If a bedridden patient with a gravitational valve system lies with a slightly elevated head, this leads to activation of the gravitational unit and this may cause underdrainage. As a result, we advise not using an anti-siphon device in a patient who is bedridden for a long period 1).
1)
Kaestner S, Kruschat T, Nitzsche N, Deinsberger W. Gravitational shunt units may cause under-drainage in bedridden patients. Acta Neurochir (Wien). 2009 Mar;151(3):217-21; discussion 221. doi: 10.1007/s00701-009-0215-7. Epub 2009 Feb 24. PMID: 19238319.