Oxygen can probably suppress respiratory drive in some (but by no means all)
CO2 retainers. However, the chief signs of respiratory depression are slow,
shallow respirations, not puffing along at a rate of 30, using accessory
muscles. If minute ventilation drops in one of these patients, it is due to
respiratory muscle acidosis more often than central respiratory depression.
CO2 certainly does rise initially in patients given oxygen. This is the Bohr
effect and happens in vivo. Usually it drops later as you treat the
respiratory muscle acidosis.
If you are going to give controlled oxygen, titrate it to the patient rather
than giving a set percentage- regardless of how much you are giving; if the
patient's sats are 70% and they've got thoracoabdominal dyssynergy; you are
not giving enough.
I know from previous threads that I'm preaching to the converted on this
list; but I still find new SHOs in both A and E and subacute medicine
sticking with the BTS guidelines (or even wrongly assuming patients to be
CO2 retainers, so witholding oxygen from elderly patients with asthma or
pneumonia and secondary CO2 rise- and repeatedly doing unhelpful ABGs).
Sorry, rant over (for now).
Matt Dunn
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