Preoxygenation, Apnoea, Anaesthesia Clinical Trial
Official title:
Assessing the Potential Use of Optiflow as a Preoxygenation Method Prior to Anaesthesia.
Optiflow can be used to prolong the apnoea period under anaesthesia without falls in blood oxygen saturation. This study is designed to assess whether the Optiflow system can efficiently replace conventional anaesthetic mask and circuit preoxygenation so that it could be used in emergency anaesthesia. If so, it would be then be in place in the case of difficult or failed intubation to maintain oxygenation ie: it could preoxygenate and then maintain oxygenation seamlessly.
The Optiflow device is used to deliver high flow humidified oxygen/air via nasal cannulae to
patients who have respiratory problems. More recently it has been used in patients having
surgery on their larynx who are intentionally paralysed under anaesthesia where the surgeon
does not want any tubes in the airway. This makes access for surgery easier/possible. Perhaps
surprisingly, these patients can be left apnoeic for 30 minutes without falls in blood oxygen
levels. The lungs are oxygenated despite there being no in and out breathing/ventilation.
We propose that applying Optiflow at the start of an emergency anaesthetic may be beneficial
when an anaesthetist finds that tracheal intubation is impossible - a "failed intubation"
scenario. Optiflow might maintain oxygenation during further attempts at intubation or until
more skilled anaesthetists arrive.
Another alternative technique presently in common use is for the patient to breathe 100%
oxygen via a facemask and conventional anaesthetic circuit for 3 minutes before anaesthesia
starts (preoxygenation). This fills the functional residual capacity of the lungs to about
90% oxygen giving an oxygen reserve when the breathing stops until the anaesthetist breathes
for them after intubating the trachea. This can result in the maintenance of normal blood
oxygen saturation for several minutes longer than would be the case if the lungs had only
been exposed to air before anaesthesia.
The research question that we ask is whether we can just use Optiflow instead of the
conventional mask/circuit to preoxygenate patients to a similar extent and as quickly as
using a facemask so that, in emergency surgery, Optiflow could be used to preoxygenate and
then already be in place in case there is a failed intubation. This would enable Optiflow to
be used for both purposes seamlessly without the need to switch between the two methods of
oxygenation.
Research to date has not yet identified how long Optiflow needs to be in place before
anaesthesia for it to be able to produce adequate preoxygenation of the lungs before the
patient stops breathing. The 3 minute timing for preoxygenation is important in emergency
cases e.g. when there is an emergency caesarean section to be done and every minute before
the baby is delivered matters. If it were not possible to preoxygenate a patient's lungs
within 3 minutes using Optiflow we would suggest that it is inappropriate to replace
conventional mask pre-oxygenation and that its value in emergency anaesthesia might be
reduced (although it may still have potential for less urgent anaesthesia when it could be
applied for a longer duration before anaesthesia).
This will be a pilot study with 20 healthy volunteer students as subjects who will undergo an
up / down sequential analysis of Optiflow pre-oxygenation. They will be recruited from
posters inviting them to consider enrolling in the project. They will receive a subject
information sheet and will provide written informed consent.
On the study day an individual subject will lie supine on a trolley and the Optiflow device
will be fitted ready to administer humidified 100% oxygen at 45 l/min via the device's nasal
cannulae. The subject will be asked to breathe normally with a closed mouth and the device
will be switched on.
The initial test period will be for 3 minutes and at the end of that time the device will be
turned off with the subject holding their breath in inspiration and the subject will then
exhale and then breathe normally via a mouthpiece which has a side sampling port connected to
a standard gas monitor (as used during anaesthesia) which will measure and display the
expired oxygen percentage. This is known to equate very well to the alveolar oxygen fraction
in healthy subjects and the target value will be 85%.
After the initial test the subject will be asked to walk around breathing room air for at
least 15 minutes to return their lung gases to pre test normality.
The process will then be repeated as before but for a different duration:
If the target of 85% had not been reached within 3 minutes the 2nd test would be for 5
minutes.
If the 85% target had been reached the 2nd test would be reduced to 2.5 minutes.
The test runs would continue in the same up/down fashion until the minimum time required to
reach 85% had been identified (to the nearest 30 seconds) or the subject had done test runs
up to 10 minutes long without reaching the target.
If the subject wished to split the test runs over more than one session that would be
accommodated. They would also be free to leave the study at any time if they so desired.
Subsequent subjects will have an initial test duration adjusted to the average of the
previous subjects so that they may not be required to undergo as many tests (this presumes
that there is not too much variation within the population) and will be less inconvenienced.
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