Trial of a silicon device to improve delivery of oxygen to patients before surgery.

For all of the below the OxyFriend is a U shaped silicon device which sits over the bridge of the nose and extends down either side of the mouth. It has an indentation on either side to allow passage of nasal cannulae. Pre-oxygenation testing The alternatives to be tested are 1. End tidal oxygen using anaesthetic mask and circuit alone 2 vital capacity breaths, 100% oxygen circuit flow set to maximum (15 litres/min) 2. End tidal oxygen using nasal prongs at 15 litres per minute and anaesthetic mask, 2 vital capacity breaths 100% oxygen circuit flow set to maximum (15 litres/min) 3. End tidal oxygen using nasal prongs at 15 litres per minute with OxyFriend and anaesthetic mask, 2 vital capacity breaths, 100% oxygen circuit flow set to maximum (15 litres/min) The experiment will be conducted in a random order between subjects. 4 groups of 6 envelopes, each envelope in the group has a different one of the 6 possible orders of tests. 4 envelopes from one group discarded to make 20 envelopes which will be randomly selected for each test. Subjects will be tested lying down with the anaesthetic face mask held on by the same researcher for all subjects. Flow for nasal prongs via oxygen outlet will be set to 15 litres per minute measured using the flow meter of the anaesthetic machine. Prior to commencement of the experiment the participants will be familiarised with the mask, OxyFriend and nasal prongs including experiencing breathing on the anaesthetic circuit with and without the nasal prongs flowing at 15 litres/min. A technique of expiring the last breath through the filter attached to the oxygen analyser will be practiced as will taking 2 vital capacity breaths commencing with a full expiration and application of the anaesthetic mask. To commence the experiment the 1st selected configuration is placed on the subject (nasal prongs, OxyFriend with nasal prongs or neither), the nasal prongs are not connected to the oxygen at 15 litres/min until the start of the preoxygenation test. The subject breathes through a filter connected to the side stream oxygen analyser until end expired oxygen is less than or equal to 18% (facilitated by deep expirations and breath holding in expiration), this will indicate a baseline oxygen state. The anaesthetic circuit and bag will be flushed with 100% oxygen prior to the commencement of each test. The subject will breath out deeply to commence the test, at the peak of expiration the nasal prongs will be connected to the 15 litres/min oxygen flow of required, and the anaesthetic mask applied to the face. For the 2-vital capacity experiments the subjects will inhale fully, exhale fully and then inhale fully again (2 vital capacity breaths). This will take approximately 10 seconds. At full inspiration the anaesthetic face mask will be removed and the nasal prongs disconnected from the oxygen source. The subject will place the filter connected to the oxygen analyser in their mouth and breath out to the end of a normal breath (functional residual capacity). The end expired oxygen reading from the side stream oxygen analyser will be recorded. The participant will continue breathing via the filter until the expired oxygen returns to baseline (less than or equal to 18%). All participants will undergo all six tests- they will be required to have end tidal O2 return to 18 before before re-testing or pre-oxygenation. The participant will then be prepared for the next test as determined by the randomisation until 3 tests are completed. Expected duration 15 minutes for all three tests Mask leak testing Equipment required 1. Disposable Anaesthetic face mask (sized according to the participant’s face), elbow, nasal prongs and 1 breathing system filter, Oxyfriend device (All reused from preoxygenation test). 2. Non disposable Anaesthetic machine (gas delivery system, leak tested hoses, flow sensors) with instantaneous side flow oxygen analyser and spirometry capacity to display inspired and expired minute ventilation separately. Timer The alternatives to be tested are 1. Mask leak using anaesthetic facemask alone, 50% oxygen flow 2. Mask leak using anaesthetic facemask and nasal prongs, 50% oxygen flow 3. Mask leak using anaesthetic facemask, nasal prongs and OxyFriend, 50% oxygen flow Prior to the commencement the participants will be familiarised with the mask, OxyFriend and nasal prongs including experiencing breathing on the anaesthetic circuit with a low level of positive end expiratory pressure (PEEP) with and without the nasal prongs and the OxyFriend. Subjects will be tested lying down with the anaesthetic face mask held on by the same researcher for all subjects. To commence the experiment the 1st selected configuration is placed on the subject (nasal prongs, OxyFriend with nasal prongs or neither), the nasal prongs blocked at the oxygen connector end so that no flow occurs through the prongs during the test. The anaesthetic machine will be turned on and the circuit set to the ventilator setting with 4cm water PEEP and 2cm water pressure support. The anaesthetic mask is placed on the participant by the researcher to obtain the optimum seal possible. The subject will commence breaths of normal tidal volume and rate (normal breathing). The oxygen/air mix added to the circuit will be adjusted to keep the ventilator bag about 2/3 full at the end of expiration. This is to prevent increased circuit pressure by forcing open the spill valve. The oxygen flush may be used to fill the circuit if needed, at least 50% oxygen will be added to the circuit from the anaesthetic machine to ensure that the gas mix that the patient is breathing is not hypoxic (the oxygen concentration in the circuit will be continuously monitored through the oxygen side stream analyser). After 30 seconds of normal breathing the timer will be started and the test continued for 1 minute. The inspired and expired minute volume will be recorded at the end of the minute. The mask will be removed and the next configuration of prongs, OxyFriend or neither placed on the subject and the test will be repeated until all three tests are complete. Expected duration 10 minutes for all three tests