Module 14
Advanced Airway Techniques
Part 2 - New Generation Supraglottic Ventilatory Devices
Desirable Features and Optimal Methods for Testing
DESIRABLE FEATURES OF SUPRAGLOTTIC AIRWAYS - con't
The ideal airway device causes no
trauma to the airway. With existing devices the incidence of
trauma to the airway, as evidenced by blood visible on the
device, ranges from almost 0% to more than 50%. Supraglottic
airways commonly cause sore throat, dysphonia and dysphagia, but
these symptoms are usually minor and transient. The possibility
of nerve injuries is of greater concern. The ideal airway will
minimize or eliminate both of these problems. However, the
intracuff and mucosal pressures vary in intensity and location
with different supraglottic airways. The incidence of sore
throat varies with different devices from below 10% to above
40%. Clinically significant nerve injury is rare with all
supraglottic airways and the relative risks of each device are
not known.
In addition to its role in
maintaining the airway during anesthesia, a supraglottic airway
may usefully enable access to the airway. This may help with
the treatment of some intraoperative complications and may help
in the management of the difficult airway. The ILMA is designed
specifically for these roles. There are several techniques for
its use. An ET can be passed blindly through a cLMA but it
requires a long, narrow tube and some luck. Light-guided,
fiberscope-guided and catheter exchange techniques are better
but they require a short supraglottic airway that is wide enough
throughout its length. Importantly, the larynx must be reliably
visible from the airway orifice. Supraglottic airway devices
vary in length and diameter. The proximal orifice of some will
accept tubes and fiberscopes of only 5.0 mm external diameter
while others accommodate ETs of 8.5 mm internal diameter. At
their distal end, grills, bars, small orifices and difficult
angles may impede or prevent access to the trachea. The ability
to view the laryngeal inlet from the airway orifice ranges
between devices from above 90% to below 40%.
While most devices will be used
by experienced anesthesiologists, supraglottic airways may be
used by the inexperienced for anesthesia, during out of hospital
rescue, or for resuscitation. The ideal airway will therefore
be intuitive to use, have a high success rate for the naive user
and have a short learning curve. For most devices, these data
are not available but what little there is suggests that
insertion and airway maintenance by non-anesthesiologists and by
naïve users, varies considerably among devices.
Finally, there is an implicit
assumption that reusable devices may be replaced by cheaper,
single-use devices and some believe that single-use devices are
intrinsically preferable. However, many single-use devices
differ from the reusable devices they seek to replace, both in
design and in the materials used. Some of these modifications
appear minor but the implications on performance have not been
evaluated. Work on single-use laryngoscopes39
and intubation stylets35
provide evidence that changes in product material may alter
performance considerably. Data on the current versions of the
single use cLMAs and comparisons between these and the reusable
LMA are not yet available.
No single device meets all the criteria
for the ideal supraglottic airway. Indeed, some of these
criteria are mutually exclusive. For instance, a device that is
large enough to accommodate an adequately sized ET is unlikely
to be as easily inserted as a smaller device. Epiglottic bars
reduce airway obstruction but hinder instrumental access to the
trachea. A single-use device is less likely than a more
expensive reusable device to be made of the best materials to
optimize handling characteristics and minimize pharyngolaryngeal
trauma. Therefore it is likely that several different airways
will always be needed for use in different clinical situations.
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