Ординатура / Офтальмология / Английские материалы / Retinal and Vitreoretinal Diseases and Surgery_Boyd, Cortez, Sabates_2010

Air-Fluid Exchange in an

Operating Room Setting

Air or gas is used intraoperatively after vitrectomy. Air enters the eye through an infusion cannula (Figure 3), which is connected to the air pump of the vitrectomy system. Generally, the infusion line for irrigation solution and air tubes are connected by a three-way stopcock to the infusion cannula sutured at the pars plana site. Peyman developed an illuminated switch on the air pump that indicates whether the air or infusion fluid line is open.54 Because the system is independent from the room illumination,

the operating room nurse can activate the air or the infusion fluid line by pressing the switch without disturbing the dark adaptation of the surgeon. In the absence of this system, the surgeon must always be cognizant of the status of the infusion line (air, fluid, or off).

Air-exchange can be combined with internal drainage of subretinal fluid (Figure 4).54 The air pump pressure is set at approximately 50 mm Hg. The air pump maintains the IOP automatically by increasing the flow of the air inside the eye to offset leakage of any air through the sclerotomies. Air enters the eye through the infusion cannula while

aspiration of fluid in the anterior vitreous cavity is performed with either a fluted needle or extrusion handpiece. The blunt or tapered needle, called an extrusion handpiece, is usually connected by a polyethylene tube to the vitrectomy console, which provides vacuum. The needle is vented externally through an opening on the body of the instrument similar to some disposable vitreous suction cutter handpieces. Aspiration of fluid vitreous or subretinal fluid is performed when suction is applied to the tube and the operator’s index finger is used to close the aperture on the body of the instrument. When the aperture is opened, air is sucked into the system,

and the suction becomes discontinuous. If the suction line is completely clamped, the suction needle functions like a flute needle. In such a situation, the IOP forces the fluid from the vitreous cavity through the needle and out of the opening on the handle of the instrument.55

Initially, multiple bubbles enter the eye, coalescing into one bubble almost immediately. The needle is then gradually moved toward the posterior pole when a posterior retinal tear is present or toward a prepared retinotomy site. Subretinal fluid is pushed posteriorly as the air-fluid exchange progresses56 and is evacuated using a flute needle.57

Drainage of subretinal fluid through a retinotomy can be performed with an extrusion cannula, which is blunt tipped, tapered, or has a soft silicone tip or extension.58 The cannula, attached to an automated suction, is placed slightly anterior to the retinal hole until the subretinal fluid is almost completely removed. At this point, a meniscus of fluid is touched by the cannula. This maneuver is repeated until reattachment of the retina has occurred. When the fluid falls below the level of the cannula tip, a bright reflex is observed as the needle touches the fluid, which helps avoid direct contact between the needle tip and RPE. The positive pressure created by the infusion of gas and controlled linear suction applied to the cannula results in the removal of subretinal fluid via the suction needle.56

The retina must be carefully observed during the air-fluid exchange and the internal drainage of subretinal fluid. When areas of epiretinal traction exist, forceful air injection may cause existing tears to enlarge or may create large tears at the site of preexisting traction. Failure of the retina to flatten or the presence of subretinal air indicates residual traction that must be relieved before the retina will reattach. Subretinal air can migrate anteriorly and may not easily be removed. During air-fluid exchange, residual areas of traction become identifiable. Subsequent visualization of the retina after gas-fluid exchange requires a corresponding change in the viewing lens because of the difference in refractive indices between fluid and gas.

Air-Fluid Exchange in an

Outpatient Setting

Air-fluid exchange in an outpatient setting following vitrectomy is indicated for recurrent vitreous hemorrhage, to remove intravitreal blood. This procedure facilitates retinal examination, allows management of possible complications, prevents unnecessary delay in visual recovery, and may protect against re-bleeding in some instances.

Stern and Blumenkranz59 recommend the use of a 20% mixture of SF6 and air. Two milliliters of pure SF6 are placed in a 10-mL syringe. Sterile tubing, connections, and a 0.22-μm Millipore filter are attached to the gas tank to ensure sterility; 8 mL of air is withdrawn into the syringe to provide the desired concentration of gas. Gas is then expelled from the syringe until 5 mL remains. The patient is placed in a prone position in bed with the chin supported by a pillow. Following topical anesthesia, a wire lid speculum is placed in the patient’s eye. Using light from either an indirect ophthalmoscope or a flashlight held by an assistant, the surgeon stabilizes the eye with a cotton-tipped applicator, and air-fluid exchange is done through the limbus in aphakic patients.

We have found that patient stability and surgical control are achieved by using the slit-lamp chin rest to support the head. The slit-lamp light beam is used to illuminate the eye. Two acetaminophen tablets are given

30 minutes before the procedure to reduce or eliminate ocular discomfort. After the application of topical anesthesia and prior to the procedure, the cul-de-sac is irrigated first with a 10% povidone-iodine solution and then with sterile balanced saline solution. A drop of topical gentamicin is placed on the cornea, and the eye is entered through the temporal limbus with a 27-gauge, needle attached to the syringe. Before it enters the eye, the needle track is beveled to prevent escape of intraocular gas and the production of hypotony when the needle is withdrawn from the eye (Figure 5).

The needle is placed slightly below the pupil, fluid is withdrawn, and gas is injected in 0.3 mL increments. Wide variations of IOP must

be avoided to prevent severe ocular pain. Fluid collects in the bottom of the syringe. When the anterior chamber begins to fill with gas, the needle is moved inferiorly, and additional fluid is withdrawn and replaced with air. When the eye has been filled with gas, the needle is withdrawn. If hypotony is present at the end of the procedure, the IOP is re-established by injecting a 20% mixture of SF6 and air through the limbus with a 30-gauge, 0.5-inch needle in aphakic patients. The same procedure is performed through the pars plana in phakic eyes.

Retrobulbar anesthesia is used to perform this procedure in phakic eyes. The patient is placed in a prone position in bed, with the head tilted toward the side of the involved eye. Following insertion of the lid speculum, the

Figure 5: Schematic Representation of Gas-Fluid Exchange through the Limbus. Arrow indicates back-and- forth motion of the 5cc syringe piston for withdrawal of fluid and injection of gas. (Art from Jaypee Highlights Medical Publications).

needle is inserted through the temporal pars plana 4 mm behind the limbus in a shelved (self-sealing) fashion. When the needle is visualized behind the lens, the exchange is started. The needle is gradually retracted as gas accumulates in the vitreous cavity. The procedure is terminated when the vitreous cavity is filled with air, which begins to enter the syringe. Following the procedure, IOP is measured by applanation tonometry. A drop of gentamicin is placed on the eye and the

IOP is recorded 4 to 6 hours later and again the following morning.59

Positioning of the patient may vary during this procedure.60,61 The patient may be placed in a face-down position in bed or on a stretcher with the head resting on the arms, which are folded across a pillow.

In order to avoid the phenomenon of “fish eggs,” the needle is positioned so that its tip is inside the gas bubble before injection continues. This causes the bubble to enlarge with additional gas but is only applicable when two needles are used, one for injection of gas and the other for withdrawal of fluid. Careless maneuvering of the needle to place the tip within the preexisting bubble may result in inadvertent contact between the needle and lens with subsequent cataract formation.60

After the needle has been withdrawn, we usually apply a cotton swab with gentamicin ointment to the needle tract to prevent the escape of intraocular gas. Topical gentamicin drops are applied after the procedure is finished. Replacement of a needle prematurely withdrawn before completion of the procedure should be done through a separate site because insertion through the initial tract may stretch this opening and result in subsequent leaks.60 When iris becomes incarcerated in the inner corneal wound, gentle massage with a blunt instrument over the external cornea in the area of the incarceration usually releases the tissue.62 When removing blood, a 30-gauge needle may become obstructed; a 25-gauge needle creates too large a tract in the cornea, which may subsequently leak; utilization of a 27-gauge needle may be preferable.62

Nitrous Oxide Anesthesia

The use of nitrous oxide (N2O) should be discontinued within 20 minutes of intraocular injection of a gas bubble to allow depletion of N2O in significant concentrations from the periocular tissues.63,64 N2O is extremely soluble in the bloodstream, allowing rapid diffusion of the gas into the bubble resulting in dramatically increased IOP in the immediate postoperative period. After N20 anesthesia is stopped, the N20 collected in the intraocular gas bubble then diffuses back into the bloodstream, leading to subsequent hypotony.

Flying and Intraocular Gases

The majority of aircraft decompress cabin pressure to less than 8000 feet after reaching cruising altitude in an average of 27 minutes.28 A 10% gas bubble can be safely tolerated through a decompression to 8000 feet above sea level.27 Larger gas bubbles are tolerable at lower altitudes.65 Pre-flight medication with topical glaucoma agents has not been found to be helpful and can result in choroidal effusion after the plane returns to sea level. Prudence dictates that the patient be properly informed of the risks of flying before surgery is performed and that flying be avoided in the presence of significant residual bubble.

Similar precautions must be taken prior to SCUBA (self-contained underwater breathing apparatus) diving (or hyperbaric oxygen therapy). As Thompson noted, although the eye may become hypotonic during the dive, it is the rapid ascent to the surface that can result in very elevated IOP.36

Conclusions

Intraocular gases are extremely versatile and powerful tools for the vitreoretinal surgeon. Gases are able to plug retinal holes, provide internal tamponade for retinal breaks to create a firm chorioretinal adhesion, and are an invaluable intraoperative device for maintenance of the geometry of the globe and to facilitate visualization and manipulation of the retina. Each gas has unique characteristics which makes it useful for specific clinical situations. Clearly a thorough understanding of the properties and applications of these gases is needed to make an informed clinical decision regarding their use.

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Perfluorocarbon liquid (PFCL) is colourless and odourless, and has a high specific gravity and low viscosity.

First used extensively during World War II (the Manhattan Project) as handling agents for the uranium that was to become the world’s first atomic bomb, then as an artificial blood substitute, and finally as a sophisticated surgical tool in the management of complex retinal disorders, the physical and chemical properties of perfluorocarbon liquids have fascinated and aided the worlds of science and medicine for over 40 years.

The intraoperative use of PFCL in vitreoretinal surgery was introduced in 1987 by the author for the treatment of giant retinal tears, retinal detachments with proliferative vitreoretinopathy (PVR), and traumatic retinal detachments.1–6 PFCL has also been used to reposition dislocated crystalline lenses and implanted intraocular lenses (IOLs).7–10 and when a subretinal haemorrhage is removed, for example during surgery for complicated age related macular degeneration.11,12

CHEMICAL AND PHYSICAL PROPERTIES

Chemical Properties

Perfluorocarbons are fully fluorinated analogues of hydrocarbons. They form a special family of compounds whose chemical and physical properties can be most closely compared to those of inert gases. Their chemical formulas give us an indication as to their physical state. Those with 1 to 4 carbon atoms are gases. The straight-chain compounds (alkanes), C6 to C9 are liquids at room temperature, and most of the cyclic compounds in the C6 to C10 range are liquids, butwithincreasingviscosities. Perfluorocarbon liquids are colorless, odorless, non-flammable, high specific gravity (1.6 to 2.1) materials, which are chemically and biologically inert when pure. They are stable to temperatures as high as 400 to 500° Celsius.

The term “fluorocarbon” is often incorrectly used in literature to describe chlorofluorocarbon refrigerant and propellant gases (CFC’s). These materials have heteroatoms, mainly other halogens, and differ from the perfluorocarbon compounds, which contain exclusively carbon and fluorine atoms. The environmental impact associated with CFC’s is not present in perfluorocarbons.

Various types of perfluorocarbon liquids have been manufactured for use as coolants, dielectric fluids, and test fluids in the manufacture and operation of electronic devices. These perfluorocarbons are not of sufficient purity to be chemically or biologically inert. Purified compounds such as perfluorodecalin, perfluorotripropylamine and perfluorooctyl

bromide have been used in biological/medical applicationsas syntheticoxygen carriers(blood substitutes), and as X-ray and NMR imaging agents, yet in the case of cyclic compounds such as perfluorooctyl bromide, they remain chemically active.

Over many years, stimulated by the work of the author, there has been a concerted worldwide effort to investigate the potential and utility of perfluorocarbon liquids as a surgical adjunct during vitreoretinal surgery. The interest is due to several of the unique physical and chemical properties of the compounds, and has centered primarily on perfluoro-n-octane, perfluorodecalin, perfluorophenanthrene and perfluorotributylamine (Figure 1).