Pathophysiology
Summary
Right-to-left shunts such as truncus arteriosus, transposition of the great vessels (TGV), tricuspid atresia, tetralogy of Fallot (TOF), and total anomalous pulmonary venous return (TAPVR) are characterized by early onset cyanosis. Truncus arteriosus is a condition where the failure of septation results in the absence of division between the aorta and pulmonary trunk. This condition is often accompanied by a ventricular septal defect (VSD) and is caused by failures such as neural crest cell migration. Both truncus arteriosus and TOF have a connection with 22q11 deletion syndromes, like DiGeorge, due to this failed neural crest cell migration.
Transposition of the great vessels (TGV) leads to isolated deoxygenated systemic and oxygenated pulmonary circuits. This grave anomaly renders the condition incompatible with life unless there’s a concurrent left-to-right shunt, such as VSD, ASD, or PDA. On a cardiac echo, the aorta is positioned anterior to the pulmonary artery. An additional notable feature for TGV is an ‘egg on a string’ appearance of the cardiac silhouette on CXR. Maternal diabetes is a risk factor for TGV.
Tetralogy of Fallot (TOF) amalgamates four primary anatomical abnormalities: pulmonary valve stenosis, right ventricular hypertrophy—manifesting on a chest X-ray as a "boot-shaped heart’—VSD, and an overriding aorta. The degree of the pulmonary valve stenosis acts as the yardstick determining the severity of cyanosis. A hallmark of TOF is the harsh systolic crescendo/decrescendo murmur due to the pulmonary valve stenosis. Furthermore, TOF can manifest with hypercapnic spells, colloquially known as ‘tet spells’. These spells are ameliorated by squatting, a maneuver that increases systemic vascular resistance, channeling more blood into the pulmonary circulation.
Total anomalous pulmonary venous return (TAPVR) is an anomaly where the pulmonary veins drain oxygenated blood back into the right heart, bypassing the left heart. This shunt causes dilation of the right atrium and ventricle. However, an ASD partially counteracts this by supplying some oxygenated blood to the systemic circulation.
Finally, Ebstein's anomaly, which presents in infancy as cyanosis, is characterized by the ‘atrialization’ of the right ventricle – a consequence of the tricuspid valve's downward displacement and resultant abnormal dilation of the right atrium. The malformation of the tricuspid valve leads to tricuspid regurgitation, eventually progressing to right heart failure. This pathology is ominously associated with lithium exposure in utero.
Lesson Outline
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FAQs
A right-to-left shunt directs blood from the right side of the heart to the left, bypassing pulmonary oxygenation. This leads to a mixing of oxygenated and deoxygenated blood, ultimately resulting in reduced oxygen supply to tissues. The resultant hypoxia manifests as early onset cyanosis, characterized by bluish discoloration of skin and mucous membranes.
Truncus arteriosus arises from failed septation, causing a lack of separation between the aorta and pulmonary trunk. This condition is often concomitant with ventricular septal defect (VSD), a defect resulting from an abnormal connection between the ventricles. The coexistence of these defects leads to the mixing of oxygenated and deoxygenated blood, exacerbating the pathology.
Transposition of the great vessels (TGV) results in the aorta and pulmonary arteries being switched, leading to separate deoxygenated systemic and oxygenated pulmonary circuits. This condition is fatal as it prevents the circulation of oxygenated blood throughout the body. However, the presence of a left-to-right shunt like VSD, ASD, or PDA allows some mixing of the two types of blood, rendering survival possible.
Tetralogy of Fallot (TOF) results from failed neural crest cell migration and is commonly associated with 22q11 deletion syndromes such as DiGeorge syndrome. It is characterized by four primary features: pulmonary valve stenosis, right ventricular hypertrophy, ventricular septal defect, and an overriding aorta. The severity of symptoms like cyanosis is dictated by the extent of pulmonary valve stenosis. Characteristic signs include a harsh systolic crescendo/decrescendo murmur and hypercapnic spells, which can be relieved by squatting.
Total anomalous pulmonary venous return (TAPVR) is a shunting condition where pulmonary veins drain into the right atrium instead of the left. This results in dilation of the right atrium and ventricle, and the effective bypassing of the left heart that normally supplies oxygenated blood to systemic circulation. The presence of an atrial septal defect (ASD) partially counteracts the detrimental effects by allowing some oxygenated blood into the systemic circuit.
