Step down of pulmonary artery saturation in PDA (patent ductus arteriosus) is seen in:
a) Eisenmenger PDA
b) TOF (tetralogy of Fallot) with PDA
c) TGA (transposition of great arteries) with PDA
d) TAPVC (total anomalous pulmonary venous connection) with PDA
e) AP (aortopulmonary) window
Answer: c) TGA with PDA
The term step down is used when the given chamber has an oxygen saturation less than that of the proximal chamber. In TGA, aorta is connected to the pulmonary artery and has a lower oxygen saturation. Pulmonary artery is connected to the left ventricle receiving oxygenated blood from the left atrium. Hence when there is a PDA, desaturated blood enters the pulmonary artery from the aorta causing a step down in oxygen saturation compared to left ventricle which is the proximal chamber in this case.