Oxygen delivery and devices

Many hospitalised patients with Covid-19 infection develop profound hypoxaemia. Understanding the principles between oxygen delivery and the different devices that can be used to provide it is key for several reasons:

  • To optimise oxygen delivery to the patient
  • To identify patients who are deteriorating from an oxygenation/ventilation point of view and escalate appropriately

And finally, because at a time of hugely increased demand, in many hospitals the available flow of oxygen may be at a premium:

  • To use only the required amount of flow for each patient
  • To consider the use of alternative methods to generate oxygen flow and reduce demand on central ‘wall-piped’ oxygen

What O2 is my patient actually getting?

Piped oxygen from the wall outlet will provide an FiO2 of 100% at differing flow rates depending on how high you turn the tap. Each ward will be able to provide a maximum total flow e.g. 120L/min before pressure is unable to be uniformly maintained and flow rates may drop in certain areas.

For most devices the FiO2 actually delivered to the patient’s alveoli depends on the flow from the wall, the device/mask used (and therefore the air at an FiO2 21% that is being entrained into the device in addition) and the peak inspiratory flow rate of the patient (amongst other patient factors).

For example:

A simple face mask (Hudson) is delivering 10L/min of 100% O2. The patient has a normal peak inspiratory flow of 25L/min. Therefore, an additional 15L/min of room air 21% O2 will be drawn into the mask to make up the total to 25L/min.

(10 x 100) + (15 x 21) = 1315%

1315/25 = 52.6% is actually delivered to the patient’s lungs at peak inspiration.

However, if we increase the peak inspiratory flow to 50L/min because the patient has increased work of breathing then:

(10 x 100) + (40 x 21) = 1840%

1840/50 = 37% is actually delivered to the patient’s lungs.

As you can see, this is a much lower percentage than before.

Devices and Masks

Nasal Cannula: 1-5L/min (higher flows not well-tolerated due to discomfort). No reservoir.

Pros:

  • Well-tolerated
  • Patients can eat/drink

Cons:

  • Dries out mucosa with risk of epistaxis
  • Flow occurring during expiration is wasted
  • Ineffective if mouth breathing
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Simple face mask: 5-10L/min (requires a higher flow to prevent rebreathing of exhaled gases from the small reservoir created by the mask).

Pros:

  • Cheap
  • Can deliver a high FiO2 up to approx. 55%

Cons:

  • Full mask so eating and drinking requires removal
  • Lack of seal around edge of mask results in air entraining
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Non-rebreathing mask with reservoir bag: 10-15L/min (if lower, then reservoir bag will not be adequately full during inspiration). Valve between mask and bag means that O2 from reservoir is not diluted with expired air.

Pros:

  • High FiO2 up to approx. 85%
  • Good for hypoxia if ventilating well

Cons:

  • Full mask so eating and drinking requires removal
  • Lack of seal around edge of mask results in air entraining
  • If lower flows, leads to rebreathing of CO2
  • Long term can cause O2 toxicity and reabsorption atelectasis
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Venturi mask: A certain ratio of air drawn in through the port that then mixes with the 100% O2 from the wall outlet. This creates a high flow that  usually exceeds the peak inspiratory flow of the patient (don’t confuse this with the flow setting you use for the wall outlet; the total flow generated by the mask and port is higher than this) and fixes the FiO2. Excess gas and expired CO2 is flushed out via holes on the side of the mask. 

Pros:

  • Delivers fixed FiO2 which is good for patients prone to T2RF (e.g. COPD) and for stepwise weaning

Cons:

  • Higher flow rates of O2 are required to generate fixed FiO2s compared to other devices which may require relatively lower flow from the wall but deliver variable FiO2s- a problem if total demand from the ward is high and there is a risk safe levels may be exceeded
  • Full face mask
  • Changing FiO2 requires changing ports

Practical considerations for Covid-19

At each board round, ensure that the oxygen flow in L/min is documented for every patient and the total for the ward calculated. This can be compared to the limit on demand which cannot be safely exceeded without compromising availability to certain parts of the ward (your hospital Estates department should know this).

At every patient bedside encounter, check the sats and wean oxygen if possible. Aim for >90% if patient is comfortable.

Do not use Venturi masks as first line unless at risk of hypercapnic failure as they require disproportionately higher flow rates from the wall O2 outlet. Similarly nasal cannulae waste much of their flow during expiration. Try simple face masks instead to reduce total demand.

Consider early critical care review for non-invasive ventilation with CPAP or intubation and invasive ventilation if patients are requiring >6L/min.

The PEEP provided by CPAP may enable lower flow rates of O2.

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