35.2Gases

35.2.1 General Considerations

Depending on the type and concentration of the gas, it may stay in the vitreous cavity for ~2 weeks (SF6) or ~2 months (C3F8). As N2 and O2 diffuse into them, pure gases expand after their implantation, most rapidly within the first 8 h; SF6 needs ~2 days to reach its maximum expansion, C3F8 needs ~4 days. For this reason, no more than 1 ml of pure gas should be injected into an emmetropic eye.

The potential for gas expansion during general anesthesia as the N2O diffuses into the intravitreal gas4 and the “collapse” of the intravitreal gas upon termination of the general anesthesia are important issues requiring proper adjustment from both the surgeon and the anesthesiologist (see Sect. 14.2).

The beneficial effects of the gas include their surface tension, buoyancy, space occupation, and the fact that proliferating cells do not penetrate it, nor will they attach to its surface.

35.2.2 Surgical Technique5

Typically, surgeons flush the air-filled eye with a premixed concentration of the gas. This method works but wastes a lot of gas. My technique is the following, regardless of the type of gas used:6

•Prepare two 2 ml syringes. Leave one empty for air withdrawal and fill the other with 2 ml of pure (undiluted) SF6. Loosely attach a short 27 g needle to the empty syringe.

•Remove all cannulas.

•Through a superotemporal location, insert the needle attached to the empty syringe into the vitreous cavity. Aim toward the center of the eye and constantly monitor the needle,7 keeping it in the same position throughout.

•Press the cone of the needle against the orbital bone or secure it firmly with your fingers, or both.8

•Withdraw ~1.5 ml of air9 into the syringe. The nurse must announce the amount of air being removed at every 0.5 ml interval.

•Detach the syringe from the needle, and hand it to the nurse. Attach the gas-filled syringe to the needle’s cone.

4Pure SF6 will expand by ~250% as N2O penetrates it.

5See Fig. 35.1.

6My default choice is 30% SF6. This provides tamponade for a sufficiently long period (~10 days) for the laser spots to take effect.

7This means that you never look up from the microscope during the entire process.

8This is determined by the patient’s facial anatomy.

9More if the IOP was high and less if the IOP was low at the beginning of air withdrawal; more in a myopic and less in a hyperopic eye.

Fig. 35.1 Mixing the gas for tamponade. (a) The 27 g needle is loosely attached to the 2 ml syringe. The needle from this point on, until the completion of the entire mixing process, is held with the thumb and index finger of the surgeon’s nondominant hand; once the needle is inserted into the vitreous cavity, no further adjustment to the syringe’s position should be made. (b) The vitreous is entered with the needle, which is aimed toward the center of the cavity. From this point on, until the completion of the entire mixing process, the surgeon’s gaze is fixed on the tip of the needle – he never looks up from the microscope. (c) 1.5 ml of air is withdrawn, monitored by the nurse who announces the progress at every 0.5 ml interval. (d) With the needle’s cone securely fixed, the surgeon removes the air-filled needle and replaces it with a syringe that contains exactly 2 ml of gas. Such accuracy is needed to allow easy determination of the amount injected. (e) 1.5 ml of the gas is injected; the plunger is pushed with the surgeon’s palm, not with his index finger, to increase his control over the injection. (f) With the non-dominant hand’s thumb continuing to provide extra support to the syringe, a finger-tap assures that the IOP is in the normal range as the gas injection nears its completion. If the eye is soft, a small amount of extra gas is injected

•Inject ~1.5 ml of the gas as the nurse announces the progress. With your nonworking hand’s index finger tap on the eye to make sure the IOP is in the normal range.

Pearl

If a gaseous tamponade is used, the patient must be warned, both preand postoperatively, that it is normal to initially “see dark” through the bubble, but it is not normal to have severe pain. If there is significant pain, he must seek help immediately. Vision will improve soon, even while the gas bubble is still in the eye, although the interface may remain bothersome, especially when the gas-aqueous interface is right in front of the macula.

35.2.3 Gas Injection into the Nonvitrectomized Eye

Most commonly used in eyes undergoing pneumatic retinopexy (see Sect. 54.6), the pure gas compresses the gel10 and may induce PVD.

35.2.4 The Eye with Gaseous Tamponade

Even if the implanted gas is of a nonexpansile concentration, the gas (air) will expand if the atmospheric pressure drops. Patients with air/gas in their vitreous cavity should not fly or be exposed to an increase in elevation exceeding a few hundred meters.11 The acute rise of the IOP, should these warnings be neglected, can lead to blindness.

Q&A

QWhat if a patient needs a gas tamponade but must travel by airplane, lives on a mountain, or has to travel through a mountain?

ASilicone oil must be used, not gas – and this needs to determined preoperatively.

35.3PFCL12

35.3.1 Indications to Use Heavier-Than-Water Liquids

• Flipping the inverted flap of a giant retinal tear.

10Hence the high (up to 30%) rate of secondary retinal tears: the traction is weakened at where the gas bubble is but may occur elsewhere.

11In other words, it is the change in altitude that matters, not the absolute height above sea level.

12It is almost never used as a true tamponade, but for didactic reasons it makes sense to discuss PFCL use here.