OSLI Retina

September 2020

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522 Ophthalmic Surgery, Lasers & Imaging Retina | Healio.com/OSLIRetina ■ E X P E R I M E N T A L S C I E N C E ■ INTRODUCTION Pars plana vitrectomy (PPV) with gas tamponade is commonly performed in the management of rheg- matogenous retinal detachment. 1 Gas was first uti- lized by Ohm in 1911, and since then, our knowledge of gas usage has mainly been based on experimental animal model data. 2-5 The type, concentration, and volume of gas injected are all important when consid- ering efficacy, duration of tamponade, and the poten- tial side effect profile. Different gases are available (sulphahexafluoride [SF 6 ], hexafluoroethane [C 2 F 6] , or perfluoropropane [C 3 F 8 ]), variable on location, and they are usually in- jected at a "nonexpansile" concentration, to reduce the likelihood of a potentially sight-threatening in- traocular pressure (IOP) rise. Postoperatively the gas enters a brief expansile phase where equalization of the partial pressures of nitrogen, oxygen, and carbon dioxide with the blood gas partial pressures occurs. After reaching maximal volume, gas absorption then occurs over 2.5 to 9 weeks into the blood by partial pressure forces, depending on the type of gas used. 6,7 In vivo modelling of gas dynamics is challenging due to problems with accuracy of volume measure- ment and, therefore, mathematical models have been developed to further our understanding. Hutter et al. produced a model assessing pressure and fluid flow dynamics of intraocular gases in pneumatic retino- pexy which was further modified for use in vitrecto- mized eyes. 8,9 Recently, we have developed a math- ematical model to determine the concentrations of different gas tamponades in air to achieve 100% fill Effect of Aqueous Dynamics on Gas Behavior Following Retinal Reattachment Surgery James E. Neffendorf, FRCOphth; Jean-Yves Guillemaut, PhD; Joseph C. Hutter, PhD; Jason Ho, FRCOphth; Tom H. Williamson, MD, FRCOphth BACKGROUND AND OBJECTIVE: To determine how the gas concentration in air required to achieve full postoperative vitreous cavity fill varies in different aqueous outflow states. MATERIALS AND METHODS: A mathematical model was used to estimate gas dynamics. The change in gas bubble volume over time was calculated in an eye with normal aqueous outflow, ocular hy- pertension (OHT), and OHT with apraclonidine treatment. RESULTS: The concentration required was higher for all gases to achieve a full postoperative fill in OHT eyes versus normal eyes. Optimal gas con- centrations were 22.6% for SF 6 , 13.9% for C 2 F 6 , and 11.6% for C 3 F 8 . Despite this, in OHT, the fill achieved was 95%, 95%, and 94% for SF 6 , C 2 F 6 , and C 3 F 8 , respectively. With apraclonidine, per- centage fill improved for all gases. CONCLUSIONS: This is the first study to show aque- ous outflow affects bubble size and indicates eyes with reduced outflow are at risk of underfill. This can ultimately affect surgical success. [Ophthalmic Surg Lasers Imaging Retina. 2020;51:522-528.] From St. Thomas' Hospital, London, United Kingdom (JN, JH, THW); University of Surrey, Guildford, Surrey, United Kingdom (JYG); and United States Food & Drug Administration, Center for Devices and Radiological Health, Silver Spring, Maryland. Originally submitted February 24, 2020. Revision received May 11, 2020. Accepted for publication July 8, 2020. Presented at the British and Eire Association of Vitreoretinal Surgeons 2018 Annual Meeting, Liverpool, United Kingdom, November 8, 2018. The authors report no relevant financial disclosures. Address correspondence to Tom H. Williamson, MD, FRCOphth, St. Thomas' Hospital, London, UK SE1 7EH; email: tom@retinasurgery.co.uk. doi: 10.3928/23258160-20200831-07

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