supine and sitting spirometry to evaluate diaphragmatic weakness

Q: How supine and sitting spirometry helps in evaluating diaphragmatic weakness?

Answer: Measuring Vital Capacity (VC) supine followed by in sitting position can give a quick idea regarding the strength of the diaphragm

• If Vital Capacity (VC) decreased by more than 10 percent, it suggests diaphragmatic weakness. 
• If VC decreased by 15 to 25 percent, it suggests unilateral diaphragmatic paralysis. 
• And, if VC decreased by half bilateral diaphragmatic paralysis is highly suspected.

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References;

1. Kokatnur L, Rudrappa M. Diaphragmatic Palsy. Diseases. 2018;6(1):16. Published 2018 Feb 13. doi:10.3390/diseases6010016 

2. Lechtzin N, Wiener CM, Shade DM, Clawson L, Diette GB. Spirometry in the supine position improves the detection of diaphragmatic weakness in patients with amyotrophic lateral sclerosis. Chest. 2002 Feb;121(2):436-42. doi: 10.1378/chest.121.2.436. PMID: 11834654.