Development and Testing of a Mobile Phone App for Risk Estimation of Gas Volume Expansion and Intraocular Pressure Elevation in Patients With Intravitreous Gas or Air Tamponade: Interobserver Assessment Study

Background Pars plana vitrectomy (PPV) with intravitreous tamponade of gas or air has been widely used for a series of vitreoretinal diseases. It is estimated that 100,000 patients per year undergo PPV globally, and half of them were subsequently tamponaded with gas or air. According to Boyle’s law (P1V1=P2V2), patients with an intravitreous remnant of gas or air will be under high risk of intraocular pressure (IOP) elevation and subsequent vision loss owing to the expanded intravitreous gas or air when traveling post operation to a place with a significantly higher altitude. We always explain to patients why postoperative travel is potentially risky. Emergency cases of elevated IOP caused by postoperative traveling would sometimes come to surgeons. However, there have been few disease education or reference tools for both the surgeons and patients to have better communication. Objective The aim of this study was to introduce and evaluate a mobile phone app developed by surgeons (the authors) for preliminary risk estimation of volume expansion and IOP elevation in patients with intravitreous gas or air when traveling to a place of higher altitude. Methods The app was developed on the iOS and Android operating systems. Boyle’s law (P1V1=P2V2) was the theoretical basis of the app. Intravitreous gas or air volume and altitude values were independent factors to deduce the risk report. Consecutive patients underwent vitrectomy, and those with an intravitreous remnant of gas or air were recruited. The surgeons judged the vertical height of the fluid/gas interface through the dilated pupil; the patients were instructed to judge it according to their visual field when looking straight ahead and line it out on a chart included in the app. Finally, all the patients were required to fill a Likert scale–based questionnaire with 2 main items to evaluate the participants’ user experience and attitudes toward the app. Results A total of 50 patients were included (30 males and 20 females). All patients could independently operate the app to complete the test. The median heights of the fluid/gas interface independently judged by the surgeon and patients were 40% (range: 10%-75%) and 41% (range: 9%-78%), respectively (P=.63). The median altitude of the participants’ destinations was 150.0 m (range: 0-3490 m). The Bland-Altman analysis revealed a good agreement between the surgeons’ and patients’ judgments (bias of −0.3%), with 95% limits of agreement of −5.8% to 5.3%. Overall, the Likert scale revealed a positive attitude from the patients toward the app. Conclusions The app is reliable for patients to have preliminary risk estimation of intravitreous gas or air volume expansion and IOP elevation if travel to a place of higher altitude is planned. The surgeons could also use it as a platform for better disease communication.


Multimedia Appendix 1
The governing equations of the APP were developed in steps, as described below: How to determine the volume of intravitreous gas/air 16 1.The vitreous cavity is set to be an oblate spheroid, the entire volume of which is 4.5 ml.The brief calculation is as follows: V1 = (4/3)πa 2 b (1) Where V1=volume (mm 3 ) of the vitreous cavity = 4,500 mm 3 = 4.5 ml, a = length of major semiaxis (mm) = horizontal eye axis/2 ≈ 22 /2 = 11 mm, and b=length of minor semiaxis (mm)=vertical eye axis/2 ≈ 18/2 = 9 mm. 2. The vertical height of the intravitreous gas is estimated by the physician (when at the in-patient department or clinic) or the patient by him/herself (for selfassessment or when the physician's instructions are unavailable) when the patient is in a sitting position with the head held level.

Surgeon's estimation method:
The surgeon directly observes the horizontal fluid/gas interface with a preset lens through the dilated pupil and judges the vertical height of the fluid/gas interface according to anatomic landmarks (fovea, vascular arcades, optic disc, etc.).The height of the fluid/gas interface is expressed as a percentage of the vertical diameter of the vitreous cavity (e.g., when the interface is at the level of the fovea, the percentage of the interface is 50%; when at the middle of the superior vascular arcade, the percentage is about 40%.The surgeon estimates the volume according to his or her expertise and experience. Patient's estimation method: The patient judges the horizontal fluid/gas interface when he/she is looking straight forward.Because of the great differences in the refraction index, the intravitreous fluid and gas/air produce greatly different vision results for the patient (as shown in Figure 2).Intravitreous gas located in the upper part of the vitreous cavity makes the vision darker and more blurred while intravitreous fluid located in the lower part of the vitreous cavity makes vision nearly the same as normal.According to the law of photorefraction through the visual axis, clear vision (through the intravitreous fluid) is located in the patient's upper visual field, and blurred vision (through the intravitreous gas) is located in the patient's lower visual field.Therefore, the patient can easily line out the fluid/gas interface according to the two different vision types in his/her operated eye.With the fellow eye covered, the patient is instructed to assess the height (in percentages) of the fluid/gas interface according to his/her visual field when looking straight ahead.The APP includes a chart for the patient to easily line out the interface according to his/her vision (Figure 1).

Estimation of the volume of intravitreous gas/air according to the height (in percentages) of the fluid/air interface
As the vitreous cavity is set to be oblate spheroid, the equation below can be used to deduce the volume of intravitreous gas, using the height of the fluid/gas interface as the only independent factor.

How to determine atmospheric pressure 12
We decided to estimate atmospheric pressure according to a simplified equation, using different cities' altitudes as the only independent factor (with every 12 m decrease in altitude, the corresponding atmospheric pressure decreases 1 mmHg).We set the atmosphere pressure at sea level to 760 mmHg.Thus, the atmospheric pressure in other places can be calculated according to the following equation: The geographic elevations for different cities are located from Internet data (Wikipedia and Google Earth).The APP also permits patients to enter an accurate geographic elevation of his/her interested place (such as a high mountain or building) in order to make it more adaptable to different circumstances. 12,16e expansion volume of intravitreous gas can be determined by Boyle's Law (P1V1=P2V2).Following this physical law, it is possible to deduce how much the intravitreous gas bubble will expand to make the IOP normal again.

How to determine the expansion volume of the intravitreous gas
If we set the patient's normal IOP at the two places to be the same as 16 mmHg, the extreme pressure of the intravitreous gas=atmospheric pressure at the corresponding position+16 mmHg.Where V3=volume (ml) of intravitreous gas at Place 1 and V4=volume (ml) of intravitreous gas at Place 2 Thus, the theoretical expansion volume of intravitreous gas when the patient moves from Place 1 to Place 2 is as follows: Where Vexpansion = the theoretical expanding volume (ml) of intravitreous gas when the patient moves from Place 1 to Place 2, P = height (percentage) of fluid/gas interface in the vitreous cavity when the patient is at Place 1 (0 ≤ P ≤ 1), A1 = corresponding altitude (m) at Place 1, and A2 = corresponding altitude (m) at Place 2.

How to estimate the risk of gas expansion
First, we set the volume of the anterior chamber to 0.25 ml.Considering the compensation mechanism of the eye when the intravitreous gas expands, we define that risk into 5 grades (1 = very low risk; 5 = extremely high risk).We define the risk according to the ratio of the expansion volume of the intravitreous gas to the volume of the anterior chamber, that is: risk ratio = Vexpansion / 0.25 = 18 (3P 2 -2P 3 ) (A2 -A1) / (9312 -A2) Where, as in Equation 3, P=height (percentage) of fluid/gas interface in the vitreous cavity when the patient is at Place 1 (0 ≤ P ≤ 1), A1 = corresponding altitude (m) at Place 1, and A2=corresponding altitude (m) at Place 2.