INTRAOCULAR GAS EXPANSION
A gas bubble may be injected by the ophthalmologist into the posterior chamber during vitreous surgery. Intravitreal air injection will tend to flatten a detached retina and allow anatomically correct healing. The air bubble is absorbed within 5 days by gradual diffusion through adjacent tissue into the bloodstream. The bubble will increase in size if nitrous oxide is admin-istered, because nitrous oxide is 35 times more solu-ble than nitrogen in blood . Thus, it tends to diffuse into an air bubble more rapidly than nitrogen (the major component of air) is absorbed by the bloodstream. If the bubble expands after the eye is closed, intraocular pressure will rise.
Sulfur hexafluoride is an inert gas that is less soluble in blood than is nitrogen—and much less soluble than nitrous oxide. Its longer duration of action (up to 10 days) compared with an air bubble can provide a therapeutic advantage. The bubble size
doubles within 24 hr after injection, because nitrogen from inhaled air enters the bubble more rapidly than the sulfur hexafluoride diffuses into the bloodstream. Even so, unless high volumes of pure sulfur hexafluo-ride are injected, the slow bubble expansion does not typically raise intraocular pressure. If the patient is breathing nitrous oxide, however, the bubble will rapidly increase in size and may lead to intraocular hypertension. A 70% inspired nitrous oxide concen-tration will almost triple the size of a 1-mL bubble and may double the pressure in a closed eye within 30 min. Subsequent discontinuation of nitrous oxide will lead to reabsorption of the bubble, which has become a mixture of nitrous oxide and sulfur hexa-fluoride. The consequent fall in intraocular pressure may precipitate another retinal detachment.
Complications involving the intraocular expansion of gas bubbles can be avoided by dis-continuing nitrous oxide at least 15 min prior to the injection of air or sulfur hexafluoride, or by avoid-ing the use of nitrous oxide entirely. The amount of time required to eliminate nitrous oxide from the blood will depend on several factors, including fresh gas flow rate and adequacy of alveolar ventilation. Depth of anesthesia should be maintained by substi-tuting other anesthetic agents. Nitrous oxide should be avoided until the bubble is absorbed (5 days after air and 10 days after sulfur hexafluoride injec-tion). Many ophthalmologists routinely request thatnitrous oxide not be used in their patients.