Several congenital heart diseases induce right to left shunting of blood, during which deoxygenated blood from the right heart gains entry straight into the left heart. As a result, a very early onset of cyanosis and severe hypoxemia is typical in a right to left shunt. Clinical criteria, followed by imaging studies, mainly echocardiography, are used to identify the cause of symptoms.
A right to left shunt is a term describing the flow of deoxygenated blood from the right heart (previously from the inferior and superior vena cava) straight into the left heart, which predominantly occurs in the setting of several congenital heart diseases (CHDs)    :
All CHDs that induce a right to left shunt present with central cyanosis and the term cyanotic heart disease is sometimes used to describe these disorders  . Signs and symptoms may appear very early, during the neonatal period in most cases, and cyanosis could be mild or severe and life-threatening, depending on the extent of hypoxemia  . If the diagnosis is not made immediately, clubbing of the fingers and toes, polycythemia, paradoxical embolization and respiratory distress can ensue    .
Mortality rates of CHDs causing a right to left shunt are very high in the absence of an early diagnosis and appropriate treatment, and unfortunately, the diagnosis is missed in up to 25% of neonates  . For this reason, a meticulous prenatal and postnatal workup is vital in recognizing CHDs of any type, especially those that can be of life-threatening. In neonates, a properly performed physical examination immediately detects cyanosis, after which various diagnostic and imaging tests should be ordered. One of the easiest tests that might be used in the clinical setting is the evaluation of pulse oximetry, a procedure that rapidly evaluates the percentage of oxygen in systemic circulation . When a right to left shunt exists, profound hypoxemia is virtually always present, and pulse oximetry has been used as an effective neonatal screening method for tricuspid atresia, tetralogy of Fallot, and the transposition of the great vessels . Plain radiography of the chest and electrocardiography (ECG) may be useful methods as well, and if clinical suspicion is raised, imaging studies in the form of echocardiography, are the gold standard in visualizing the shunt and confirming the underlying cause    . Standard 2D, 3D, and color Doppler are all used to identify the anatomical anomaly and make the diagnosis. The benefit of ultrasonography is that it enables physicians to make a prenatal diagnosis, and tetralogy of Fallot can be diagnosed as early as 12 weeks  . In addition, it enables adequate therapeutic preparation, thus significantly reducing the risk for complications.