What postural change in FVC is suggestive of respiratory muscle weakness?

In our practice, an assisted ventilation clinic which is enriched with patients who have diagnosed neuromuscular disease and/or chronic respiratory failure of unclear etiology, we often use 12% or 200cc simply by analogy to the criteria we use to assess bronchodilator responsiveness during PFTs.

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We use a supine drop of 15% for "significance." A symptomatic ALS population Postural change of FVC predicts some respiratory symptoms in ALS had roughly this percentage change. 15% cutoff is seen in ERS statement as well 2019 ERS.

I agree, clinical signals may be sooner and certainly vary by patholo...