Just a quick update. My husband is home from the hospital. There was no surgery, heart attack, tumor, hemorrhage, or (new) blood clots, all of which I've gone through before with him. Still, being my husband, he can't just have a simple diagnosis. It turns out that he has a rare blood disorder called polycythemia. His bone marrow produces too much hemoglobin, so his blood is sludgy with iron, doesn't circulate well, and leaves deposits of iron in veins throughout his body, further blocking circulation.
The treatment for this rare condition is one of the oldest medical procedures known to man, practiced by the ancient Greeks and Aztecs - bloodletting! They call it phlebotomy now, and use tidy syringes and tubing and bags. But it's still basically the same thing people did for hundreds of years to rid the body of its "bad humors" - letting gravity drain blood out of a person's arm.
And this simple procedure could, possibly, completely resolve all the health problems that have been plaguing him for years. So I'm very happy. Also very worn out. Thanks for all your kind messages and good wishes.