**2. Epidemiology**

TGA is the second most common congenital cyanotic heart defect (CCHD) and the commonest one occurring during the 1st week of life [1]. According to an estimation, about 1153 infants are born with TGA annually in the USA. The prevalence is assessed to be 2.3–4.7 per 10,000 live births [2, 3]. TGA accounts for approximately 3 percent of all congenital heart disease (CHD) disorders and almost 20 percent of all cyanotic CHD defects [2]. Overall, the incidence of DTGA is 5–10% of all CHDs, whereas that of LTGA is <1%, and 0.02 to 0.07 per 1000 live births. About 90% of the cases present as an isolated defect and the disorder is rarely associated with extracardiac anomalies. The occurrence of non-cardiac congenital lesions in TGA at <10% is significantly lower than those in other CHDs [4]. DTGA is not associated with any identifiable syndromes or genetic abnormality. It is notable that 80 percent of the patients with DiGeorge syndrome display 22q11 deletion and conotruncal lesions, but they rarely suffer from DTGA [5]. TGA in family members is uncommon, and the prevalence of CHD in the siblings of affected babies is not different from the general population at 0.3 percent [6]. TGA has a documented association with maternal diabetes mellitus, and it is more common in males to females in a 3:1 ratio in the DTGA form. The pathogenesis is multifactorial, and a combination of genetic and environmental factors is believed to play a role.

### **3. Embryology**

While the exact embryology is undefined, TGA is hypothesized to be secondary to a developmental aberration in the morphogenesis of the bilateral sub arterial conus. During the first month of fetal life, the subaortic conus and sub pulmonary conus, which represent the preliminary great arteries, are normally positioned above the right ventricle. At approximately 30 to 34 days of fetal life, the subaortic CONUS is resorbed, and the aortic valve migrates inferiorly and posteriorly into the left ventricle. The sub pulmonary conus does not resorb, and the pulmonary valve retains its association with the right ventricle [7]. In D-TGA, the sub pulmonary CONUS is resorbed abnormally, and the pulmonary valve migrates posteriorly. Simultaneously, the unresorbed subaortic conus forces the aortic valve to move anteriorly and get engaged with the morphologic right ventricle. The origin and course of the coronary arteries vary and are determined by the movement of the subaortic conus [8].

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**Figure 1.** *Normal heart.*

*Transposition of Great Arteries*

**4. Anatomy**

*DOI: http://dx.doi.org/10.5772/intechopen.99205*

circuit and deoxygenated blood in the systemic circuit.

Under normal conditions, Ao originates from the LV and is situated posteroinferior and to the right of the main pulmonary artery (**Figure 1**). In DTGA, Ao arises from the right ventricle (RV) and is positioned anteroinferior and to the right of PA, which is connected to the LV (**Figure 2**) [7]. The atria are normally positioned with atrial situs solitus, which is associated with atrioventricular concordance, d-looping of the ventricles, and ventriculo-arterial discordance. The aortic valve is anteroinferior and to the pulmonary valve's right instead of being posteroinferior and right as in normal conditions. The pulmonary and systemic circulations run independent of and parallel to each other with no mixing of the oxygenated and deoxygenated blood. In LTGA, the Ao is anterior, and to the left of PA, and the ventricles are inverted with atrioventricular discordance. The morphological RV is positioned to the left and the morphological LV to the right with Ao originating from the RV and PA from LV (**Figure 3**). The blood is pumped into the systemic circuit via RV and the pulmonary circuit via LV in this form. The systemic and pulmonary circulations are not impaired, and a shunt is not needed to mix blood. The two types of TGA are, therefore, hemodynamically different entities. In DTGA, with complete transposition and atrioventricular concordance, the lifesaving communication between the two parallel circulations is achieved interarterially via PDA, which connects the Ao with PA, or intra atrially via atrial septal defect (ASD) or PFO. Ventricular septal defect (VSD) is present in about 50% of the cases and provides another source of mixing between the oxygenated blood in the pulmonary

Anatomically, DTGA may occur as an isolated defect (simple) or in combination with other cardiac lesions (complex). The commonest anomaly found in DTGA is VSD, often associated with other cardiac lesions, such as pulmonic valve stenosis
