Module 14
Advanced Airway Techniques
Part 2 - New Generation Supraglottic Ventilatory Devices
Desirable Features and Optimal Methods for Testing
SUGGESTED NEW METHODS OF ASSESSMENT
New airway devices, indeed all
new equipment, should undergo mandatory assessment of
manufacturing quality and clinical performance before
marketing. The function of supraglottic airways should be
evaluated in bench models and in vivo. The characteristics of
the ideal supraglottic airway, outlined above, provide a
checklist against which function can be assessed. A three stage
performance evaluation of new devices would be best.
Stage 1.
Bench evaluation using mannequins or models designed to test
function and safety.
Stage 2*.
A rigorous cohort study to determine whether the device is effective and safe.
Stage 3*.
A randomized controlled trial against the current gold standard
for the procedure the new device is expected to be used for (cLMA, pLMA, ILMA).
* Stages 2 and 3 require
ethical approval and written patient consent.
1. Stage 1: Bench Evaluation
Bench models include airway
mannequins and others such as those specifically designed to
test aspiration risk.38
The problem with all these non-clinical models is determining
whether function in models mimics that in patients. The
existing mannequins are not designed for this role and the
performance of supraglottic airways in them is not
representative of their performance in patients. With the
increasing use of supraglottic airways during resuscitation,
during out of hospital rescue, and by non-anesthesiologists,
there is an urgent need for realistic mannequins to be developed
for testing and training. Several companies are developing
these mannequins, which hopefully will enable reliable bench
testing of supraglottic airway devices. Functions that can be
tested with these mannequins include: ease of insertion,
laryngeal seal, airway resistance, stability of the device in
different head and neck positions, ease of passage of a gastric
tube, positioning of the airway over the larynx, and suitability
for fiberscopic or catheter exchange techniques. Learning
curves and use by non-anesthesiologists can be examined.
Comparisons can be made between new and existing devices.
Suitable models may be constructed to assess airway protection
and protection from aspiration. Bench testing might lead to
further development of a device before starting clinical
studies.
2. Stage 2: Cohort Study
A cohort study might be used for
the first assessment of clinical performance in patients. A
cohort study enables full clinical evaluation of the new device
under routine clinical conditions. Such a study enables
examination of all the functions that can be tested with bench
tests (with perhaps the exception of aspiration protection). It
also enables assessment of function during spontaneous
respiration and determination of any airway trauma or
pharyngolaryngeal morbidity. The cohort must be large enough to
enable identification of common problems, but unless it is very
large it will not detect uncommon or rare problems. For
instance, for an event that does not occur in a cohort study of
n cases, the 95% confidence interval for frequency of that event
is approximately 1 in 3/n.34
For example, if no nerve injuries occur in a cohort study of 100
cases, the upper limit of the 95% confidence interval for risk
of nerve injury is 1 in 33. A cohort of more than 100 would be
a reasonable compromise between being large enough to identify
important uncommon events and still remain a practical size.
3. Stage 3: Randomized Control Trial
After successful completion of
bench and cohort evaluation, the need for further modifications
of the device should be considered. Significant modifications
will necessitate repetition of the early evaluations. On
successful completion of the early evaluations, the new device
should be compared with its best existing competitor. In most
cases this would be the cLMA. Such an evaluation should be a
randomized controlled trial (RCT) of adequate size to identify
clinically important differences in function. The previous
evaluations would have indicated any important differences in
function between the new and standard device (e.g., significant
differences in airway seal pressure) and enable power
calculations to determine appropriate study size. However,
trials of at least 100 patients would provide more comprehensive
and clinically useful comparisons. Economic evaluation of cost
effectiveness of the new device might also take place at this
stage.
Data from the three phases of evaluation
might then be used to determine what role the new airway device
has in the market. It might be licensed for only one aspect of
airway care (e.g., for spontaneous breathing only, for
controlled ventilation in patients with good pulmonary
compliance, or for airway maintenance when tracheal access was
likely to be necessary). License extensions might be granted in
the light of further research.
The use of the above methodology
would still result in only 200-300 uses of the device in
patients before release to market. This number would not be
enough to identify uncommon and perhaps unexpected problems,
complications or advantages. Therefore, the proposed method of
evaluation does not obviate the need for post-marketing
surveillance or reporting of adverse incidents. A formal method
of such evaluation could be developed. For instance, the first
5000 devices used after marketing might have evaluation cards
attached, to be returned after use. Alternatively, the
manufacturer might be required to actively seek reports of all
adverse incidents for the first two years after release (similar
to the ‘Yellow Card system’ for new drugs that applies in the
UK).
The structured nature of the
evaluation would have advantages for manufacturer, clinician and
patient. For successful devices, the manufacturer would have
robust data to support performance claims and a clearer vision
of the likely advantages and roles of the new device. This
would enhance marketing and raise credibility. For devices that
performed poorly, the manufacturer could avoid the expense of
large-scale production and marketing of devices that would
ultimately fail to achieve market share. The clinician would
have better evidence on which to base personal evaluation.
Researchers would have clearer ideas of how a new device might
be evaluated to further define function and investigate wider
indications for use. Finally, the patient would be less likely
to be exposed to unnecessary risk during the use of a new
device.
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