Cyanotic congenital heart disease: tetralogy of Fallot and TGA

Overview

Congenital heart disease (CHD) affects about 8 per 1000 births. Lesions are first sorted by whether the child is cyanosed (on examination or pulse oximetry), then by the lung blood flow on chest X-ray, and then by the ECG axis and hypertrophy pattern. Echocardiography confirms the diagnosis.

Cyanosis in CHD comes from deoxygenated blood reaching the aorta, either through a right-to-left shunt or because the circulations run in parallel. Two lesions dominate PGET questions: tetralogy of Fallot (TOF) and transposition of the great arteries (TGA).

Classification by anatomical features

Approximate share of all CHD (Akbar)

Causes and associations

• Most CHD has no single known cause; it is thought to be multifactorial, with the insult in the first weeks of gestation.
• Maternal factors: rubella and cytomegalovirus, diabetes, drugs (anticonvulsants, folate antagonists, oestrogens), lithium (Ebstein anomaly).
• Syndromes: Down syndrome (AV canal defect), Turner syndrome (coarctation, bicuspid aortic valve), DiGeorge syndrome (aortic arch anomalies), Marfan syndrome.
• TGA is commoner in infants of diabetic mothers and in boys (3:1).

Tetralogy of Fallot

Components

1. Pulmonary stenosis (right ventricular outflow obstruction), mostly infundibular (50-70%), often with a valve element too.
2. Ventricular septal defect (large).
3. Aorta overriding the septum.
4. Right ventricular hypertrophy, which follows from the outflow obstruction.

An ASD is also present in about 15%, and the aortic arch is right-sided in about 25%.

Pathophysiology

• Blood leaving the right ventricle meets the narrowed outflow and takes the easier route across the VSD into the aorta.
• The shunt grows when systemic vascular resistance falls (for example with exertion) or when the infundibulum goes into spasm.
• The VSD is large, so the two ventricles have equal pressure and the right ventricle is not overloaded with volume. Heart failure does not occur in uncomplicated TOF.

Clinical features

• Many babies are not blue at birth. Cyanosis deepens over the first months as the infundibular narrowing progresses.
• Poor growth, clubbing and breathlessness on effort.
• Squatting: older children squat after exertion. This kinks the leg arteries, raises systemic vascular resistance and reduces venous return, so less blood crosses right to left.
• Examination: left parasternal heave; heart size usually normal; a systolic thrill in about half; single second sound (soft or absent P2); a harsh ejection systolic murmur at the left sternal edge (third space). The VSD itself makes no murmur.
• The murmur comes from flow through the stenosis, so the more severe the stenosis, the shorter and quieter the murmur.

Hypercyanotic (tet) spells

• Mainly in the first 2 years of life.
• Typically on waking in the morning or after a bout of crying.
• The infant becomes restless and breathes fast and deep, cyanosis deepens, breathing becomes gasping, and the child may faint. Death can occur.
• During the spell the murmur softens or vanishes, because little blood now passes through the outflow tract.

Management of a spell

Also correct dehydration and anaemia. Any spell is a reason to bring surgery forward.

Investigations

General management

• Keep the child well hydrated (dehydration thickens the blood and risks thrombosis).
• Keep warm to limit oxygen use; avoid hypoglycaemia.
• Iron supplements for relative iron deficiency, so haemoglobin stays in a useful range.
• Partial exchange transfusion (or phlebotomy) only for severe polycythaemia.
• Good dental hygiene and endocarditis prophylaxis.
• Adequate calories to support growth; psychological support for the family.

Surgery

After repair, right bundle branch block and ventricular ectopics are common; late sudden death from arrhythmia can occur.

Complications

Prognosis

• Severe forms are deeply blue at birth and may die in a spell.
• Many who survive the first year seem to settle as collateral vessels develop, but few reach their twenties without surgery.
• Repair before school age gives good long-term survival; arrhythmias remain the main late risk.

Transposition of the great arteries (TGA)

Anatomy and parallel circulation

• The right ventricle gives off the aorta and the left ventricle gives off the pulmonary artery.
• Blue blood from the body returns to the right heart and goes straight back to the body through the aorta. Red blood from the lungs returns to the left heart and goes straight back to the lungs.
• The two circuits run in parallel instead of in series.
• Survival depends on mixing through a patent foramen ovale, PDA or VSD: the lesion is shunt dependent.
• Associated lesions: VSD, PDA, pulmonary stenosis.

Clinical features

TGA with intact ventricular septum (commonest serious form):

• Fetal oxygenation is near normal, so the baby looks well at birth and is often large (some about 4 kg).
• Within hours to the first days, as the duct closes, mixing through the foramen ovale alone is not enough: deep cyanosis and fast breathing.
• Often little or no murmur.
• Heart failure is not the main problem; hypoxaemia is.

TGA with VSD: more mixing, so cyanosis is milder (noticed within the first month) but heart failure is common; the murmur resembles a large VSD.

Investigations

Management

Medical (emergency):

• Start prostaglandin E1 infusion immediately (0.05-0.20 microgram/kg/minute) to keep the duct open.
• Correct acidosis, hypoglycaemia and hypothermia.
• Treat heart failure if a VSD is present.

Procedures:

• Balloon atrial septostomy (Rashkind procedure): a balloon catheter tears the atrial septum at the foramen ovale to improve mixing; the first palliative step.
• Arterial switch operation: the treatment of choice for TGA with intact septum; should be offered by about 2 weeks of age when the anatomy is suitable.
• Atrial switch (Mustard procedure): an older approach using an atrial baffle to redirect venous blood.

Prognosis

• Untreated: about 30% die in the first week, 50% in the first month and over 90% in the first year.
• After an uncomplicated arterial switch, survival is about 90-95%.

Other cyanotic lesions in brief

Tricuspid atresia

• No tricuspid valve, so the right ventricle is small. All venous blood must cross the atrial septum to the left atrium; a VSD or PDA is needed for lung blood flow.
• Marked cyanosis from birth, single S2, often no significant murmur.
• ECG: left axis deviation (superior axis), large right atrium, LVH.
• X-ray: normal or slightly large heart with reduced lung markings.
• Management: PGE1 if lung flow is duct dependent, a Blalock-Taussig shunt, then bidirectional Glenn and Fontan operations.

Truncus arteriosus

• One great artery leaves the heart above a large VSD and gives rise to the aorta, coronary and pulmonary arteries.
• Usually mild cyanosis with early heart failure as lung resistance falls; bounding pulses.
• Single S2, ejection click, systolic murmur at the left sternal edge.
• ECG: biventricular hypertrophy. X-ray: cardiomegaly with increased lung markings.
• Surgery before 6 months (VSD closure and a conduit from the right ventricle to the pulmonary arteries) to prevent pulmonary vascular disease.

Total anomalous pulmonary venous return (TAPVR)

• All pulmonary veins drain to the right side of the heart (supracardiac, cardiac, infracardiac or mixed). An atrial communication is essential.
• Unobstructed: heavy lung flow, cardiomegaly, heart failure, mild cyanosis; widely split S2.
• Obstructed (the veins drain into a systemic vein, either above or below the diaphragm): small heart, severe cyanosis and pulmonary venous congestion; needs immediate surgery.
• X-ray (supracardiac): "snowman" or "figure of 8" heart from dilated left and right innominate veins and the right SVC.
• ECG: right atrial enlargement and RVH.

Ebstein anomaly

• Tricuspid valve displaced down into the right ventricle; tricuspid regurgitation; linked to maternal lithium.
• Associated with WPW syndrome and SVT.
• X-ray: massive "wall-to-wall" (box-shaped) heart with reduced lung markings. ECG: right atrial enlargement, RBBB.

Quick comparison

Quick revision

• Cyanotic with low lung flow: TOF, tricuspid atresia; with high lung flow: TGA, truncus, TAPVR.
• TOF: PS + VSD + overriding aorta + RVH; PS severity sets cyanosis; no heart failure.
• TOF signs: squatting, clubbing, single S2, ejection murmur at left sternal edge, boot-shaped heart, RAD.
• Tet spell: knee-chest, oxygen, morphine 0.2 mg/kg SC, bicarbonate, IV propranolol; oral propranolol to prevent.
• TOF complications: stroke (<2 years), brain abscess (>2 years), endocarditis, polycythaemia.
• TGA: aorta from RV; parallel circuits; PGE1, balloon septostomy, arterial switch by 2 weeks.
• Tricuspid atresia: left axis deviation; TAPVR: snowman; Ebstein: wall-to-wall heart, lithium.

Written by The House Job from standard references (Basis of Paediatrics by Pervez Akbar; Nelson Essentials of Paediatrics).

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