During pars plana vitrectomy for rhegmatogenous retinal detachment repair, SF6 gas is used as a tamponade agent. At what percentage concentration (in air) should SF6 be injected to achieve iso-expansile concentration (no net expansion or contraction)?

• A 14% SF6
• B 25% SF6
• C 50% SF6
• D 20% SF6 ✓

Explanation

SF6 (sulphur hexafluoride) is iso-expansile at 20% concentration in air. Below this, the gas contracts; above this, it expands as nitrogen diffuses in. C3F8 (perfluoropropane) is iso-expansile at 14%. Air is not expansile. Knowing iso-expansile concentrations prevents dangerous IOP spikes from over-expansile gas. Patients must be warned against air travel and nitrous oxide anaesthesia until gas resorbs.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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