Boyle’s Law (P1V1 = P2V2) was the prerequisite to ensure the prediction accuracy of gas expansion. The calculation depended on gas volume and altitude values. The vitreous cavity was set to be an oblate spheroid (whole volume = 4.5 ml). The APP calculated the intravitreous gas/air volume based on the vertical height of fluid/gas interface (in percentage), when the patient was in sitting position with the head flat. Finally, the ratio (expanded volume of intravitreous gas/normal volume of the anterior chamber) was transferred by the APP to report a final risk estimation of IOP elevation, provided the patient was transported directly to another place of higher altitude.