20: Pre-oxygenation before airway procedures – March 2015
The airway is simply a pipe to allow air flow in and out between the mouth/nose and the alveoli. Airway and breathing/ventilation usually go hand in hand together. As such, any airway procedure should enhance the patient’s ventilation in some way otherwise why do it? Importantly though, the performance of airway procedures can compromise the ventilation if they are done poorly or take too long. This includes suctioning of the airway and insertion of devices such as an endotracheal tube. One adjunct to minimise risk is pre-oxygenation.
Once oxygen saturation falls to 90% the oxyhaemoglobin dissociation curve drops sharply. Further deterioration can then be quick and easy. To minimise this risk pre-oxygenation is used to increase the oxygen reservoir available before any procedure occurs. There are three basic principles to this: increase oxygen in the alveoli, remove nitrogen from the alveoli and increase oxygen in the blood. The focus is on the first two since once the haemoglobin are saturated only a small amount of oxygen can be usefully available dissolved in the blood.
The best method of pre-oxygenating before a longer procedure such as intubation uses a bag/valve/mask non re-breathing system. Re-breathing systems will not remove as much nitrogen even if they can remove carbon dioxide. Non re-breathing systems with a high inspiratory fraction of oxygen not only increase oxygen they also remove nitrogen. Since around 80% of air is nitrogen, a lot is trapped within the alveoli that can be replaced with oxygen. The more oxygen in the alveoli, the longer it will take before it stops diffusing into the blood.
Pre-oxygenation should take at least three minutes. After this you can be confident that unless there is interfering pulmonary disease suitable pre-oxygenation should have taken place. It will be important to maintain a good facial seal with the mask to ensure no outside air enters the mask and all exhaled gas is removed. Pulmonary diseases may not allow the oxygen saturation to rise to 100%. It is also important not to allow the patient to breathe room air even once as the nitrogen from the air can fill the alveoli again.
The ideal position for pre-oxygenation is not supine, particularly if the patient is obese or has pulmonary disease. Where possible a semi upright position of around twenty degrees will provide for a longer period before desaturation occurs. The head should be in the sniffing position to optimise airway diameter and maximise likelihood of visualisation during the procedure.
In a healthy adult there can be about 450ml of oxygen in the alveoli during inspiration. Pre-oxygenation may be able to increase this to as much as 3000ml. Each minute 250ml will be used. If not replaced desaturation will occur. The time taken to be problematic will vary. Healthy adults may take up to eight minutes if no ventilation occurs. Sicker adults can take half this with critical or obese patients less than three minutes. The application of a nasal cannula with high flow oxygen at 15L/min attached during the procedure can help delay desaturation. This is only ‘insurance’ and should not mean the procedure takes any longer.
Jeff Kenneally – www.Prehemt.com