Awake tracheal intubation with the aid of a fiberoptic
device was first described by Murphy in
1967, [5] who used a choledochoscope
to facilitate nasotracheal intubation in patients with difficult airway.
Since then, numerous subsequent authors have described the
anesthetic techniques and experiences with awake FOB guided intubation. It
offers several advantages over use of FOB after induction of general anesthesia
in patients with cervical spine instability:
Patient remains in a neutral
position, minimizing the risk of neurological deterioration;
patient's
neurological status can be assessed after intubation, and
spontaneous
ventilation is preserved. [3]
There are multiple ways of anesthetizing the airway to
facilitate the performance of awake FOB guided intubation. Among them, topical
anesthesia with nebulized LA, gargles,
lozenges, sprays, airway blocks and LA through the working channel of FOB
is commonly used.
Administration of lignocaine through nebulization for
anesthesia of upper airway and larynx has also been previously studied. In
their study Cullen et al. [6] found that lignocaine
nebulization decreased the discomfort of nasogastric tube insertion.
In 2007, Techanivate et al. [7] found adequate upper airway
anesthesia with 2% lignocaine
nebulization and topical cocaine application to the nose for fiberoptic
nasotracheal application.
In our study, the time
taken to perform FOB guided intubation was significantly more in the
nebulization group as compared to the nerve blocks group.
The topical anesthesia group received 20 ml of 4% lignocaine via nebulization followed by a 3 ml transtracheal injection.
On the other hand, the nerve block group received bilateral glossopharyngeal and superior
laryngeal nerve blocks along with the transtracheal injection.
Kundra et al. [8] also compared two methods of anesthetizing
the airway for awake fiberoptic nasotracheal intubation. One of the groups
received 4 ml of 4% lignocaine
through nebulization and the other received airway blocks (translaryngeal, bilateral superior laryngeal and lignocaine soaked
cotton swabs in the nose).
Although the time taken to intubate was similar in both
groups, patients who received lignocaine nebulization for airway anesthesia had
to undergo significantly higher stress during the insertion of endotracheal
tube through the glottis.
The grimace scores
as well as the mean HR and BP in the nebulization group were significantly
higher during endotracheal tube insertion.
Patient comfort was
better in the nerve blocks group as compared with the nebulization group in
our study, as deduced by the coughing/gagging episodes as well as the patient
assessment of procedure recall.
These findings are similar to those reported by Graham et
al. [9] in 1992. They compared three different methods to provide airway
anesthesia during FOB. All patients received benzocaine lozenges, lignocaine sprays for posterior pharynx and lignocaine
jelly for nasal passages along with either 4 ml of 2.5% cocaine injection through FOB working channel,
transtracheal injection of the same amount of cocaine or nebulized 4 ml of 4%
lignocaine.
They reported that the transtracheal injection of cocaine
provided significantly superior patient comfort and less coughing episodes as
compared with the rest of the techniques. The findings reported by Reasoner et
al. [3] were also similar. Although, there was no difference in the number of
coughing/gagging episodes between the two study groups, patient recall of the procedure was more in the nebulization group.
Kundra et al. [8] also reported higher grimace scores, mean HR and BP during insertion of
endotracheal tube in patients who received lignocaine via nebulization as
compared to nerve blocks.
In our study, vocal cord visibility and ease of intubation
as assessed by the bronchoscopist were better in the nerve block group as
compared with the nebulization group. This finding is similar to that observed
by Graham et al. [9]
They reported that the bronchoscopist preferred
transtracheal instillation of LA as compared to LA nebulization or LA
instillation through the working port of FOB.
However, Reasoner et al. [3] did not find any difference in
the quality of airway anesthesia between nebulized LA and nerve blocks as
assessed by a blind observer/bronchoscopist.
Gal [10] reported that lignocaine
mist produced as an aerosol during ultrasonic nebulization causes airway
irritation in subjects as evidenced by coughing.
Later however, it results in bronchodilatation due to its membrane stabilizing action. No such
adverse effects caused by lignocaine mist were noted in our study.
The maximum total dose of lignocaine used in our study was 400 mg through nebulization. Such a
dose of lignocaine has been safely used and reported for FOB in many previous
studies.
In 1997, Parkes et al. [11] used 6 mg/kg of 10% lignocaine solution through nebulization mask for
fiberoptic intubation.
The serum lignocaine levels measured remained below the
accepted threshold of 5 mg/l at all
times (highest levels obtained were 0.45 mg/l).
Similarly, Langmack et al. [12] measured the serum lignocaine
levels in 51 asthmatic volunteers undergoing FOB with topical lignocaine. The
average total dose used was 600 mg (8.2
mg/kg), which was found to be safe in all patients as assessed by serum
lignocaine concentrations.
However, in 1993, Wu et al. [13] have reported seizures in a patient after administration
of a total dose of 300 mg of topical lignocaine during FOB. The serum
lignocaine concentrations were found to be well above the acceptable toxic
limits.
Hence, a constant lookout for signs and symptoms of
lignocaine toxicity is mandatory while using large doses.
The limitations of our study are that it is an unblinded
study allowing some amount of bias. Furthermore, serum lignocaine levels were
not measured due to nonavailability of this facility at our center.
Given the results of the study and the above discussion, the
following conclusions may be drawn. The performance of bilateral superior laryngeal and transtracheal recurrent laryngeal
nerve blocks provides adequate airway anesthesia to aid in awake FOB guided
intubation.
Furthermore, 10 ml of
4% lignocaine through ultrasonic nebulizer may not provide acceptable
conditions for bronchoscopy, but a higher dose might be able to adequately
anesthetize the airway.
However, a lower dose of lignocaine through nebulization
along with supplemental lignocaine instillation through the working channel of
FOB might provide adequate airway anesthesia.
More studies need to be performed to determine the amount of
lignocaine, which can be used for nebulization with serum lignocaine levels.
