The Slower the Better: On the Instability of Gas Jets in a Model of Pneumatic Retinopexy

PURPOSE. To investigate the effect of injection technique parameters on the formation of multiple gas bubbles in a porcine eye model for pneumatic retinopexy. METHODS. Three hundred twenty-four adult porcine eyes were injected with 0.4 mL of C3F8 with variations in the depth of injection, speed of injection, and size of needle bore. The number of gas bubbles in the eye was assessed with indirect ophthalmoscopy. RESULTS. Shallow injections resulted in a higher incidence of a single bubble than did deep injections (P Ͻ 0.001; Fisher exact and Wilcoxon rank sum tests). Slow injections were significantly advantageous in producing a single gas bubble during shallow as well as during deep injections (P Ͻ 0.001, Fisher exact and Wilcoxon rank sum tests). With a shallow needle insertion, the slow speed of injection produced a single bubble in 75.9% of the eyes, whereas moderately brisk injections resulted in one bubble in 50.9% of the eyes. During deep needle insertion, 44.4% of the eyes had one bubble if the gas was injected slowly and only 11.1% had a single bubble with moderately brisk gas injections. The bore of the needle did not significantly change the number of bubbles during deep or shallow injections. CONCLUSIONS. The factors that were found to be important in reducing the formation of multiple gas bubbles in the eye were shallow depth of injection and slow speed of gas delivery. (Invest Ophthalmol Vis Sci. 2007;48:2734 -2737) DOI:10.1167/ iovs.06-1384 P neumatic retinopexy (PR) has been established as a treatment modality for rhegmatogenous retinal detachment (RD). This outpatient procedure consists of transconjunctival cryopexy with intravitreal gas injection, followed by head positioning. Alternatively, laser photocoagulation can be applied around the tear instead of cryopexy when the retina is attached 1 or 2 days after the initial gas injection. One potential complication of PR may occur if small bubbles ("fish eggs") are formed in the vitreous cavity during gas injection. This occurrence may facilitate migration of one of the bubbles into the subretinal space. 1,2 Subretinal gas in the vicinity of the tear will prevent sufficient chorioretinal scarring, leading to the persistence of the RD. 3 In addition, the subretinal gas bubble can shift and detach more areas of the retina, including the macula. 5-7 We designed an experiment to test the effect of injection technique parameters on the formation of multiple gas bubbles in a porcine eye model. The effect of the depth of injection, speed of injection, and the needle bore on the formation of gas bubbles was studied. METHODS Three hundred fifty adult porcine eyes were purchased (Clougherty Packing Co., Los Angeles, CA). Of the 350 eyes, 26 were not used because of either corneal scars (22 eyes) or ruptured globe (2 eyes). Each study eye was mounted in an artificial head, and secured with pins to prevent movement. A volume of 0.4 mL perfluoropropane (C3F8) gas (Alcon Surgical, Fort Worth, TX) was placed in a 1.0-mL tuberculin syringe, fitted with a 25-, 27-, or a 30-gauge needle (BD Biosciences, Franklin Lakes, NJ). To make the injection site uppermost, the artificial head was placed supine with a 45°tilt. The needle was then passed into the eye perpendicular to the sclera, on the temporal side 3.0 mm posterior to the limbus. Injection of the entire volume of gas was performed, and the needle was withdrawn from the eye with the plunger held down. The depth of the needle during gas injection was approximately 3 mm (one-fourth needle length) for the shallow injections (after initial insertion of 6 to 8 mm to penetrate the anterior hyaloid) and 12 mm for the deep injections (full needle length). The speed of injection had two variations. Fast injections were performed in a moderately brisk fashion. The injection was given smoothly and quickly but not with excessive force. Slow injections, in contrast, were performed in a period of 8 seconds (timing by counting "one-one thousand, two-one thousand. . . ,"). Immediately after the gas injection, an anterior chamber paracentesis was performed with a 30-gauge needle mounted on a 1.0-mL syringe without a plunger, to reduce the pressure associated corneal edema. An indirect ophthalmoscope with a 20-D lens was used to assess the number of gas bubbles in the vitreous cavity. We performed a total of 324 gas injections: 216 shallow injections and 108 deep injections. Of the 216 shallow injections, 108 injections were performed slowly and 108 were delivered in a moderately brisk fashion. At each injection speed, 36 injections were made with each needle size (25-, 27-, and 30-gauge). Of the 108 deep injections, 54 injections were performed slowly and 54 were given briskly. For every injection speed, 18 injections were performed with each needle size (25-, 27-, and 30-gauge). RESULT