A 23-year-old man arrives at the ED after a self-inflicted gunshot wound to the head. He responds to painful stimuli with extensor posturing. He is breathing spontaneously with saturations of 100% on a non-rebreather at 10 liters/min. He is premedicated, paralyzed, and induced. An intern makes two long unsuccessful attempts at intubation, followed by a resident who successfully intubates the patient over a bougie. The saturation never drops below 100% despite seven minutes of chemical paralysis and no interceding mechanical ventilation.
One of the fundamental principles of rapid sequence intubation (RSI), and arguably the most important, is preoxygenation. Preoxygenation creates the reserve that allows us to paralyze a patient chemically yet withhold positive-pressure ventilation, limiting the risk of gastric insufflation and subsequent aspiration. Preoxygenation also provides the time necessary for controlled successful laryngoscopy.
Kung et al divided 100 healthy patients undergoing OR intubation between preoxygenation and no preoxygenation. Even among healthy patients in this controlled setting, a substantial number of patients who did not receive preoxygenation suffered desaturation compared with none of the preoxygenated patients. (Anaesth Intesive Care 1991;19:192.)
In healthy patients undergoing OR intubation, a substantial number not receiving preoxygenation suffered desaturation compared with none of those preoxygenated
Preoxygenation is predicated on basic respiratory physiology. The air we all breathe is only 21% oxygen, regardless of your location or altitude; the remaining 79% is nearly all nitrogen. If all of the nitrogen in your lungs were replaced with oxygen, you would have nearly five times the oxygen you have now. Preoxygenation is referred to as de-nitrogenation for this reason.
Patients with healthy lungs and adequate functional residual capacity may develop enough reserve to survive up to eight minutes of apnea without desaturation. (Anesthesiology 1997;87:979; see figure.) This certainly calls into question the lore of routinely holding your breath or limiting intubation attempts to 20 seconds. When preoxygenated patients with normal saturations are paralyzed for intubation, like the case above, several attempts (or one very long attempt) at laryngoscopy may be made without the need for intervening mechanical ventilation or supplemental oxygenation. This takes great restraint. I have observed many clinicians appropriately withhold positive-pressure ventilation between administration of paralytics and the first intubation attempt yet rush to ventilate the patient despite normal saturations after the first intubation attempt fails.
Unfortunately, many emergency department patients cannot tolerate eight minutes of apnea. (Anesthesiology 1997;87:979.) Common clinical variables that affect the amount of apnea a patient can withstand include age, obesity, pregnancy, metabolism, lung disease, baseline saturations, and acute illness. Children, for instance, achieve preoxygenation faster, but have shorter apnea tolerance in large part because of their increased basal metabolism. (Brit J Anaesth 1996;77:333; Paediatr Anaesth 1998;8:292; J Clin Anesth 1995;7:93.) This is also true of acutely ill patients, especially hyperdynamic patients with septic shock and severe alcohol withdrawal. Obese and pregnant patients have less reserve in large part because of limited functional residual capacity.
Patients with underlying lung disease, especially with acute or acute-on-chronic disease resulting in hypoxemia, are a particularly scary group to intubate because they are prone to very rapid desaturation. They are already hypoxic and on the steep part of the saturation curve. (See figure.) We teach our residents that patients with a saturation of 100% after preoxygenation have an adequate reserve, that those with less than 100% but above 90% have a limited reserve, and those with less than 90% have no reserve. (Ann Emerg Med 2006;47:581.) In the first group, the goal should be no positive-pressure ventilation, though some patients, particularly those with high-risk characteristics, may still desaturate.
In the second group, the clinician should plan to optimize first-pass success (The AirwayCam Guide to Intubation and Practical Emergency Airway Management. AirwayCam Technologies, Inc., Wayne, PA; 2004), and anticipate that some patients will require careful positive-pressure ventilation if the intubation attempt is prolonged. In the last group, the clinician should be prepared to provide immediate optimal bag-valve-mask ventilation and to place a rescue airway if saturations cannot be maintained.
In most cases, preoxygenation will be accomplished with a tight-fitting non-rebreather mask with 10–15 liters/min flow for at least three minutes. (Brit J Anaesth 1995;75:777; Anesth Analg 2001;92:1337.) Such a system delivers 70% to 90% oxygen, and is sufficient for most patients. Because of passive flow, the mask may be left on after paralysis until laryngoscopy. (Am J Emerg Med 2005;23:864.) For those patients with limited or no reserve, a self-inflating or anesthesia bag with reservoir connected to high-flow oxygen can be used to provide 100% oxygen. It is extremely important that the clinician recognize that all self-inflating bags have some type of valve. If the bag and mask are held over the face without opening this valve, the patient is not receiving any oxygen! Unless the patient requires assisted respirations, the mask can be gently placed over the face and the valve fluttered with gentle bag squeezing, timed to the patient's respirations until paralysis is achieved.
An alternative for preoxygenation is the use of deep or vital capacity breaths. As few as four such breaths are comparable with a few minutes of oxygen at lower tidal volumes while six or more such breaths may actually be superior to traditional methods. (Brit J Anaesth 1990;64:517; Anaesthesia 1994;49:629.) A single vital capacity breath after forced exhalation may even provide adequate preoxygenation. (Can J Anaesth 2000;47:1144.) Most ED patients undergoing intubation cannot cooperate with vital capacity breaths, but these approaches are very applicable to patients about to undergo moderate or deep procedural sedation.
Research has demonstrated that preoxygenation is more successful for most patients with at least 20 degrees of head elevation. (Anesth Analg 1993;77:1081; Brit J Anaesth 2005;95:706. [Epub] Sept. 2, 2005.) This is especially true for obese patients, but does not seem to apply to pregnant woman.LK (Anesth Analg 1993;77:1081; Brit J Anaesth 2005;95:706. [Epub] Sept. 2, 2005.) This highlights yet another reason that all trauma patients in spinal precautions should be considered difficult intubations. This also emphasizes the importance of keeping patients, particularly those with respiratory distress, in their position of comfort as long as their mental status allows.
Preoxygenation is a critical step in RSI that will often negate the need for interposed positive-pressure ventilation. It usually will be accomplished with a tight-fitting non-rebreather mask on 10 to 15 liters/minute for at least three minutes, ideally with at least 20 degrees of head elevation. After preoxygenation, patients may be categorized as having adequate, limited, or no reserve with corresponding preparations made.