While breathing room air, oxygen saturation drops precipitously to below 90% within about a minute of the start of apnea in the average healthy adult. As we saw in a previous blog post, preoxygenation is one of the most important safety measures we can use prior to induction of anesthesia and in preparation for intubation. Adequate preoxygenation can more than double the time to hypoxia during open airway apnea, allowing more time for intubation to occur. However, increasing the time to critical hypoxia from 1 minute to 2 or 3 minutes with preoxygeation, as important as that is, can still be too short if the intubation turns out to be truly challenging. Apneic oxygenation is an easy technique to increase the time to desaturation significantly. However you have to know how to optimally provide it in order to safeguard your patient
Which Patients Are At Risk Of Rapid Oxygen Desaturation?
Many factors predispose to rapid onset of critical hypoxia including:
- Decrease in FRC from any cause
- Higher metabolic rate
- Lower Hgb Concentration
- Lower Alveolar Concentration of Oxygen
The patients groups most likely to have these characteristics include:
- Morbid obesity
- Small children
- Lung Disease/Shunting
- Serious Illness
Why Are Morbidly Obese Patients At Risk of Rapid Desaturation?
Morbidly obese patients cause a great deal of concern when planning an intubation. In addition to being at higher risk of rapid desaturation, the morbidly obese are also most likely to be in the difficult to intubate category. Morbidly obese patients will often desaturate quickly with apnea for many reasons:
- increased oxygen demand, CO2 production, and alveolar ventilation because metabolic rate is proportional to body weight
- increased minute ventilation required to maintain a normal CO2 level.
- fatty tissue over the chest decreases chest wall compliance
- increased abdominal mass forces the diaphragm upwards and reduces lung volumes
- inspiratory and expiratory reserve volumes are decreased, which leads to a lower functional residual capacity and vital capacity
- FRC is further decreased when supine and especially when in Trendelenburg
- some morbidly obese patient’s (BMI greater than 40), already have baseline relative hypoxemia because of these physiologic changes.
What Is Apneic Oxygenation
One technique that can markedly delay the onset of hypoxia during apnea is apneic oxygenation.
Once any period of apnea starts (even holding your own breath), the body will still continue to remove oxygen from the lungs as long as there is oxygen present. A nonfebrile adult at rest uses roughly 3 ml/kg of oxygen per minute. A child can use 6 ml/kg/min. Fever, seizure, and increased activity raise that oxygen consumption significantly When we inhale room air with a concentration of 21%, we usually exhale gas containing a concentration of about 15% oxygen. In between, the average 80kg adult removes about 240 ml (3 ml/kg X 80kg) of oxygen from his lungs. If metabolic rate goes up, then oxygen will be removed from the lungs at a much faster rate.
The oxygen saturation only starts to decrease when the store of oxygen in the lungs is depleted and the PaO2 is of the order of about 60mmHg corresponding to an O2Sat of about 90%. Once saturation reaches this level it drops rapidly, about 30% every minute. . Once the O2 sat falls below 90%, arterial oxygen drops quickly into the dangerously hypoxic range as fewer and fewer oxygen molecules are bound to Hgb. We want to try to keep O2 saturation above 90%.
As oxygen is removed from the alveoli, the gas pressure in the alveoli goes down. If the airway is open, gas from the oropharynx will be sucked down into the lungs. If that gas contains a good concentration of oxygen, then oxygen in the alveoli will be replenished and onset of hypoxia is delayed.
This is the main reason why an intubator will continue to hold the airway open with the oxygen mask sealed in place while waiting for induction medications to take effect — even if they are performing a true rapid sequence induction and not actively ventilating the patient. The intubator knows that oxygen will continue to passively enter the lungs from the oropharynx during this time. This is passive apneic oxygenation.
Proactive Apneic Oxygenation
However, we can be more proactive about providing apneic oxygenation than just holding the airway open. We can use nasal prongs to provide a continuous flow of oxygen into the pharynx while the patient is apneic.
One study (1) looked at 30 obese patients with BMI between 30-35. They provided 5 liter per minute nasal oxygen flow during a prolonged apneic period. They simulated difficult laryngoscopy by gently inserting the laryngoscope, visualizing the larynx, and then delaying tracheal intubation until 6 minutes after succinylcholine administration or when arterial oxygen saturation (SpO ) decreased to 95%. Patients who received oxygen during apnea maintained SpO ≥95% for significantly longer than controls (5.29 vs. 3.49 minutes) and had significantly greater mean SpO nadir (94.3% vs. 87.7%). Time to desaturation to an SpO of 100% after initiation of ventilation with 100% oxygen did not differ significantly between groups.
When you are managing a difficult intubation, extending the time to critical hypoxia to as much as 5 and a half minutes is tremendous. Can we do better?
Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE) is a more aggressive technique for providing high-flow, positive-pressure, humidified oxygen via nasal canula. In an observational, cross-sectional study (2), researchers in the U.K. used the THRIVE technique in 25 patients with known or anticipated difficult airways who were undergoing general anesthesia for otolaryngology procedures.
They elevated the head to 40 degrees (thereby increasing baseline functional residual capacity and improving preoxygenation). They preoxygenated with 70 L/minute for 10 minutes using a commercial device (OptiFlow) to deliver high-flow, humidified oxygen via a modified nasal cannula. After induction, they lowered the head to 20 degrees head-up and continued the nasal cannula flow to provide apneic oxygenation until intubation had been successful. The mean time to intubation was 17 minutes and none of the patients desaturated below 90% despite apnea.
Now not everyone is going to have a device capable to providing humidified nasal oxygen at 70 liter flow. But even short term 15 liter unhumidified nasal prong flow has been shown to be tolerated and effective in delaying onset of critical hypoxia.
What About Hypercarbia?
Of course during apnea, not only is oxygen dropping by carbon dioxide is rising. While apneic oxygenation increases the time to hypoxia, it does not change the rate of rise in carbon dioxide. Compared to 250 ml/min of oxygen moving out of the alveoli to the bloodstream, only eight to 20 ml/minute of carbon dioxide moves into the alveoli during apnea, with the remainder being buffered in the bloodstream. This typically causes a rise of 8-16 mmHg carbon dioxide during the first minute and then 3-4 mmHg carbon dioxide per minute thereafter. A rising PCO2 will progressively drop pH.
As we noted when we were discussing preoxygenation, you can increase the time to significant hypercarbia by asking your patient to hyperventilate for a minute or so prior to induction or hyperventilating your patient manually with bag-valve-mask after induction of unconsciousness.
However, use of higher flow (> 15 L/min) nasal prong oxygen during apnea may actually help slow carbon dioxide rise by washing out by washing out deadspace and stenting open airways allowing better gas washout.
When Should We Consider Proactive Apneic Oxygenation?
We should certainly be using passive apneic oxygenation on all patients by keeping an open airway with an oxygen mask with a good seal even if we are not ventilating the patient. Who should get nasal prong proactive apneic oxygenation?
We should be considering the use of nasal prong oxygen flow for apneic oxygenation in patients at risk of rapid development of hypoxia. Patients at risk include those who are critically ill, obese, pregnant, those with potentially or known difficult airways and even the complicated pediatric patient. Any technique that can potentially delay the onset of critical hypoxia in our critical patients is worth a look.
May The Force Be With You
Christine E. Whitten MD, Author Anyone Can Intubate 5th Edition
Pediatric Airway Management A Step by Step Guide
Ramachandran SK et al. Apneic oxygenation during prolonged laryngoscopy in obese patients: A randomized, controlled trial of nasal oxygen administration. J Clin Anesth 2010 May; 22:164.
Patel A and Nouraei SAR. Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE): A physiological method of increasing apnoea time in patients with difficult airways. Anaesthesia 2014 Nov 10; [e-pub ahead of print](http://dx.doi.org/10.1111/anae.12923)
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