Gas Selection

Intraocular gas works by temporarily closing retinal breaks via the surface tension properties of the bubble meniscus. Blocking the movement of liquid vitreous into the subretinal space allows the retinal pigment epithelium to actively pump fluid from the sub-retinal space and flatten the detachment. Once the neurosensory retina is in apposition to the pigment epithelial layer, the adhesive properties of cryopexy or laser retinopexy permanently close the break(s). The most commonly utilized gases are air, sulfur hexa-fluoride (SF6), and perfluoropropane (C3F8). Choice of gas is based upon volume issues, arc length of contact/bubble size requirements, and bubble duration (Table 4.1) [7]. The tamponade must last until the laser or cryopexy adhesion is strong enough to resist reopening - generally 3-5 days for laser and 5-7 days for cryopexy. Air is non-expansile and quickly disappears from the eye. Sulfur hexafluoride and C3F8 are expansile and have longer half-lives. In general, a 1-ml final bubble size tamponades a 120° arc length, which is sufficient for most cases of PR (Table 4.2) [8]. There is, however, considerable variation based on the axial length/ size of the globe, so that larger bubbles are required in myopic individuals to achieve the desired arc length of contact.

Table 4.1. Gas characteristics (from [7])

Expansion

Half-life Isoexpansile

Standard volume

(days)

concentration

(pure gas)

Air 0

1

100%

0.5-1.0 ml

SF6 2.0-2.5x

2.5

18%

0.5 ml

C3F8 4x

4

14%

0.3 ml

Table 4.2. Bubble size/tamponade arc. Arc varies with axial length/size

of globe (from [8])

Final bubble size

Arc of contact

0.30 ml

90°

1.0 ml

120°

3.0 ml

180°

The volume of the initial injection is a key consideration in gas selection. Intravitreal injections of 0.1-0.2 ml usually result in modest IOP rises, but increasing the initial volume beyond this point dramatically increases pressure. Paracentesis provides potential space within the globe, but is usually limited to 0.1-0.2 ml. As a result of these considerations, the authors routinely use 0.25 ml of pure (100%) C3F8 gas, which provides a small initial injected volume and a large final volume (~i ml) sufficient to tamponade most breaks.

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