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E. B. Fox, K. Lalwani, R. Fu, L. Kelly, B. Edmunds; The Effect of Nitrous Oxide on Intraocular Pressure in Healthy Adults. Invest. Ophthalmol. Vis. Sci. 2010;51(13):562.
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Most anesthetic agents reduce IOP. Although nitrous oxide (N2O) is commonly used for sedation, there are no studies of its usefulness in EUAs where accurate IOP measurements are critical. This is the first of several studies investigating the effect of N2O on IOP and involves healthy adults.
After IRB approval and informed consent, 20 healthy adult volunteers underwent an eye and pre-anesthetic exam. IOP was measured by Tonopen. IOP was measured at baseline (time (tB)) and after 3,6,9, and 12 minutes on a 70% N2O/O2 mixture (time steady state(tSS)1-4). Volunteers then breathed oxygen and room air, and IOP was measured 5,10, and 15 minutes after N2O cessation (time room air(tRA)1-3). Blood pressure (BP), end tidal(Et)N2O, and activity level (AL, sedation/agitation) were recorded with each IOP measurement. Power calculations predicted detection of a 2.8 mmHg difference in IOP between any two time points; 80% power, n=20. A linear mixed model assessed change in IOP over time, significance set at p<0.05. BP, AL, and sex were included in the model.
There were no significant differences between IOP at baseline and IOP at any of the time points. Significant differences in IOP were noted between tSS1-tSS2, tSS1-tSS3, and tSS2-tRA3 (Table 1). IOP in sedated patients was significantly lower than in agitated patients (-1.29mmHg,p=0.013).
Inhaled N2O does not cause significant IOP changes compared to baseline in healthy volunteers. There was a low baseline mean IOP and small statistically significant differences between internal time points. It is unclear if these differences would persist as absolute or proportional differences in populations with higher IOP or diseases such as glaucoma. Future studies will assess the effect of N2O on IOP in children, subjects with higher IOPs, and patients with glaucoma.Table 1. Effect of N2O on IOP(mmHg)Significant difference between two time points, p-value: *=0.01,#=0.046,&=0.035,^=0.004. 1Adjusted for BP and sex (least square means). 2Adjusted for BP, sex, and AL. 3Unavailable for tB and tRA1-3 as confounded by AL.
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