My upright is 57% and my flat is 45% lung function I have RLD and a paralysed Diaphragm. I was given Seretide before they found RLD and then consultant told me it would not work now?
Lying flat is going to affect the way the lung function as they rely on negative pressure.
Inhalation is created by the dome shaped muscle, the diaphragm, flattening out. The diaphragm also seals the bottom of the cavity (the plural cavity) that contains the lungs.
As the diaphragm flattens out, it increases the volume of the pleural cavity. As the cavity increases in size, the same amount of gas that was in your lungs/cavity a moment ago does not increase and you have the same amount of gas filling a larger cavity which decreases the pressure as compared to barometric pressure.
As a result, you don't pull air in, air is actually pushed into lower (negative) pressure inside of your lungs by the area of greater pressure that is outside of your lungs.
So when your organs are sloshing about when flat it will affect results.