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Testing For POSH


Phoenix Rising Founder
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.


Senior Member
Brisbane, Australia
The research now points to POSH occurring in any patient group that has POTS as a symptom, so not something that's more prevalent in the ME/CFS population;

Postural Hyperventilation as a Cause of Postural Tachycardia Syndrome: Increased Systemic Vascular Resistance and Decreased Cardiac Output When Upright in All Postural Tachycardia Syndrome Variants;
[ We conclude that all POTS is related to decreased CO, decreased central blood volume, and increased systemic vascular resistance and that a variant of POTS is consequent to postural hyperventilation. ]

It sounds very similar to me based on #2. I was tested for POSH based on my CO2 levels. Although HYCH also includes #3, abnormal cerebral blood flow:

"HYCH was defined by: (1) Symptoms of OI; (2) Orthostatic hypocapnia (low ET-CO2); (3) Abnormal decline in orthostatic CBFv due to hypocapnia; 4) Absence of tachycardia, orthostatic hypotension, or other causes of low CBFv or hypocapnia."


Senior Member
@TinaYesen , it’s interesting when different people or groups converge on the same idea, perhaps from different angles or fields.

Looks like Dr Natelson is still looking at this topic:

I am a bit too tired to read his articles today, but will look at them soon.

Apart from Peter Novak, the other autonomic researcher I noticed who has looked at CO2 is Julian M Stewart.

On a personal note, I’m about to have capnography testing as part of another test. I don’t think I have hypocapnia, but we will see!