BREATHING: Why is being tested laying down so important?

Linda asks of Dr. Carter - When I was seeing Dr. Charlie Chan he told me that when I get my breathing checked for MIP (maximum inspiratory pressure) and MEP (maximum expiratory pressure) I should have it done sitting and lying down. He stressed the horizontal position and told me that because I didn't have the benefit of gravity when lying down I'd breathe more shallowly and it was good for the docs to know that when lying down I took in and especially pushed out quite a bit less air (one litre less). I've told folks to be tested lying down as well as sitting up but their doctors just poopoo that mainly because it comes from me and I'm not a doctor. Could you please answer the question (Why be tested lying down as well as sitting?) or suggest that people with CMT have their breathing tested both sitting and lying down and tell us WHY so I can print it in the CMT Newsletter with your name on it and then, maybe, pulmonary specialists will listen. Mine did and it made a big difference in how he saw my breathing and sleep problems.
Dr. Carter answers: You and Dr. Chan are absolutely right. People with CMT and NMD (neuromuscular disease) in general should have MIP and MEP done sitting and lying down. The horizontal position puts your diaphragm at a disadvantage by removing the effects of gravity. When you are upright, gravity helps lower the diaphragm and fill your lungs with air. This is why people often have breathing problems at night, which can be helped by sleeping sitting up or propped up with pillows. The heart also has a harder time pumping when you are lying flat, which further compounds the problem.

By checking your MIP and MEP lying down, your physician gets a better idea of how you might be breathing at night. A more extensive, expensive way of doing this is with a formal sleep study where they can observe you sleeping and check the amount of O2 in your blood, etc. Any physician who doesn't agree with this has forgotten basic physiology!