In message <8AFB10D0D3D6D3119C27009027E77D39085766@PWHEXG1>, Robert
Anthony COCKS <[log in to unmask]> writes
>Gautam
>
> Re your comment: "I tell them that hypoxia kills in minutes,
>CO2 retention takes longer"
>
>
>I agree - but your comment about the timescale of CO2 retention may not
>apply. The respiratory physicians are not talking about acute CO2 retention
>in these patients, but chronic. With chronically high pCO2, upward changes
>in pCO2 cease to be a stimulus to respiration, and the only thing acting as
>a stimulus is lowering of pO2. Their argument is that giving Oxygen removes
>the only stimulus to breathing (hypoxic drive) which remains in these
>patients.
>
>Fine theory, but I've never seen any COPD patient with acute-on-chronic
>respiratory failure stop breathing in A&E when given the Oxygen they so
>obviously need.
I have. Or at least seen pts getting sleepier and sleepier as their CO2
builds. This takes hours but it is more than theory and it can kill,
slowly, un-dramtically and potentially un-noticed.
Don't get me wrong. I still think giving high FiO2 early to a pt with an
acute exacerbation of COPD is a GOOD thing, and I will happily go toe-
to-toe with any respiratory physician who thinks otherwise. But anyone
applying high FiO2 (incl pre-hospital) should be aware of the potential
risk and aim to get gases done sooner rather than later, and to repeat
them regularly until a steady state is established. The aim is to apply
as much oxygen as is needed without driving up CO2. If in doubt, assume
hypoxia and treat aggressively with high FiO2.
There is an inevitable trade off in such patients between their acute
need for oxygen and the longer term risk of hypercapnia. We have seen
such patients existing reasonably well for a number of years with a paO2
of 8kPa and a paCO2 of 6. Artificially putting their paO2 up to 9 with
60% FiO2 can, and DOES, sometimes allow the slippery slope of CO2
retention and reduced respiratory drive to set in.
You correctly mention that acidosis is a powerful respiratory stimulant,
but CO2 is an even more powerful anaesthetic gas once you get up towards
10kPa. Perhaps someone could tell me if the acidotic drive is reset in
COPD pts the way hypercapnic drive is.
A final morbid thought: In the end-stage patient where all else has
failed, if the choice is between slowly falling asleep peacefully due to
CO2 retention whilst oxygen-replete, or thrashing around in a distressed
hypoxic death-throw with normal CO2, I know what I'd prefer. Give me O2
every time.
Dr G Ray
Staff Grade
A&E
Sussex
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