April 20, 2014

Explain it: Preoxygenation

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One of the main consequences of paralyzing or heavily sedating patients is that they become APNEIC. As we have all witnessed, it is during this apnea period, that hypoxia happens. This talk is going to be all about setting the patient and provider, you, up for success during apnea. We’ll break it down into seven questions.

 Question 1.  What is the best source of high FIO2 for preoxygenation?

A non-rebreather mask or a bag valve mask? The answer – A Non-rebreather mask with the O2 flow rate turned up to 30-60L/minute. What’s the standard rate these usually get turned to? 15, maybe 20 L/min. But that is going to deliver 60-70% FiO2 at best. At the higher flow rate, O2 delivery is at least 90% if not more. Preoxygenate- very high flow, non-rebreather. But what about the bag valve mask? This delivers O2 only if there’s a tight seal and you deliver a squeeze of the bag. For the most part, pre-oxygenating with a standard BVM is going to give the patient room air.

 Question 2. How long should the patient be preoxygenated?

The real answer is as long as possible. But ideally, at least Three minutes of tidal volume breathing with high FiO2 (as mentioned above) or take 8 full inspiratory capacity breaths. The 8 full breaths are even more effective if the patient completely exhales before the first breath.

 Question 3. Can Non invasive positive pressure ventilation improve preoxygenation?

If the patient can’t get sats above 93-95% with tidal volume breathing or 8 full inspiratory capacity breaths, they have a higher chance of desaturating during apnea. What they probably have is shunt physiology. What is shunt physiology? This is when the alveoli are perfused but not ventilated. Common shunt conditions are pulmonary edema and pneumonia.

If you think your patient might have shunt physiology or their sats won’t go above 93-95%, consider non-invasive positive pressure ventilation.

 Question 4. What’s the best position for preoxygenation?

First off, what’s the worst? Supine. It’s harder to take deep breaths, and the posterior lungs can become atelectatic (shunt physiology)

Preoxygenation with 20-25 degrees of head elevation is what’s been studied and has been found to improve safe apnea time when compared with supine positioning. This is going to be even more pronounced in patients with high BMI. If there is suspected spinal trauma, then you can use reverse Trendelenberg to elevate the head.

 Question 5. How long does it take to desaturate after pre-oxygenation?

What are we really doing with preoxygenation? We’re not significantly increasing the amount of oxygen in blood by much. We’re making a reservoir of O2 in the lungs.

So how long does it take? It’s really difficult to predict. In some studies, critical patients can desaturate in less than 30 seconds, a healthy patient …  as long as 8 minutes.

 Question 6. Can apneic oxygenation extend the duration of safe apnea?

Apneic oxygenation? Yes indeed. It really exists. It’s an important concept to know because of the types of procedures we do in the ED, including intubation. In the apneic patient, it is possible to maintain oxygenation even without breathing movement. Each minute, the alveoli, all on their own, will move about 250ml of O2 into the bloodstream. This creates low pressure so gas will then flow from pharynx to alveoli. What this means for us is that in the apneic patient, we can maintain some oxygenation without assisted ventilation. Since we’re talking about intubation, how do we get in oxygen if we’re working in the mouth? Go through the nose – nasal cannula. By increasing the NC flow rate to 15L/min, it’s possible to get close to 100% FiO2 in the pharynx. In the awake patient, even 5L/min via nasal cannula is hard to tolerate, so have the NC in place and when the patient’s sedated, crank up the oxygen. As a side note, a facemask will deliver high FiO2 to a breathing patient but is not effective in apnea.

There is no disadvantage to using a high flow nasal cannula for intubation. So if you have a patient with shunt, such as with CHF, use positive pressure to start preoxygenation to try and relieve the shunt or high flow NRB if there is no shunt. When the patient becomes sedated, turn on the high flow nasal cannula. Keep one O2 port (christmas tree) saved for the NC and turn it up when the patient won’t care if they’re being blasted with dry air up their nose.

Who should have a nasal cannula in place during intubation? Everybody. It’s cheap and what’s the downside? You can’t always predict who will be a challenging intubation or how long it will take to desaturate.

 Question 7. When and how should we provide manual ventilations during  the apneic period?

What will assisted respiration do for the patient?

  1. Ventilation
  2. Oxygenation

1. Ventilation (CO2 management).  CO2 rises 8-16mm Hg in the first minute of apnea and then 3mm Hg each minutes after, so the effects on ventilation are minimal in most intubations.. Ventilation becomes important when even small CO2 changes make a difference like severe metabolic acidosis and increased ICP.

2. Oxygenation (O2 Management). When a patient can only get sats up to 90%, they are on the steep part of the hemoglobin dissociation curve and there’s a fair chance we’re going to have to bag them at some point. Why do we dislike bagging during apnea? Because we don’t want to overwhelm the LES. (It only takes 25 cm H2O) A better and safer option is to connect a ventilator to the facemask of your BVM. Because the vent has inspiratory pressure control and can deliver PEEP. It will also give respirations at a nice slow rate. This is one case where a machine is probably better than a human.


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Comments

  1. Minh Le Cong says:

    great post, Rob! I predict everyone will be doing the nasal oxygenation by end of the year. ..perhaps not all the anaesthetists but those looking after bloody sick patients will move to this simple technique. Spend some more oxygen during your RSI…it has little downside apart from using up oxygen supply..which might be a problem if you have to bring your own oxygen to the party..

    • Rob Orman says:

      Thanks, Minh! I completely agree.. the rapid part of RSI shouldn’t be in the preparation of the patient.

      • Rob,
        This works no problem in our resus rooms but often we’ll need to quickly intubate in a smaller room with only a single oxygen hook up. Respiratory can bring down an 02 splitter, but how would you handle pre-ox with one hook up?

        • Rob Orman says:

          Steve-

          The NRB is key in preoxygenation. After the patient is sedated and/or apneic, switch out to nasal cannula. It will be a bit of a circus if you then need to BVM and switch out yet again. An extra O2 hookup will only be needed for a few minutes (hopefully) and if you have some extra portable tanks, they will do the trick.

  2. Seth Trueger says:

    great review! 2 points to add:

    1) not enough oxygen ports? throw an oxygen tank under the bed. then no switching needed during the critical time (avoid the circus)

    2) I’ve tried on NC at 15L. tickles a little but tolerable (and that was without meds). for me, that’s enough to put it at 15L before we start doing anything important (again, avoid the circus). if the patient fights or there’s another problem, then turn it down. but I don’t want critical brain cells thinking about shifting nobs or o2 ports.

Trackbacks

  1. This paper is SO DAMN IMPORTANT, and I want to tell you all about it, but now I don’t have too. Why? You ask.Because our buddy over at ERCAST, Rob Orman, isn’t satisfied with owning the anus and has decided to own the oxygen.

Awesome article, I know - please share your erudite thoughts...