A 5-year-old boy was shot with a BB gun at a family barbecue. He is otherwise healthy and previously experienced an uncomplicated general anesthetic for bilateral myringotomy and tube placement.
How is normal intraocular pressure maintained?
Normal intraocular pressure (IOP) is 10–20 mm Hg with a difference of up to 5 mm Hg between the two eyes. Diurnal variation results in 2–3 mm Hg higher IOP each morning owing to mydriasis, supine position, and eyelid pressure during sleep. In addition, the supine position can increase IOP by 1–6 mm Hg. IOP is dependent on aqueous humor dynamics (most important factor), choroidal blood volume, central venous pressure (CVP), and extraocular muscle (EOM) tone.
Aqueous humor dynamics
The eye is divided into anterior and posterior chambers by the iris. The eye has 250 μL of aqueous humor; one third of the aqueous humor is in the anterior chamber, and two thirds is in the posterior chamber. Aqueous humor is necessary for proper light refraction, allowing for vision. Additionally, it delivers oxygen and glucose to the avascular lens and cornea.
Aqueous humor is produced by diffusion, filtration, and active secretion from choroidal capillaries in the ciliary process located in the posterior chamber. It flows from the posterior chamber, around the iris, and into the anterior chamber. Elimination occurs through the space of Fontana and Schlemm canal at the iridocorneal angle. Changes in intraocular fluid dynamics can cause significant alterations of IOP because the globe is essentially noncompliant and surrounded by the rigid orbit. Decreases in cross-sectional area of the space of Fontana and Schlemm canal, as occur with mydriatic drugs, increase IOP. Even arterial pulsations cause measurable IOP changes.
Choroidal blood volume
Sudden increases in systemic blood pressure and CVP increase choroidal blood volume, resulting in elevated IOP because of the comparatively slow elimination process. Although choroidal vessels do not autoregulate, the factors altering cerebral blood flow affect blood volume in the eye. Vasodilation occurs with hypercarbia, hypoxemia, and increases in metabolic rate. Similar to cerebral blood flow, there is a linear relationship between arterial partial pressure of carbon dioxide (PaCO 2 ) and blood flow. Hypoventilation, with the resulting elevation in PaCO 2 , increases IOP.
Central venous pressure
Significant increases in CVP can temporarily inhibit the flow of aqueous humor through the Schlemm canal causing an increase in IOP.
Extraocular muscle tone
EOMs differ from skeletal muscles. Compared with skeletal muscles, EOMs can rapidly and precisely control contraction and relaxation and resist fatigue. Skeletal muscles are innervated by a single axon to the midbelly of each fiber. EOMs have single and multiple neuronal innervations allowing for focal contractions. Increased EOM tone increases IOP.
What are the anesthetic considerations for repair of open eye injuries?
The primary anesthetic goal for repair of open eye injuries is avoidance of sudden increases in IOP and the resultant extrusion of vitreous humor. Increases in IOP may be seen with increased venous and arterial pressure, coughing, bucking, Valsalva maneuver, increased PaCO 2 , and the supine position. The following maneuvers can reduce or prevent increases in IOP:
Increase venous drainage by elevating the head
Prevent increases in IOP
Minimize preoperative sedation to avoid increases in PaCO 2 secondary to sedation and hypoventilation
Minimize hemodynamic response to laryngoscopy and intubation by ensuring deep anesthesia before airway manipulation
Hyperventilate before intubation and during surgery
Minimize bucking and coughing during laryngoscopy and intubation by providing adequate depth of anesthesia and muscle relaxation
Use anesthetic techniques designed to provide smooth emergence without coughing or bucking while the endotracheal tube is still in place
To allow for unobstructed access to the surgical field, oral RAE tubes are popular choices for securing the airway. Supraglottic airways, such as flexible laryngeal mask airways, have also been used. Because airways are inaccessible during eye surgery, care should be taken to secure the artificial airway adequately.
Box 40-1 outlines considerations during various phases of general anesthesia.