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.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.