General Anesthesia
General anesthesia for ear surgery requires attention to preservation
of the facial nerve, the effect of N2
O in the middle ear, extremes of
head positioning, the possibility of air emboli, blood loss, nausea, and vomiting.
Positioning of the Patient
Avoid extremes of neck extension or head torsion. Injuries can
occur to the brachial plexus or cervical spine (see Chapter
28
). Patients with limited carotid artery blood flow are especially vulnerable
to further decreases in blood flow from exaggerated neck positions.
Preservation of the Facial Nerve
Preservation of the facial nerve is more easily accomplished and
confirmed if the patient is not totally paralyzed. If a muscle relaxant is used,
the effects should be monitored to ensure that at least 10% to 20% of muscle response
remains. Otologic surgical procedures are associated with a 0.6% to 3.0% incidence
of facial nerve paralysis. Intraoperative monitoring of evoked facial electromyographic
activity may assist in functional preservation of the facial nerve during surgery.
[245]
Nitrous Oxide and Middle Ear Pressure
The middle ear and paranasal sinuses consist of open, nonventilated
spaces. The middle ear space is vented intermittently when the eustachian tube is
opened. When inhaled in high concentrations, N2
O enters the air cavities
faster than nitrogen can leave. The result is an increase in pressure.[246]
After discontinuation of N2
O, the gas is rapidly reabsorbed.
Sustained, marked, negative middle ear pressure may develop. Such pressure may
contribute to the development of serous otitis, disarticulation of the stapes, and
impaired hearing.[232]
Patterson and Bartlett[247]
reported hearing impairment caused by hematotympanum and
disarticulation of the stapes struts. N2
O anesthesia may be hazardous
to hearing in patients who have previously undergone reconstructive middle ear surgery.
Transient worsening of middle ear function, rapid increases in
middle ear pressure proportional to the inhaled concentration of N2
O,
nausea and vomiting, and rupture of the tympanic membrane have all been associated
with elevated middle ear pressure and abnormal eustachian tube function. Susceptible
patients include those with previous otologic surgery, acute or chronic otitis media,
sinusitis, upper respiratory tract infection, enlarged adenoids, and pathologic conditions
of the nasopharynx.
A bulging eardrum and "lifting off" of the tympanic membrane graft
can occur during tympanoplasty surgery. There is no evidence that using N2
O
(50%) for general anesthesia for type 1 tympanoplasties will interfere with graft
placement or outcome of the surgical procedure.[248]
The anesthetist should limit the concentration of N2
O to 50% and discontinue
administration 15 minutes before closure of the middle ear. The decrease in pressure
can be avoided by flushing the middle ear cavity with air before closure of the tympanic
membrane.
