Preoxygenation device settings required to create high FiO2 in the ED

Below are the device settings required to provide a high FiO2 during Preoxygenation (Preox)

Non-rebreather bag (NRB)

All NRB’s have a valve between the mask and the reservoir bag preventing expired air entering the reservoir, but NRB’s also have 2 expiratory ports in the mask and both must be also covered by one way valves to provide high FiO2 at 15L/min. However when using such a set up, if the oxygen source is disconnected the patient can asphyxiate as there is no avenue to breathe in room air. Consequently NRB’s used in ED generally have one of their expiratory ports not covered by a valve. This allows the entrainment of room air and reduces the FiO2 that is provided at 15L/min to approximately 60-70%, but at a rate of >30L/min can still provide high FiO2. Most ED gas flow regulators only have markings up to 15L/min, even though the regulator valve can continue to be opened up and provide greater flow rates. The issue is that you don’t know exactly what that flow rate is for your given flow regulator and this may vary between regions.

However regardless of whether the expiratory ports on the face mask are covered by valves or not, even with the NRB set at 15L/min, the addition of nasal cannula also set to 15L/min will combine to provide a high FiO2 (by replacing expired air from the nasopharynx with oxygen).

Bag-Valve-Mask (BVM)

This only provides high FiO2 to the spontaneously breathing patient if there is  a one way valve present on the expiratory port and a good mask to face seal is obtained. Many frequently used BVM’s in ED’s lack the one way valve on the expiratory port and provide an FiO2 close to room air. The addition of a PEEP valve as well as providing PEEP, will also function as a one way valve on the expiratory port if it lacks one.

However it appears that the addition of nasal cannula at 15L/min can compensate and provide high FiO2 even in the absence of an expiratory valve. Additionally such nasal cannula flow can convert the PEEP provided by a PEEP valve into CPAP.

In addition many BVM’s use an duck-billed inspiratory valve which requires significant negative pressure (created by the patient) to open it. As a result if there are small leaks around the mask the patient will entrain room air instead of opening the inspiratory valve sufficiently. Alternatively if one has a perfect seal and the patient is making weak inspiratory efforts, this could result in minimal opening of the inspiratory valve and low oxygen delivery or asphyxiation in a worst case scenario. Careful monitoring of patient respiratory effort combined with visual confirmation of sufficient collapse and re-inflation of the reservoir bag is critical for safe Preox using a BVM.

Given nasal cannula set to 15L/min are rapidly becoming standard of care for apnoeic oxygenation, their additional use during the preoxygenation phase when using a NRB or BVM seems like a no brainer (assuming the patient tolerates it).

Non Invasive Ventilation:

Set the FiO2 to 100% and ensure a good mask-face seal. NIV also allows you to deliver CPAP or BiPAP during Preox. NIV can be delivered by a standalone NIV machine or using a Ventilator set to a NIV mode.

For more information see Monash University’s Demonstration of Factors Influencing Performance of Oxygen Delivery Devices by Dr Nicholas Chrimes.

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