What neuromuscular blocking agents are appropriate for the patient
who is at risk for loss of intraocular contents? How does one minimize the
deleterious side-effects of these agents in the presence of a rup-tured globe?
Selection of a neuromuscular blocking agent for
the patient at increased risk for both aspiration of abdominal contents and
extrusion of ocular contents has long been controversial. For many years,
succinylcholine, which is recognized to increase IOP by causing tonic
contractions of extraocular muscles, was thought to be contraindicated, and the
use of nondepolarizing agents was recommended. The dilemma occurred at a time
when only relatively long acting agents, such as pancuronium, d-tubocurarine,
and dimethylcurarine were available. Over the last 15 years, with the
introduction of the intermediate-acting agents, suitable substitutes have
become available. Caveats to the use of these agents under the circumstances of
this case suggest that very large doses (4 times the ED95) of these
drugs must be used to effect good intubating conditions. Consequently, there
may be a prolonged duration of neuromuscular blockade. Using larger intubating
doses of rocuronium offers intubating times and conditions similar to
succinylcholine but prolongs the duration of action to similar times as
vecuronium. Using similar doses of mivac-urium has a shorter duration of action
than rocuronium but mivacurium has a longer onset time that may be cru-cial to
securing the airway. In general, the issue of succinyl-choline and the open
globe is no longer controversial. Clinicians may rest assured that there are no
contraindica-tions to its use. Two separate studies by Libonati et al. (1985)
and Donlon (1986) demonstrated that succinylcholine offers ideal intubating
conditions with minimal risk. This is true provided that defasciculation with a
nondepolarizing neuro-muscular blocker and administration of lidocaine and
fen-tanyl were accomplished before intubation.
The ideal neuromuscular blocking agent will be
one that offers rapid onset, short duration, and absence of fas-ciculations.
Prolonged or difficult laryngoscopy will have a more deleterious effect on
raising IOP and subsequent extrusion of intraocular contents than the choice of
neuro-muscular blocking agent itself. It is important to remember during
preoxygenation that suboptimal placement of the facemask can result in pressure
on the globe, predisposing to extrusion of intraocular contents.
In this particular patient with a smoking
history, airway reactivity increases the risk of coughing during patient
management. The use of intravenous lidocaine and fentanyl will aid in cough suppression.
Equally important is the timing of laryngoscopy and endotracheal intubation,
which requires use of a neuromuscular blockade monitor. Although
laryngotracheal topical anesthesia may reduce airway reactivity, it is probably
ill advised. The act of spraying the larynx and trachea with local anesthesia
in the awake state may cause retching or coughing. Abolition of
laryn-gotracheal reflexes increases the risk of gastric content aspi-ration
before endotracheal intubation and after extubation in both the awake and
Intravenous induction agents, such as propofol,
thiopen-tal, and methohexital, reduce IOP. Narcotics, sedatives, and major
tranquilizers also lower IOP, as long as ventilation is controlled because
increases in PaCO2 will raise IOP.
Ketamine may cause nystagmus and blepharospasm,
resulting in suboptimal surgical conditions. Etomidate, which reduces IOP, may
cause myoclonus, which may ultimately result in an increase in IOP.
Additionally, both ketamine and etomidate have been associated with higher
rates of postoperative nausea and vomiting, which also detract from their
usefulness in the case of the ruptured globe.
As previously stated, any form of respiratory
acidosis will increase IOP, and respiratory alkalosis will decrease IOP. Normal
ranges of PaCO2 will have little effect on IOP. Interestingly,
metabolic acidosis actually reduces IOP, whereas metabolic alkalosis will
Hypoxemia and hyperthermia will lead to
increased IOP. Hypothermia will reduce IOP.