA conduit.

    • Placement of RV–PA conduit improves systemic and coronary perfusion because there is no diastolic coronary run-off; as well, growth of the branch PAs improves due to pulsatile flow.

Stage II

  • Performed at 4–6 months.

  • Cavopulmonary/Glenn shunt and take down of systemic shunt/conduit.

Stage III

See Fig. 15.8.

Fig. 15.8 Schematic representation of hypoplastic left heart following Stage III Fontan palliation. Ao aorta; IVC inferior vena cava; LA left atrium; LV left ventricle; PA pulmonary artery; RA right atrium; RV right ventricle; SVC superior vena cava.

Fig. 15.8 Schematic representation of hypoplastic left heart following Stage III Fontan palliation. Ao aorta; IVC inferior vena cava; LA left atrium; LV left ventricle; PA pulmonary artery; RA right atrium; RV right ventricle; SVC superior vena cava.

  • Performed between 3–5 years.

  • Completion of a Fontan circulation, usually an extracardiac conduit.

Natural history

  • Around 70% of patients undergoing Norwood repair now reach adulthood.

  • Long-term complications are those of any Fontan, plus:

    • Coarctation repair site—recoarctation?

    • LPA—stenosis?

    • ASD—restrictive?

    • Coronaries from diminutive aorta—ischaemia?

    • Systemic RV—dysfunction?

    • Systemic tricuspid valve—regurgitation?