Pressure Changes in Poorly Compliant Spaces

The entrance of nitrous oxide into gas cavities surrounded by poorly compliant walls can increase the pressure on those walls. Nitrous oxide administration can impose unwanted increases in intraocular pressure after intravitreal sulfur hexafluoride injection.^[61] Other examples include the gas space created by pneumoencephalography (now rare as a deliberate procedure) and the natural gas space in the middle ear. Pressures in the head or middle ear may rise by 20 to 50 mm Hg because of the ingress of nitrous oxide at a faster rate than air can be removed.^{[62] [63]} Recognition of this problem has decreased the use of nitrous oxide for tympanoplasty because the increased pressure may displace the graft. Increased middle ear pressure may adversely affect postoperative hearing.^[64] The capacity of nitrous oxide to expand the gas in the middle ear has also been used to elevate an adherent atelectatic tympanic membrane off the promontory and the ossicles.^[65]