Starting Point: We do a bad job at preoxygenation
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We could do better at preoxygenating patients before emergent intubations. In my experience the most commonly used device for preoxygenation is a bag-valve mask, which has many pitfalls. We often accept a poor mask seal in efforts to remain gentle with awake patients. Many masks don’t provide oxygen unless the operator is squeezing the reservoir in synchrony with the patient’s breaths, which may be difficult to coordinate. Alternatively, PEEP valves can be used to ensure the delivery of oxygen, but this approach requires a good mask seal and is underutilized. The operator is usually distracted with other tasks. The final result is often suboptimal.
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Preoxygenation is critical for emergent airway management. It increases safe apnea time before desaturation. Additionally, good preoxygenation is a prerequisite to effective apneic oxygenation (apneic oxygenation depends on a high concentration of oxygen extending from the nasal cannula to the alveoli). Combining high-quality preoxygenation with apneic oxygenation can really extend safe apnea time and improve first-pass success.
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There are better alternatives to bag-mask ventilation but they’re logistically problematic
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There are a lot of effective ways to preoxygenate patients. The challenge is developing a system which is simple enough to function well in an emergency. When your patient starts vomiting, self-extubates, develops bradycardia, and desaturates to 70% you don’t want to be searching for special equipment or setting up something fancy. Two of the best approaches are noninvasive ventilation or a non-rebreather mask, but even these systems can be tricky to operate under duress.
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Noninvasive ventilation is great for preoxygenation. However, this is logistically difficult to set up in emergent situations. Depending on who the respiratory therapist is, they may attempt to murder you (“You want me to put the patient on BiPap for ten minutes, just so we can take it off when you intubate??”).
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Another approach is using a non-rebreather reservoir mask with the flow rate set as high as possible (Weingart & Levitan 2012). This is great technique but it does involve juggling three devices (the reservoir face-mask, a nasal cannula for apneic oxygenation, and a bag-valve mask if needed). If you don’t have three oxygen sources available, it can get tricky ensuring that each device is connected to oxygen at the appropriate time.
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Nasal Cannula for Preoxygenation & Apneic Oxygenation: How to do it
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- Apply a regular nasal cannula to the patient.
- If time allows, tape the cannula to the patient's cheeks with the prongs well seated in the nostrils. You don’t want it coming out in the heat of battle. This also discourages people from removing the cannula when you're about to intubate (1).
- Crank the oxygen flow to 15 liters per minute. Then keep turning up the oxygen flow rate until the resuscitation room starts sounding like a wind tunnel. This should get you to about 30-45 liters/minute flow.
- The patient should be preoxygenated within about five minutes.
- Leave the cannula on throughout the entire intubation. After the patient has been paralyzed, decrease the flow rate to 15 liters/minute; this will provide apneic oxygenation (4).
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Why it works
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Recently, commercial high-flow nasal cannula systems have become widely accepted as an way to provide very high levels of oxygen and a small amount of PEEP. This is based on applying high flow rates of oxygen via nasal cannula (i.e., 15-70 liters/min flow). Oxygen is heated and humidified for patient comfort.
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By cranking the flow rate of a regular nasal cannula well past 15 liters/min, the nasal cannula will function in a similar fashion. Delivering 100% oxygen at 30-45 liter/minute flow should provide >90% effective inspired oxygen concentration which is quite adequate for preoxygenation (2)(Ward 2013).
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Advantages
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The main advantage of this approach is that it is extremely simple and fast. The only materials required are a nasal cannula and oxygen source, which is universally available and often already attached to the patient. Getting this set up requires seconds. Once the nasal cannula is running, this leaves everyone free to divert attention to other issues (drugs, hemodynamics, equipment, etc.). From a human-factors standpoint, anything which simplifies the intubation process will reduce chaos and increase the overall likelihood of success.
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Apneic oxygenation requires excellent preoxygenation and maintence of a high concentration of oxygen extending from the nasal cannula to the alveoli. By keeping the nasal cannula on throughout the entire process, the transition from preoxygenation to apneic oxygenation is seamless and high concentrations of oxygen in the upper airway are undisrupted.
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This technique allows for preoxygenation in some very challenging situations. Preoxygenation with nasal cannula may be performed while a patient is lying on their side and actively vomiting. This technique may be used in patients who cannot accept a mask due to facial surgery or trauma.
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Disadvantages and Risks
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The primary disadvantage is that the nasal cannula is uncomfortable at this flow rate without heating or humidification. Critically ill patients often have an altered mental status at the time of intubation, so they may tolerate this surprisingly well. Patients who are very air-hungry may also tolerate this. A bit of reassurance and coaching can help. If this fails, sedation such as ketamine is effective (i.e., delayed sequence intubation).
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For patients with nasal obstruction this technique could be problematic, although the obstruction may respond to nasal trumpets. In practice, it is extremely uncommon for both nostrils to be sufficiently obstructed to impair a high-flow nasal cannula device.
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Depending on the specifications of the flow meter, there theoretically could be a risk of barotrauma. Commercial high-flow nasal cannula systems provide flow rates up to 70 liters/min in adults with excellent safety – therefore the highest safe flow rate is above70 liters/min (3). By targeting a flow rate of 30-45 liters/minute, there is a large margin of safety. Many flow meters provide a maximal flow of 60 liters/min (likewise providing a margin of safety), but others do allow for flows as high as 90-100 liters/min. It may be useful to check the specifications of the flow meter used at your hospital (see below). If your flow meter goes up to 90-100 liters/min then it should be turned up a fair amount above 15 liters/min but nowhere near maximal flow rate in order to target a flow rate of 30-45 liters/min. (Before putting the cannula on the patient, experiment with the flow meter to get a sense of what it sounds like to be at 15 liter/min versus maximal flow rate, how far the dial adjusts, and tune it up accordingly.)
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Other uses of nasal cannula oxygen at 6-45 liters/min flow
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For a patient on a regular nasal cannula who suddenly desaturates, immediately adjusting the flow rate to 15 liters/min or higher can temporarily maintain oxygenation while working on a longer-term solution (i.e., noninvasive ventilation, a commercial heated/humidified high-flow nasal cannula, intubation, etc.). The traditional teaching that a nasal cannula can’t gohigher than 6 liters/minute or 15 liters/minute is a myth. High flow rates without heating and humidification should probably be limited to short periods of time to prevent nasal irritation. Nonetheless, this is a handy and powerful way to oxygenate crashing patients.
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Conclusions
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Nasal cannula oxygen cranked up to roughly 30-45 liters/min flow is a simple, easy, fast, and very powerful approach to preoxygenation and apneic oxygenation. This may be set up in seconds, and once in place it will preoxygenate the patient without requiring any further attention. The main drawback is that if the patient is awake and alert they may find the cannula irritating and try to remove it. This technique is especially useful in emergencies when more sophisticated equipment isn’t immediately available, when there are only a few operators, or if a facial mask cannot be used.
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Notes
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(1) No matter how much the staff is educated about apneic oxygenation, there is always one person in the room who will try to take off the nasal cannula when you're about to intubate (Murphy's Law of Apneic Oxygenation). Taping the cannula in place helps remind everyone that the cannula should remain on throughout the entire procedure. If someone does try to remove the cannula, the tape will slow them down enough for you to intervene.
(2) Why 30-45 liters/min flow? 15 liters/min flow will generally achieve high concentrations of oxygen (probably >85% in most cases). However, if the patient has a high minute ventilation they probably require a higher flow rate (For example, if a patient is tachypneic with a minute ventilation of 15 liters/minute, it's unrealistic to expect all 15 liters/minute of oxygen to make its way from the nasal cannulae to the patient's alveoli). Additionally, a higher flow rate may generate a small amount of PEEP. Overall, it may be safer to err on the higher side and push the flow rate up to the 30-45 liter/min range.
(3) There are reports of barotrauma in neonates and children, and lower flow rates are probably needed here. I have little experience and knowledge with the pediatric population as an adult intensivist.
(4) The only problem that could realistically occur with barotrauma would be if the patient had a nasal cannula running at a very high rate and was ventilated aggressively with a bag-valve mask simultaneously; the combination of these two maneuvers could prevent back-flow of gas out of the upper airway and increase pressures. In order to avoid the possibility of this ever happening, an assistant should decrease the flow rate of the nasal cannula to 15 liters/minute after the patient is paralyzed. Thus, if the patient requires reoxygenation with a bag-valve mask, only 15 liters/min will be running and this would be safe. High flow rates may be requires for preoxygenation to keep up with a patient's intrinsic minute ventilation, but 15 liters/minute is quite adequate for apneic oxygenation.
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Expert Commentary from Scott Weingart
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Since this is one of my most controversial posts, I asked Dr. Weingart to provide his thoughts on the matter:
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My Response:
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Safety of high-flow nasal cannula
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The safety of applying high flows of gas to the nose is well established from a barotrauma standpoint, with commercial devices designed to deliver up to 70 liter/min of heated and humidified oxygen. At Genius Generalour devices deliver up to 60 liters/minute flow, and we routinely use this flow rate for days at a time without any problems.
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Ironically, high-flow nasal cannula may be safer than some other accepted practices in respiratory therapy. Consider, for example, the universal practice of ventilating an intubated patient using a self-inflating bag. If a patient with airway obstruction (i.e., asthma or COPD) is vigorously and frequently bagged, it is easy to cause gas trapping and high intrathoracic pressures resulting in pneumothorax or hypotension. If this technique was introduced today, it would probably meet strong resistance because it delivers an unknown amount of pressure to the lungs.
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Delivering gas at high flow rates without heating and humidification causes nasal irritation. This has been a minor issue however, as most of the patients we are intubating are extremely sick and past the point of noticing this. Emergency intubation is a high-stakes procedure with risks including death, aspiration, hypoxemia, and airway trauma. If transient nasal irritation allows optimization of the procedure and reduction of these complications, this may be worth it.
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Efficacy of high-flow nasal cannula oxygen for preoxygneation
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The experiment which Dr. Weingart proposes is well-designed. Similar experiments have been done using high-flow humidified nasal cannula. Since the oxygen concentration in non-humidified nasal cannula is higher than humidified nasal cannula (760mm vs. 713mm), a non-humidified nasal cannula would be expected to generate similar or higher levels of oxygen. The only difference is that heated humidified systems have slightly different size nasal prongs.
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Sim 2008 studied thirteen volunteers breathing at rest, and also breathing with their chest bound to induce tachypnea and simulate respiratory distress. Effective inhaled FiO2 was measured using a 10-cm cannula passed through the nares. Nasal cannula at 40 liters/minute flow achieved high FiO2 under both conditions. Note that nasal cannula achieved a higher FiO2 than a non-rebreather facemask set at 110 liters/minute flow, which has been established to deliver an adequate amount of oxygen for preoxygenation.
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Chanques 2013 placed patients on high-flow nasal cannula and measured inhaled tracheal FiO2 using a cannula placed through a tracheostomy site. High-flow nasal cannula achieved inspired FiO2 close to 90% at flow rates of 30-45 liters/minute:
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Bottom line? Commercial high-flow nasal cannula at 30-45 liters/min achieves around 90% inhaled FiO2, depending on how high the flow is adjusted (i.e., closer to 45 liters/min will increase the FiO2). At a flow of 30 liters/minute, some patients breathing with an open mouth will receive lower amounts of oxygen. Further research is required to confirm this using standard nasal cannulas, determine required flow rates, and investigate how these could be achieved using a flowmeter. Nonetheless, this may be an improvement over a bag-valve mask, which achieves an FiO2 of 55-95% under ideal conditions depending on whether the mask model allows entrainment of room air (Kwei 2006).
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Conclusions
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Emergency airway management requires immediate treatment of extremely unstable patients, often with little time to prepare. It is critical to approach these situations with an organized and methodical mindset. Much has been written about airway algorithms. However, it is often hypoxemia which threatens these patients rather than the presence or absence of a definitive airway. Therefore, it is also important to have a methodical approach to preoxygenation and apneic oxygenation.
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Below is an example of one possible algorithm for emergency preoxygenation and apneic oxygenation. Combining a nasal cannula at 15 liters/minute with a nonrebreather facemask at 15 liters/minute is an excellent technique. An alternative is to combine a nasal cannula with a bag-valve mask equipped with a PEEP valve, which generates high levels of FiO2 and some positive pressure to recruit alveoli (Weingart 2012). If these approaches are unavailable or fail (i.e., due to poor mask seal), cranking up a standard nasal cannula to 30-45 liters/minute could be used as an alternative approach.
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The above algorithm does not incorporate some powerful techniques which may be difficult to set up on an emergent basis (i.e., noninvasive ventilation). If these approaches are immediately available to you, then your algorithm would be different. As with intubation, there are a variety of possible algorithms. The details of any algorithm are less important than simply havingan algorithm with an organized approach. It is seldom necessary to go past Plans A or B, but patients and circumstances vary (for example, a patient with active vomiting who is being orally suctioned may require preoxygenation via a solely nasal route). Designing your own algorithm which is appropriate for your practice is a useful exercise to improve cognitive readiness. Remember Murphy's Laws:
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I would like to thank Dr. Weingart for sharing his expertise with us. I've found this process to be very informative and hope others will as well.
Image credits: https://upload.wikimedia.org/wikipedia/commons/f/ff/Nasal_cannula.png
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Josh, Great comments! FiO2 studies are great, but ETO2 is what we really want to ensure adequate denitrogenation. Hopefully someone can get this done.
I was wondering if someone could give the specifics of where they found the information regarding the use of a regular nasal cannula past the 15LPM (around the 30+LPM) rate would achieve 90% inspiratory levels. I reviewed the study cited but was only able to find this information when it came to using a High flow nasal cannula. Just looking for clarification. Thanks
Dear Professor Farkas,
Do you know the Double Trunk Mask.
http://rc.rcjournal.com/content/64/8/908
If yes, do you have used this device ?
F Duprez PhD