Gas Concentration Errors in Vitreoretinal Surgery: Insights from a Model of Gas Mass Transfer Dynamics.

Abstract

Introduction: Concentration mix errors of fluorinated gases commonly used in vitreoretinal surgery can lead to catastrophic elevations in intraocular pressure. The timing and optimal management of this complication is poorly understood. This study aims to evaluate the impact of controlled venting on intraocular pressure (IOP) elevation following simulated gas mix errors.

Methods: A validated mass transfer model of intraocular gas dynamics was used to simulate the properties of "isovolumetric" gas concentrations of 20% SF6, 16% C2F6 and 12% C3F8, along with the most common concentration mix errors: double concentration, 10/3 (3.33x) concentration and 100% gas. Simulations were performed on eyes with vitreous cavity volumes of 4.0 ml, 7.2 ml, and 10.0 ml (representing hyperopic to highly myopic eyes). The model calculated gas volume and IOP over time, both before and after controlled venting events to normalise IOP.

Results: All gas concentration errors produced complete gas fill and rapid IOP elevations. Only double-concentration errors generated IOPs potentially manageable with medical therapy (peak 60 mmHg for 40% SF₆, 75 mmHg for 32% C₂F₆, 61 mmHg for 24% C₃F₈). Simulated 10/3 (3.33x) and 100% concentrations resulted in extreme IOP elevations incompatible with ocular perfusion. Controlled gas venting at day 1, normalised IOP for SF₆ but repeated venting procedures were required for C₂F₆ and C₃F₈ due to continued gas re-expansion. Eyes with larger vitreous cavities experienced prolonged IOP elevation and delayed equilibration after venting.

Conclusion: Gas concentration mix errors produce rapid and often unsalvageable IOP rises. While limited SF₆ over-concentrations may be temporarily stabilised with medical therapy and controlled venting, most scenarios, especially involving C₂F₆ or C₃F₈, require urgent surgical complete gas exchange.