OSLI Retina

September 2020

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September 2020 ยท Vol. 51, No. 9 523 of the vitreous cavity postoperatively. 10 This model demonstrated that variation in axial length and the size of the eye does not appear to significantly alter the gas concentration required. 10 Another influencing factor on gas behavior and total fill is the aqueous outflow ability of the eye. This is important because gas requires aqueous to leave the eye to allow it to fully expand and form a complete tamponade. It has been shown that eyes with ocular hypertension have a reduced trabecular meshwork outflow and uveoscleral outflow rate, and therefore it is possible that such patients could have a reduced maximal gas bubble size. They therefore run the risk of being under-filled at the time of sur- gery with the inherent risk of reduced tamponade time. 11 This has the potential to adversely affect sur- gical outcome in certain situations. Furthermore, drugs such as apraclonidine (Iopidine; Novartis, Basel, Switzerland), which alter aqueous dynamics, theoretically may influence gas dynamics. We have further improved our mathematical model to control for aqueous fluid dynamics and quantify its effect on required gas concentrations to achieve 100% fill. In particular, we were interested in assessing the effect of ocular hypertension and apraclonidine treatment on gas dynamics. Figure 1. Model predictions after injection of 20% SF 6 or C 3 F 8 following vitrectomy. Change in gas volume with time after injection of 20% SF 6 or C 3 F 8 (at different concentrations) following vitrectomy. For each of the eyes considered, measurements reported by Jacobs et al. are shown as a symbol, whereas the prediction made using our model is shown as a continuous line of matching grayscale. 14 An average eye volume of 7.2 mL was assumed, unless the maximum volume reported in the original paper was larger, in which case it was used instead. The initial volume being unknown, it was set to 90% of the first measurement to allow for gas expansion.

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