Okay. I've got the answer. When my psychiatrist told me that Nuedexta should potentiate Namenda as well as dextromethorphan (DM), he wasn't speaking of quinidine blocking the metabolism of Namenda at all, but rather that DM and Nasmenda should compliment each other at the receptor level.

And here's what the FDA Approved Prescribing Information for Namenda has to say on the topic of Namenda – Memantine Interaction:

NMDA receptor antagonists (memantine):A drug interaction study was conducted between a higher combination dose of dextromethorphan/quinidine (dextromethorphan hydrobromide 30 mg/quinidine sulfate 30 mg) and memantine 20 mg/day to investigate the pharmacokinetic and pharmacodynamic interactions in 52 healthy subjects. Both dextromethorphan and memantine are antagonists of the N-methyl-D-aspartate (NMDA) receptor which could theoretically result in an additive effect at NMDA receptors and potentially an increased incidence of adverse events. There was no significant difference in the plasma concentrations of dextromethorphan and dextrorphan before and after the administration of memantine. Plasma concentrations of quinidine increased 20-30% when memantine was added to dextromethorphan hydrobromide 30mg/quinidine sulfate 30mg. [Emphasis added.]

The moral of the story, once again, is that even with what remains, at 10 mg, a very low dose of quinidine - even when increased 20-30% - anyone contemplating the use of Niedexta, should/must get pre-clearance from their internist to rule out the possibility of a "long Q - T interval" on the EKG.

I hope this is helpful.

Mike