What are the possible vascular channels for a catheter on the left side of the spine if the situs is solitus?

Most likely it is an arterial channel, with catheter in the aorta. Alternate possibility is the catheter in a hemiazygous system.

What can cause lowering of pulmonary venous saturation in a child during cardiac catheterisation?

Most likely cause is hypoventilation due to sedation.

In which condition can oxygen saturation in all four cardiac chambers be equal?

Saturation in all four cardiac chambers will be equal in total anomalous pulmonary venous connection (TAPVC). But due to streaming from SVC to RV and IVC to LA, LV saturation will be less than that of RV in supracardiac form of TAPVC.

In which condition can the ventricular saturation and saturation in both great vessels be equal?

In single ventricle, since there is complete mixing of left atrial and right atrial blood, saturations in aorta and pulmonary artery are equal.

What can cause an obligatory right to left shunt at the atrial level?

In tricuspid atresia, there is an obligatory right to left shunt at the atrial level. Similar situation occurs in TAPVC as well.