Oxygenation can be further optimised by performing apnoeic oxygenation in addition to preoxygenation.Pulmonary shunting (blood flow to the non-oxygenated, collapsed lung units) then occurs resulting in much more rapid desaturation than otherwise predicted - even if pre-oxygenation is performed.This occurs due to resorption atelectasis as oxygen transfers from the lungs into the pulmonary circulation.FRC will decrease in a healthy adult patient by about 250 mL during the first minute after airway occlusion.In patients who develop airway occlusion, desaturation will occur more rapidly due to loss of functional residual capacity (FRC) high metabolic rate, fasciculations from suxamethonium) In some critically ill patients critical desaturation may occur immediately despite attempts at preoxygenationįactors that decrease safe apnoea time include:.In a healthy preoxygenated patient the safe apnea time is up to 8 minutes, compared to ~1 min if they were breathing room air.SaO2 88% to 90% marks the upper inflection point on the oxygen-haemoglobin dissociation curve beyond which further decreases in PaO2 leads to a rapid decline in SaO2 (~ 30% every minute).an alternative term in use is ‘duration of apnoea without desaturation’ (DAWD).Safe apnoea time is the duration of time until critical arterial desaturation (SaO2 88% to 90%) occurs following cessation of breathing/ventilation an ETO2 of 90% may not be achievable in some critically ill patients regardless of the means of preoxygenation.optimal preoxygenation is achieved when ETO2 = 90%.ETO2 is typically used in the operating theatre setting and is rarely available elsewhere.of little importance due to the low solubility of oxygen in bloodĮnd tidal O2 (ETO2) monitoring is the gold standard test in clinical practice for assessing denitrogenation of the lungs during preoxygenation:.this maximises oxygen content of the blood by ensuring haemoglobin is fully saturated.achieve as close to SaO2 100% as possible.when a patient breathes 100% oxygen, this washes out the nitrogen, increasing the oxygen in the lungs to ~3,000 mL.when breathing room air (79% nitrogen) ~450 mL of oxygen is present in the lungs of an average healthy adult.
Oxygen consumption during apnea is approximately 200-250 mL/min (~3 mL/kg/min) in healthy adults.Denitrogenation involves using oxygen to wash out the nitrogen contained in lungs after breathing room air, resulting in a larger alveolar oxygen reservoir.Safe apnoea time is the duration of time following cessation of breathing/ventilation until critical arterial desaturation occurs (typically considered SaO2 88% to 90% in clinical settings).The primary mechanism is ‘denitrogenation’ of the lungs, however maximal preoxygenation is achieved when the alveolar, arterial, tissue, and venous compartments are all filled with oxygen.Preoxygenation is the administration of oxygen to a patient prior to intubation to extend ‘the safe apnoea time’.
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