Intraocular gas tamponade is an essential part of vitreoretinal surgical procedures. Air, sulphur hexafluoride (SF6), perfluoro propane (C3F8) and perfluoro ethane (C2F6) are commonly used for any vitreoretinal surgical procedure that needs intraocular tamponade. In recent years, with the development of vitreoretinal surgical techniques and devices, gas tamponade is becoming widely used whereas silicone oil tamponade is losing its popularity [1]. Usage of gas tamponades brings certain safety issues. Overuse of gases may cause uncontrolled intraocular pressure (IOP) rise and consequently, retinal vascular occlusion. In addition, diminished optic nerve perfusion may cause visual loss, especially in glaucoma patients. On the other hand, inadequate amounts of gas tamponade result in retinal re-detachment and surgical failure. Generally, surgeons prefer to measure the end of surgery IOP with the palpation of the globe when they walk on this thin red line. Considering the issues mentioned above, quantitative measurement of the IOP yields more reliable results than digital assessment by the palpation of the globe. However, quantitative measurements need to be standardised because of the deviations in gas or air-filled eyes.

Aronowitz, in 1976, investigated the relationship of the intraocular gas amount and IOP in an experimental model of five cadaver eyes, which consisted of an intraocular barometer, intravitreal gas delivery system, and Schiotz tonometer [2]. He reported indentation tonometer to underestimate the IOP in direct proportion to the intraocular gas amount. Although he made a comprehensive interpretation depending on Boyle's law and Friedenwald relationship, the lack of in vivo information was the limitation of the study. Some other studies compared different tonometers in gas-filled eyes postoperatively. However, these studies had no reference intraocular measurement methods showing actual IOP [3], [4], [5]. Moon performed in vivo measurements with Tono-Pen and compared with Goldmann applanation tonometer (GAT) [6].

This study aims to procure a quantitative IOP measurement method by creating correction tables for gas-filled eyes to achieve a safe IOP level at the end of the surgery. Regression analysis was based on Schiotz and Tono-Pen tonometers as they are easily accessible by vitreoretinal surgeons and can be used in sterile operating room conditions.