Abstract
Precis. A mathematical model is described of the physical properties of intraocular gases providing a guide to the correct gas concentrations to achieve 100% fill of the vitreous cavity postoperatively. A table for the instruction of surgeons is provided and the effects of different axial lengths examined.<br /> <br /> <br /> ABSTRACT<br /> <br /> Purpose – To determine the concentrations of different gas tamponades in air to achieve 100% fill of the vitreous cavity postoperatively and to examine the influence of eye volume on these concentrations.<br /> <br /> Methods – A mathematical model of the mass transfer dynamics of tamponade and blood gases (O2, N2, CO2) when injected into the eye was used. Mass transfer surface areas were calculated from published anatomical data. The model has been calibrated from published volumetric decay and composition results for three gases sulphahexafluoride, SF6, hexafluoroethane, C2F6, or perfluoropropane, C3F8. The concentrations of these gases (in air) required to achieve 100% fill of the vitreous cavity postoperatively without an intra-ocular pressure rise were determined. The concentrations were calculated for three volumes of the vitreous cavity to test if ocular size influenced the results.<br /> <br /> Results – A table of gas concentrations was produced. In a simulation of pars plana vitrectomy operations in which an 80% to 85% fill of the vitreous cavity with gas was achieved at surgery, the concentrations of the three gases in air to achieve 100% fill postoperatively were 10-13% for C3F8, 12-15% for C2F6 and 19-25% for SF6. These were similar to the so-called ''non-expansive'' concentrations used in the clinical setting. The calculations were repeated for three different sizes of eye. Aiming for an 80% fill at surgery and 100% postoperatively, an eye with a 4ml vitreous cavity required 24% SF6, 15% C2F6 or 13% C3F8; 7.2ml required 25% SF6, 15% C2F6 or 13% C3F8; and 10ml required 25% SF6, 16% C2F6 or 13% C3F8. When using 100% gas (for example, employed in pneumatic retinopexy), in order to achieve 100% fill postoperatively, the minimum vitreous cavity fill at surgery was 43% for SF6, 29% for C2F6 and 25% for C3F8 and was only minimally changed by variation in the size of the eye.<br /> <br /> Conclusions – A table has been produced which could be used for surgical innovation in gas usage in the vitreous cavity. It provides concentrations for different percentage fills, which will achieve a moment post-operatively with a full fill of the cavity without a pressure rise. Variation in axial length and size of the eye does not appear to alter the values in the table significantly. Those using pneumatic retinopexy need to increase the volume of gas injected with increased size of the eye in order to match the percentage fill of the vitreous cavity recommended for a given tamponade agent.