Transposition of the great vessels pathophysiology
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- In the fifth week of gestation, opposing pairs of ridges form in the truncus arteriosus.
- These ridges are termed the right superior truncus swelling and the left inferior truncus swelling.
- The right superior truncus swelling grows distally and to the left while the left inferior truncus swelling grows distally and to the right.
- The result is twisting of the swellings around each other and the foreshadowing of the anatomically normal spiral septum.
- Simultaneously, swellings in the dorsal and ventral walls of the conus cordis appear and grow toward each other and distally.
- Eventually, these swellings fuse with each other, as well as the truncus septum, thus dividing the conus cordis into anterolateral (right ventricular outflow tract) and posteromedial (left ventricular outflow tract) portions.
- Equally important to septal formation, is the migration of neural crest cells through pharyngeal arches three, four, and six, and to the heart.
- There, they contribute to endocardial cushion formation in the truncus arteriosus and conus cordis, as well as lengthening of the outflow tracts.
- Any insult to the migration of neural crest cells can cause tetralogy of Fallot, truncus arteriosus, and TGA.
- Right atrium (RA) is connected to a morphologic right ventricle (RV).
- In a normal heart, the great arteries (aorta and pulmonary arteries) are concordant with the morphologic LV and RV.
- This is termed ventriculo-arterial concordance.
- In addition, the aorta and pulmonary trunk ascend in a spiral relationship.
Transposition of the great vessels
- In the TGA the aorta arises from the morphologic right ventricle via a subaortic infundibulum and the pulmonary artery arises from the morphologic left ventricle, without a subpulmonary infundibulum.
- These ventriculoarterial connection is known as ventriculoarterial discordance.
- As a consequence, there is a a fibrous continuity between the mitral and pulmonary valve, but no continuity between the tricuspid and aortic valve.
- The abnormal origin of the great arteries results in an altered spiral relationship.
- Therefore, the aorta and pulmonary artery run parallel to each other
- In normal heart thus the circulation is in series.
- However, in transposition of the great vessels circulation is in parallel
|dextro-Transposition of the great vessels||Referred to as complete or uncorrected transposition of the great arteries identifying the single discordance between ventricles and great arteries|
|levo-Transposition of the great vessels||Referred to as congenitally corrected transposition, identifying a double discordance (atrioventicular and ventriculo arterial)|
- The fetus circulation in-utero is different compared to the extra-uterine circulation.
- The fetus tolerates a D-TGA well in-utero due to this difference in circulation.
- The high resistance in the pulmonary circulation compared to the placenta, allows the blood to flow to the descending aorta rather than to the lung.
- Due to this the fetus gets blood with a higher oxygen tension.
- Fetal Ciculation→Oxygen-rich blood from placenta(drains to)→Umbilical vein (drains to)→Right atrium (drains to) →Fossa ovalis(drains to)→Left ventricle (drains to)→The pulmonary artery(drains to)→Ductus arteriosus(drains to)→Descending aorta.Pathophysiology in Dextro-TGA in extra-uterine life-
- In normal cardiac anatomy, the aorta is positioned posterior and to the right of the main pulmonary artery.
- Aorta being positioned anterior and slightly rightward of the pulmonary artery.
- These changes cause the aorta to arise from the right ventricle and the pulmonary artery from the left ventricle (ventriculoarterial discordance).
- In Uncorrected D-TGA the systemic and pulmonary circulations are parallel circuits which means that the deoxygenated systemic venous blood comes to the right ventricle and inplace of going to the lungs, drains back to the systemic circulation via the aorta.
- Similarly, oxygenated pulmonary venous blood is recirculated to the lungs via the pulmonary artery.
- This parallel circulation is incompatible to life.
- For a child with dextro-TGA to survive, a communication between the two parallel circuits is necessary.
- Various connections that allow mixing in these patients are: patent foramen ovale, ventricular septal defect, atrial septal defect,patent ductus arteriosus or the bronchopulmonary collateral circulation.
- Ventricular septal defect (VSD) occurs (in about 50%) of patients with D-TGA. Patients with a VSD may have other cardiac anomalies like pulmonary stenosis or atresia, overriding of atrioventricular valve, and coarctation of aorta.
- Left ventricular outflow tract obstruction is common in D-TGA and is present in up to 25 percent of patients.
Below is an image depicting the abnormal flow in the large vessels of the heart.
- Levo-TGA (L-TGA) is a lesser known form of TGA.
- The left ventricle is positioned to the right of the right ventricle (opposite sides of the heart).
- The pulmonary trunk and aorta arise in their anatomically correct orientations, however, since the ventricles are reversed, the aorta is fused with the right ventricle, and the pulmonary trunk is combined with the left ventricle.
- The resultant flow of blood in a patient with L-TGA is as follows:
- Deoxygenated blood enters the anatomically correct right atrium, passes through the mitral valve into the left ventricle, and is pumped into the pulmonary trunk to the lungs.
- From the lungs, the oxygenated blood enters the left atrium, passes through the tricuspid valve, and into the right ventricle where blood is then pumped into the aorta.
- Since the flow of blood in patients with L-TGA passes through the normal systemic and pulmonary circuits, L-TGA is sometimes termed anatomically correct TGA.
Conditions associated with TGA include:
- Ventricular septal defect
- Pulmonary stenosis
- Left atrioventricular valve regurgitation (tricuspid or systemic)
- Complete heart block