Fortunately, the cardiac MRI did not bear out the cardiologist's hypothesis of amyloidosis caused by the multiple myeloma. It revealed no evidence of any kind of damage to the heart muscles. This is very good news, but leaves us back at square one, in terms of diagnosing my symptoms. The next step down this road would involve substantially more invasive diagnostic techniques, but no one involved wants to go there at this point.
So we have agreed on a kind of passive-aggressive strategy for dealing with this situation. I will continue with the daily 20 mg dose of Lasix to control the fluid retention. Meanwhile, I will attempt to gradually wean myself from the supplemental oxygen. I will don the hose only when walking, or otherwise exerting myself in some way, but remove it when at rest. If this goes well then the next step would be to attempt to lose the oxygen altogether. That would of course be a very good thing, but whether it would be accompanied by corresponding improvements in blood pressure and heart rate is anyone's guess.