Hi there --
This is an odd observation by your doctor and I would think he'd have been more specific as to his meaning and the possible cause. I can make a couple guesses, however you really need to follow up with him and request a more thorough explanation.
One reason this may be happening is that many people with Marfan's simply have, due to structural abnormalities, less vital capacity (lung filling capacity) than others. However, if this were the case you should have noticed some compromise, so that doesn't seem to be the likely answer. Another possibility would be a pneumothorax (collapsed lung), which would of course yield this finding, but would likely also be accompanied by some discomfort or pain, shortness of breath, exercsise intolerance, etc. I can't imagine this not being explored and explained. I have to assume your doctor gave you some sort of pulmonary function test, and if so perhaps you know the numerical results? Also you could get your hands on an Asthma Check device, a cheap little plastic airflow device that pretty accurately show what your lung capacity is (you should be able to blow at least a 350 on one of these, which would be an adequate two-lung-power reading). Perhaps your doctor would be willing to give you one of these, since he seems to think you're running at 1/2 capacity but has no interest in taking any action or explaining such a weird finding. If not they can be purchased in some drugstores and on line for less than $20.00.
I hope this is helpful to you. Please follow up with us here as needed. Good luck to you.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist