POSH is what Dr. Natelson calls Postural Orthostatic Syndrome Of Hyperventilation. He discovered it a couple of years ago - particularly in older ME/CFS patients. In POSH people with ME/CFS breathe normally when they are lying down but when they stand up people breathe too deeply and hyperventilate. this doesn't appear to be due to anxiety - it's a purely physiological process. It reduces carbon dioxide levels in the blood thus causing a host of symptoms including fatigue. Here's A Test for Hyperventilation When Standing (Postural Orthostatic Syndrome of Hyperventilation (POSH)). Stand up straight for 8 minutes. If you start to feel ill rest for a while and then repeat the test later. This time breath into a paper bag held across your nose and your mouth. If the symptoms you had without the bag dont show up when you are breathing into the bag then you may be unconsciously hyperventilating.