My Dad Has COPD, Use To SATS In The Mid

My Dad has COPD, use to SATS in the mid 80's, today, the Doctors office didn't call back. I am questioning what does it mean when he lays down and his Sats drop even more, he is on 6-8 Liters of O2 and heart rate now is 105, he barely moves and Sats drop, this has been over the last 2 days. Was in the hospital 2 weeks ago with COPD exc., GI bleed resolving and has poor lungs, main questions now, what does elevated HR mean and SATs drop when he lays down, Lung sounds clear, I used a stethoscope, no crackles, no rales.
YYYY@YYYY Sorry, thought this was free advice, we can call er, Thanks