I've never heard of that being done unless it's part of imaging or other procedure (like hysteroscopy). While 58340 describes the activity, it is not for the reason indicated in the description of the code. If there was a hysteroscopy done, this is included in the hysteroscopy.
58340 Catheterization and introduction of saline or contrast material for saline infusion sonohysterography (SIS) or hysterosalpingography
If there was no sonohysterography or hysterosalpingography, I do not feel that code is accurate. There is no better code I am aware of.
In this scenario, I see your options as:
1) Unlisted
2) Add -22 to the EMB for the additional work of flushing the uterus
I personally try to avoid unlisted if there is another reasonable alternative and would recommend option 2.