My grandfather had covid pneumonia and his HRCT score was 1225. He was discharged about 40 days ago, but for the past 56 days, his oxygen level drops to 92-93 while he's asleep and goes back up to 96-97 once he's awake. Is this something dangerous? What can we do to help him? Everything else looks fine except his D-dimer, which is high at 1.06 when it should be below 0.5, although his CRP is normal. Can you guide us on what steps to take?