Metaprolol 10 mg is an extremely low dose, and while it's true the imipramine can interact with it, causing an effective increase in the available amount in your bloodstream, the only reason this would be problematic would be because your blood pressure could drop too low. This isn't happening, and it's hardly surprising since that's such a low dose of metaprolol. Your doctor might consider titrating (adjusting) the metaprolol dose upward to a level where it is doing the job it's intended to do, then monitoring your BP for a while to make sure it's okay. This relationship should remain fairly stable at any given dose. He might consider doubling the dose by having you take 10 mg. twice per day (every 12 hours), since plain metaprolol (and 10 mg would be the plain, not long-acting type), is generally a twice-per-day drug anyway, and 10 mg. once per day is hardly going to make a dent in the BP, especially since it's not doing much at all presently. It is safe to take these meds in combination, and the imipramine is probably the most problematic of all of them, not because of the beta blocker interaction, but because of potential side effects which you appear to not be having anyway. Again, the interaction with metaprolol should remain fairly static, so increasing the dose of metaprolol carefully (and at twice per day) should slowly bring the BP within a satisfactory range. Right now it's not even close.
Hope this helps. Good luck to you.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist