Low flow devices nasal cannulae simple face masks and reservoir bag masks

Nasal cannulae are commonly used because they are convenient and comfortable. They deliver 2-4 litres per minute of 100% oxygen in addition to the air a person is breathing. If a person is breathing slowly with 10 breaths per minute and 500 ml tidal volumes, the minute volume (or inspiratory flow rate) will be only 5 litres per minute. So the patient receives 2 litres per minute of 100% oxygen plus 3 litres per minute of air. Two-fifths of what the patient is breathing is 100% oxygen. However, a person breathing quickly (40 breaths per minute and 750 ml tidal volumes) will

Venturi valve colour

Flow rate

Concentration (%)

(litres per minute)

Blue

2

24

White

4

28

Yellow

6

35

Red

8

40

Green

12

60

Figure 2.1 Different oxygen masks. (a) Nasal cannulae; (b) simple face mask; (c) reservoir bag mask; (d) Venturi mask. Reproduced with permission from Intersurgical Complete Respiratory Systems, Wokingham, Berkshire

Table 2.1 Which mask for which patient?

Oxygen device

Patients

Nasal cannulae

Simple face masks and masks with reservoir bag (> 5 litres/min aiming for SaO2 > 94%) Venturi masks

Patients with otherwise normal physiology (vital signs), for example, postoperative, slightly low SaO2, long-term oxygen therapy Higher concentrations required and controlled O2 not necessary, for example, severe asthma, acute left ventricular failure, pneumonia, trauma, severe sepsis

Controlled oxygen therapy required, for example, patients with COPD

have a minute volume of 30 litres per minute, so 2 litres per minute of 100% oxygen should be given plus 28 litres per minute air. Only two-thirtieths of what the patient is breathing is 100% oxygen. The person breathing slowly receives a large proportion of oxygen whilst the person breathing quickly receives much less. Thus nasal cannulae deliver a variable concentration of oxygen depending on how the patient is breathing. It is possible to estimate the concentration of oxygen by the calculations in Box 2.1.

Box 2.1 Calculation of oxygen concentration

• Minute volume: 5 litres per minute (for example, 10 breaths per minute x 500 ml per breath)

• O2 flow rate: 2 litres per minute

Inspired O2 concentration = 2 litres per minute of 100% O2 + 3 litres per minute air

• Minute volume: 30 litres per minute (for example, 40 breaths per minute x 750 ml/breath)

• O2 flow rate: 2 litres per minute

Inspired O2 concentration = 2 litres per minute of 100% O2 + 28 litres per minute air

In some cases the oxygen will not be enough and in some cases it could be dangerously high, for example in some patients who retain carbon dioxide. There are several case reports of these patients becoming unconscious with hypercapnia once they have been given nasal cannulae, especially during an acute exacerbation of their illness. There is no way of accurately knowing how much oxygen is being delivered.

Simple face masks (also called Hudson or MC masks) deliver up to 50% oxygen when set to 15 litres per minute. Like nasal cannulae, the concentration is variable depending on the fit of the mask and how the patient is breathing. Sometimes patients are given 2 litres per minute through face masks because they have COPD. Significant rebreathing of carbon dioxide can occur if the oxygen is set to <5 litres per minute because exhaled air is not adequately flushed from the mask.

Face masks with a reservoir bag can deliver 70-80% oxygen at 15 litres per minute but this again depends on the fit of the mask and how the patient is breathing. A reservoir of oxygen is held in the bag. A non-rebreathe valve separates the bag and the mask so that the patient can inhale oxygen from it but not exhale back into it. Sometimes these masks are called "non rebreathe bag masks" for this reason. It is impossible for a patient to receive 100% oxygen via these masks for the simple reason that there is no air-tight seal between mask and patient. The reservoir bag should be filled with oxygen before the mask is placed on the patient. The bag should not deflate by more than two-thirds with each breath.

Nasal cannulae, simple face masks, and reservoir bag masks all deliver variable oxygen concentrations - or uncontrolled oxygen therapy.

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