My Lola had a shunt and has MVD. My suggestion to you is DO NOT let them do exploratory surgery. Take a deep breath, do some research and figure out what you need to do next. The greatest help to me was to join the Yahoo group for liver disease -
Liver_Shunt_And_MVD_Support : Liver Shunt & MVD/HMD Support
they are a wealth of information and can give you numerous resources that may be helpful. I took Lola to University of TN to have her shunt repaired and she is now like a different dog. I was told a Portogram and the Ultrasound was not always conclusive. We did the scintigraphy and repair at UT for the same amount of money it would have been to have all of the tests run at my local vet. I'm not saying don't run the tests, just make sure you need to have them done. The lactulose should not cause diarrhea - only soft stools. I would cut back on the lactulose...the above group will be a great help to you - they were much more help to me than my local vet. Good luck!