Preoxygenation, Reoxygenation, and Delayed Sequence Intubation in the Emergency Department

Scott D. Weingart, MD

Disclosures

J Emerg Med. 2011;40(6):661-7. 

In This Article

Reoxygenation

If the first pass at intubation fails and the patient's oxygen saturation drops below 90–95%, reoxygenation is required before any further intubation attempts. The standard method for reoxygenation is to ventilate the patient with a BVM apparatus attached to high-flow O2. Skilled practitioners will also place an oropharyngeal airway and, if there is any difficulty, nasopharyngeal airways as well. Even in skilled hands, this method can be problematic; when performed by a novice, it can be deadly.

Every BVM breath during reoxygenation potentially puts the patient at risk for gastric insufflation and aspiration. Ideally, the patient would receive the minimum number of ventilations to achieve reoxygenation and these breaths would be delivered in a slow, gentle manner to avoid overcoming the lower esophageal sphincter opening pressure of ~20–25 cm H2O.[22] However, studies show the difficulty of maintaining these goals during the stressful environment of an emergency resuscitation.[23,24]

In addition to changes in time perception when stressed, another possible explanation for this is a misunderstanding of the effects of increased ventilations on oxygen saturation. Ventilating the patient at increased respiratory rates will not raise the oxygen saturation any faster than at a controlled rate. In Figure 3, the effects of alveolar ventilation on oxygenation can be appreciated. At a fiO2 of 0.5, only ~ 500 mL/min of ventilation must reach the alveoli to generate a high PaO2. At a fiO2 of 1.0, even less alveolar ventilation must occur to yield a PaO2 > 500 mm Hg. Even assuming a high fraction of dead space in a patient undergoing resuscitation, this means that to achieve reoxygenation with the buffer of a high PaO2, only 3–4 breaths/min are needed. Given this information, the rate of 10 breaths/min recommended by most resuscitation guidelines seems reasonable and safe, offering at least double the required number of breaths. Ten slow (1.5–2 s per breath), low tidal volume breaths per minute would seem the optimum rate for reoxygenation. Yet, when the patient has desaturated, we often witness rates as high as 60–120 breaths/min.

Figure 3.

Alveolar ventilation vs. alveolar oxygenation. When breathing room air, approximately 3 L must reach the alveoli to maintain a PaO2 > 100 mm Hg. If the fiO2 is increased to 0.5, only 1 L/min is needed to generate a PaO2 > 500 mm Hg. If the fiO2 is increased beyond 0.5, even less alveolar ventilation is needed.

Beyond ensuring the proper rate and timing of ventilations, ideal mask seal is also imperative or the ventilations will not reach the alveoli. During our training, we are still taught how to correctly hold the mask of the BVM with one hand, but this is an inferior method that often does not achieve an adequate seal. Two providers are needed for reliably effective BVM ventilation: one to hold the mask with two hands and a second person to squeeze the bag.

Standard BVMs cannot provide PEEP, which, as we have previously discussed, is the only effective means to treat shunt during emergent intubation. In patients who required CPAP for preoxygenation, to attempt to reoxygenate with zero PEEP is illogical and often unsuccessful. PEEP valves are available that fit on the exhalation port of most BVM devices. These strain valves allow the generation of some PEEP by occluding the exhalation port to a selectable extent, but the PEEP disappears with continued gas absorption or with any loss of mask seal. Despite these disadvantages, when no other options exist, PEEP valves can have dramatic effects on reoxygenation.

There is, however, another commonly available solution to the problems of BVM reoxygenation: the standard ED mechanical ventilator as a reoxygenation device. This same ventilator can be used for the non-invasive preoxygenation as mentioned above and therefore it is advantageous to have at the bedside a standard ventilator rather than a non-invasive ventilation machine for the intubation of a high-risk patient.

The ventilator provides guaranteed slow, low tidal volume breaths. PEEP can be added and titrated to the patient's requirements. A single provider can hold the two-hand mask seal while the ventilator delivers the respirations, freeing up a practitioner. Ventilator settings for reoxygenation are shown in Figure 4. Two studies have compared handheld ventilators to BVMs for non-intubated ventilations; these studies have shown the handheld ventilator to be safe and that it may be associated with fewer complications.[25,26] The improved valve structure and more precise settings of a standard rather than handheld ventilator make it even more desirable. For this strategy to be successful, the clinicians must be able to set up the ventilator themselves without having to wait for a therapist to be paged down to the ED.

Figure 4.

The steps of non-invasive ventilation for preoxygenation, using the ventilator for reoxygenation, and delayed sequence intubation (DSI).

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