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Highest possible oxygen concentrations

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#1 Sun, 20/10/2019 - 13:32
doc.deano
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Last seen: 3 weeks 1 day ago
Joined: 20/11/2017 - 20:02

Highest possible oxygen concentrations

Hello

I am a Consultant Anaesthetist and also have Cluster Headaches. Oxygen is a recognised treatment. This post helps explain how we can achieve the high inspiratory concentrations that are most desirable. It is not to explain how oxygen works.

It is very difficult to get 100% oxygen ("pure") into the lungs but we can get close. The speed at which we breathe in can be above 60 litres per minute and up to 120 litres a minute when stressed (in pain). No plain cylinder flows come anywhere near this - even in hospitals where maximum from wall outlests is 15 litres a minuite. So when we start our breath air is "pulled in" from elsewhere and this dilutes the oxygen concentration. Our pattern of breathing also makes a difference. Rapid shallow breaths will not be as effective as slower deeper breathing. (In medical-speak it is alveolar ventilation we are attempting to achieve). What can we do to offset this?

A tight fitting mask will discourage air being sucked in and a resevoir bag that fills (with oxygen) has its contents "pulled in" in preference to outside air. It is important the resevoir bag does not completely collpase while inspiring. In reality some air will always get in to dilute the oxygen coming from the cylinder. Slow deep  breathing to minimise this can help increase concentrations.

A demand valve greatly increases flows and shuts off the oxygen when not inspiring, so the cylinder should last longer (than without).

Everything I read states that 100% oxygen is delivered. This is true but getting it into the lungs for gas exchange will decrease the % that actually arrives there. The only way to achieve 100% oxygen concentration is to have a tube passed into the lungs and deliver 100% oxygen down that! (Intubation). Not recommended unles having an operation anyway!

And finally; it is thought that high oxygen decreases cerebral blood flow. It so happens that breathing rapidly also decreases the carbon dioxide in the blood which also causes a decrease in cerebral blood flow (in the short term) due to vasoconstriction (narrowing of blood vessels). Breathing  fast can cause other unpleasant feelings and even fainting so be careful. Good luck all.

Mon, 04/11/2019 - 14:47
ElizabethK
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Last seen: 1 week 12 hours ago
Joined: 26/02/2014 - 14:26

Thank you - very informative!

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