Preoxygenation (PreOx) of patients is performed to extend the period of safe apnoea before the patient desaturates. This is a critical safety step before any procedure where apnoea is expected (eg intubation) or a possibility (eg procedural sedation).
PreOx is achieved through 3 main mechanisms:
– Preoxygenation of Blood
This is the maximisation of dissolved oxygen in blood. This is by far the least important PreOx mechanism as oxygen is poorly soluble in blood.
– Preoxygenation of Haemoglobin
Bringing the oxygen saturation as close as possible to 100%. In healthy lungs this can be achieved very quickly.
– Denitrogenation of the lungs
Washing out the nitrogen content of the lungs by replacing it with oxygen. This provides a large oxygen reservoir that will diffuse into the bloodstream in the event of apnoea. This is the most important aspect of PreOx, as the denitrogenated lungs represent 95% of the oxygen reservoir in apnoea compared to only 5% in blood. Denitrogenation can be achieved in 2 ways using a high FiO2 (Fraction of Inspired Oxygen) source (i.e. >90% FiO2).
1. Having the patient take 8 vital capacity (full) breaths in/out. This is obviously only achievable with an awake and compliant patient eg prior to procedural sedation
2. The patient takes normal tidal volume breaths for at least 3 minutes. This is usually the only option in Emergency Department (ED) intubations.
So it is not sufficient to just achieve good saturations. You must wait the required time for denitrogenation to occur. In the operating room, this time can be calculated using gas analysers of the expired oxygen content, however these are not generally available in Emergency Departments (ED). Therefore the Emergency practitioner should rely on the time estimate of at least 3 minutes as their marker of denitrogenation.
Devices that can provide a high FiO2 gas source for Preox in ED:
1. A Non-Rebreather face mask with reservoir bag (NRB)
2. A Bag-Valve-Mask (BVM)
3. Non-Invasive Ventilation (NIV)
Positioning for Preoxygenation
Ideally to facilitiate preoxygenation the patient should be positioned in the semi-recumbent position with head elevation of approximately 30 degrees. They can then be repositioned to facility intubation after preoxygenation.
If a patient has C-spine precautions in place, the Reverse Trendelenberg position can be used for PreOx.