Canagirl,
I am into my 8th year of Lidocaine Infusions, have them every 4 weeks. Ketamine I think 4-5 years, but things are hazy today. No reduction in effectiveness as they are both anaesthetics, so work differently to other meds.
I noted down their actions a while ago, can find Lidocaine now: Blocks sodium channels in neuronal cell membrane - reducing transmission of pain. I read recent Thread about Sodium Channel research.
Ketamine is on the WHO Essential Medicines list it is so versatile and powerful.

Dave.

Here's the above Thread:
http://neurotalk.psychcentral.com/thread218667.html