In addition to the ergonomic difficulties of face-mask ventilation, problems creating an effective seal, and issues delivering oxygen into the lungs at low pressure, mask ventilation in a supine position has many disadvantages in terms of oxygenation.
Explore This IssueACEP Now: Vol 36 – No 01 – January 2017
In a flat position, the abdominal contents push the diaphragm upward, reducing the functional residual capacity of the lungs. Additionally, the posterior lung segments collapse.
Unlike pressurization of the oropharynx, pressurization of the nasopharynx causes passive opening of the airway as the soft palate is pushed away from the posterior pharynx (see Figure 1). Combining nasal oxygen with pulling on the mandible is an incredibly easy and fast way to open the upper airway. Oxygen shoots from the nasopharynx, down into the upper airway, and into the trachea. In the patient who is upright, the diaphragm drops and the lungs expand. Through the miracle of hemoglobin, oxygen is drawn down the trachea as it gets absorbed across the alveolar capillary membrane even without positive pressure ventilation (apneic oxygenation).
I used to bag patients as my initial response to hypoxemia in the emergency setting. Now, I put Oxygen On, Pull on the mandible, and Sit the patient up (OOPS). I have done this in the setting of oversedation and narcotic overdose, which resulted in complete apnea, and oxygenation improves quickly. I sometimes augment nasal oxygen at the top of the flow meter 15+ liters with a non-rebreather to boost oxygen flow >30 lpm.
In cardiac arrest, I used to bag patients while preparing to intubate. Now, I use passive apneic oxygenation and, if necessary, place an LMA-type device to run the initial portion of the code.
My current use of mask ventilation is only when I want to deliver some positive end-expiratory pressure (PEEP; PEEP valves should be on every BVM). This is generally only used when inducing patients for intubation. I gently ventilate for a couple of breaths when I use muscle relaxants to confirm that I can bag the patient and to expand the alveoli during the onset phase of muscle relaxants. I always do so in a head-elevated position (at least ear-to-sternal notch). I am careful to use low pressure, low volumes, and low rates, except in situations of compensatory respiratory alkalosis. My use of face-mask ventilation in these settings is generally with a nasal cannula, which helps stent the airway open and augment flow. I choose to perform face-mask ventilation in this situation, as opposed to an LMA, because I am worried about the LMA device being inserted too early, which could trigger active vomiting before rapid-sequence intubation medications kick in.