Figure 2.

A subeustachian pouch when present and the coronary sinus occur in the same anatomic plane between the eustachian ridge and the tricuspid annulus. When a guiding sheath or catheter is pulled back with counterclockwise torque from the ventricle, the catheter may fall into the pouch. When it appears to the operator that the coronary sinus has not been cannulated in that plane, failing to recognize the presence of the pouch may result in the operator pulling further back behind the eustachian ridge when cannulating the CS would then become impossible.