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What is the Theoretical Basis for Preoxygenation Before Endotracheal Intubation?

Maintenance of hemoglobin oxygen saturation during airway management is critical to patient safety. Desaturation to below 70% oxygen saturation puts patients at risk for arrhythmias, hemodynamic decompensation, hypoxic brain injury, and death. A major challenge for emergency physicians is prompt intubation to ensure airway patency without severe hypoxia or aspiration. In patients without pulmonary disease, the risk of hypoxic saturation from preoxygenation followed by endotracheal intubation is low if hemoglobin is adequate, metabolic demands are low, and pulse oximetry is 100% with room air inhalation .

In contrast, septic patients with multilobar pneumonia who have developed hypoxemia are at immediate risk of critical tissue hypoxia at the time of endotracheal tube intubation despite high-flow 100% oxygen. Preoxygenation allows a safety buffer during hypopneas and apneas, and it prolongs the safe duration of apnea, defined as the time until the 88%-90% oxygen saturation level is reached, to allow for definitive airway placement. When patients are below this oxygen saturation level, their state is on the steeper part of the oxyhemoglobin dissociation curve and can drop to critical levels of oxygen saturation in an instant.

1. The basis of pre-oxygenation before endotracheal intubation

Standard induction of anesthesia in patients undergoing elective surgery is by administration of sedatives, artificial ventilation, muscle relaxants, and continued artificial ventilation until a definitive airway is placed. Preoxygenation in these patients is not mandatory because ventilation is continuous throughout the induction period and the patients have normal physiologic and metabolic demands.

Rapid induction anesthesia has been developed in the field of anesthesia for surgical patients with high risk of aspiration due to intestinal disease, physical fitness, and critical illness. This technique administers both sedation and muscle relaxants, with no ventilation (unless hypoxemia prevention is required) until the muscle relaxant is effective. This approach has been applied to emergency departments where all patients requiring airway management should be considered at risk for aspiration. The default technique is rapid induction intubation.

Before RSI, patients inhaled with room air, during the period before endotracheal intubation after administration of sedatives/muscle relaxants, will have a drop in oxygen saturation over a period of 45-60 seconds. In the 1950s, anesthesiologists realized that the safest way to perform RSI was to fill the patient's alveoli with high concentrations of inspired oxygen prior to intubation. Studies have shown that the time to desaturation is significantly increased if preoxygenation prior to endotracheal intubation is given with 100% oxygen rather than room air.

2. Goals of preoxygenation of endotracheal intubation in the emergency room

(1) Make the patient's oxygen saturation as close to 100% as possible.

(2) Preoxygenation of endotracheal intubation to denitrogenate the residual lung volume (maximize the oxygen storage of the lung).

(3) Denitrogenation and maximizing blood oxygenation.

The first two goals are necessary, while the third goal has little effect on increasing the duration of safe apnoea because oxygen is poorly soluble in blood and its oxygen storage capacity is relatively small compared to lungs.

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