HOW TO INJECT GAS INTO THE EYE AT THE END OF YOUR PARS PLANA VITRECTOMY PROCEDURE
In this EyeQpoint, we will discuss one of the methods (there are more than one) that we find practical and most effective.
At the end of the surgery, the vitreous cavity is filled with air. We are going to explain the closed technique, where we first remove the two upper trocars and leave only the infusion line. Once you remove the trocars, you must ensure no leakage from the sclerotomies. In a previous video, we discussed how to deal with sclerotomies that do not efficiently seal.
https://www.youtube.com/watch?v=kFQaYKCdcBs&t=85s
Keep the intraocular pressure of the air infusion between 28-30 mmHg. The lower the pressure, the less risk of damage to the optic nerve, especially in patients diagnosed with glaucoma.
Next is the gas dilution. In these surgeries, the gas is applied at a non-expansible percentage. Table 1 shows each gas's characteristics. For cases like rhegmatogenous retinal detachment, we usually use C3F8 at a non-expansible concentration between 12% to 16%.
Table 1. Gas used as tamponade agents in vitreo-retina surgery. ¹⁻³
*Rarely used in clinical practice.
Figure 1. Steps for dilution of 12-15% C3F8 gas. From left to right, look at how 100% of the gas filled the 20cc or the total of the syringe. The gas is thrown away in the middle of the image, leaving only 2.5 to 3-cc. Then, on the right, in the same syringe, air is pulled in till it reaches 20cc. Now you have a 12-15% C3F8 concentration.
The gas is drawn with a filter connected to a 20 or 50-cc syringe. We recommend using at least 20-cc to achieve a complete washout in the vitreous cavity once the gas is applied. The syringe is filled with 20-cc of 100% C3F8 gas, and then it needs to be diluted. If I have 20-cc of gas at 100% and I want it between 12 to 16%, we usually move the plunger,
throwing away the excess gas until we are left with 2.5 to 3-cc, then we add air again to the syringe until we reach 20-cc again. This way, the gas is diluted to its non-expansible concentration. Figure 1 shows how the gas is prepared in a 20-cc syringe before it is injected into the eye. Remember that the syringe at this moment has an air filter at the top all the time.
The next step is essential, and the assistant might get nervous because it needs to be done quickly and without pauses. The infusion line is disconnected from the machine, and momentarily, the eye stops receiving pressure, which can cause decompensation. Connect the syringe with the diluted gas to the infusion line as quickly as possible. Be careful, as the movements can pull the infusion line out. Do not take too much time; remember, the eye is no longer receiving active infusion.
Now, the gas is partially insufflated to give the eye a good tone, and then you can puncture the upper portion with a 30G needle 4 mm from the limbus. The needle is connected to an open syringe with a small amount of BSS, which helps verify an outflow between the air in the vitreous cavity and the injected gas. Once the syringe is in place, your assistant begins to inject slowly (Figure 2). We recommend holding the base of the infusion trocar to ensure it does not move and there is no occlusion or anything that might alter the continuous flow of the gas. It is also recommended that your assistant tell you every 5-cc of gas injected into the eye.
Once you have only 5-cc in the syringe, stop and remove the needle and the trocar simultaneously to achieve a good seal and prevent the eye from decompensating. The remaining 5-cc is good to keep in mind in case there is leakage, allowing more gas to be injected. If you do it with a 50-cc syringe, the procedure is the same; only here you need to have 5 to 6-cc of pure gas before refilling the syringe with air up to 50-cc again.
Figure 2. The gas is introduced through the infusion cannula (a), and the remnant of air in the vitreous cavity is washed out through the syringe (b).
When injecting, you can pause till 10-cc in the syringe in case the eye does not have good pressure, and you can also inject a little more if needed. Now, you can also do it the other way around, that is, put only the initial amount of gas in the syringes that you use, for example, in the 20-cc syringe, put only 2.5 to 3 cc and then fill the rest with air up to 20-cc, the same with the 50-cc syringe, fill only 5-6 cc of gas and fill the rest with air up to 50-cc. We believe that doing like in the first step explanation helps maintain a more precise percentage of gas dilution.
Finally, ensure there is no gas leakage from the sclerotomies, and corroborate a good globe digital tone.
Look at our illustrative video for more details and a better understanding of the procedure. We know that sometimes it is better to see it than read it.
a
b
References
1. Parinya Srihatrai SWAFPR. Intraocular Tamponades and Vitreous Substitutes. VITREORETINAL SURGERY ONLINE.
2. Foster WJ, Chou T. Physical mechanisms of gas and perfluoron retinopexy and sub-retinal fluid displacement. Phys Med Biol. 2004;49(13):2989-2997. doi:10.1088/0031-9155/49/13/015
3. Schachat AP WCHDWPFKSD et al. Retina. (Ryan, ed.).; 2017.