Thread by @just1doctorwala

let's get a little more technical with this explanation

so we know that atmospheric air is 21%

but when the lungs are damaged we need to give oxygen support in higher concentrations to overcome the damage and allow oxygenation

we use different delivery devices for this

https://twitter.com/just1doctorwala/status/1385210054538715143

Nasal cannulae
Deliver 24-30% oxygen
Flow rate 1-4L/min (4L will dry the nose, 2L is more comfortable)

Hudson mask
Delivers 30-40% oxygen
Flow rate 5-10L/min

Venturi mask
Delivers 24-60% oxygen
Different colours deliver different rates
Flow rate: Varies with colour. The correct flow rate to use with each colour it is shown on mask, along with the percentage of oxygen delivered.

Types:
BLUE = 2-4L/min = 24% O2
WHITE = 4-6L/min = 28% O2
YELLOW = 8-10L/min = 35% O2
RED = 10-12L/min = 40% O2
GREEN = 12-15L/min = 60% O2

Non-rebreather mask
Delivers 85-90% oxygen
15L flow rate
Bag on mask with valves stopping almost all rebreathing of expired air
Used for acutely unwell patients

Note that saturations should be maintained at 94-98%, not 100%.
Do not keep patients on 15L for longer than necessary as over-oxygenating for prolonged periods can be harmful.

Non-invasive ventilation (CPAP/BiPAP)
CPAP (continuous positive airways pressure)
High pressure air/oxygen with a tight-fitting mask
Positive pressure all the time to help keep airways open (split them)
Used in acute pulmonary oedema and sleep apnoea

BiPAP (bilevel positive airways pressure)
High positive pressure on inspiration and lower positive pressure on expiration
Used in exacerbations of COPD and ARDS

Invasive ventilation
Fully controlled oxygen delivery up to 100%
A ventilation bag or machine is attached to an artificial airway to ventilate lungs.
Used in intensive care and theatre

Thus as you can see anything more than 21% requires oxygen cylinders