His brain was cooked by the bleeding...there’s no way the guy can even return to the senate. It’s too bad the governor of his state can’t declare him unfit...and put a republican in his seat.
Only by an act of Congress could his seat be declared vacant because of incapacity. The last time they did this, IIRC, was when MD Congresswoman Gladys Noon Spellman went into a coma and never regained consciousness (back in 1981). She lingered for 7 years before her death. Steny Hoyer (now the rodent Majority Leader) won the vacancy.
Back in the early ‘70s, another Senator from South Dakota (Republican Karl Mundt), also was incapacitated for his last few years in office. He never returned to DC and died a year after his term expired. The reverse political problem occurred at the time, as SD had a Democrat Governor (and SD in the early ‘70s was trending rodent). A Democrat ended up winning Mundt’s seat, anyhow.
Yeah, that pesky Constitution again. Johnson could be in a vegitative state and he'd still be able to retain his office. There is no method for removing a sitting senator except through impeachment.