I’m not in a good position at present to look into this, but perhaps someone’s interest may be piqued.
It occurs to me that the recent oxygen shortages in various countries can be alleviated by recycling the oxygen at the point of consumption by scrubbing out the CO2. The way oxygen is used now is analogous to open circuit SCUBA (exhaled gases are discarded); what I’m suggesting is akin to closed circuit rebreathing.
In CCR the exhaled gases are passed through the scrubber which absorbs the CO2. Additional O2 is added to make up for the amount actually used, and the resulting mix is inhaled.
Normally the absorbent is a mixture of sodium and calcium hydroxides in granular form. This may not be readily available. An alternative may be portland cement (which contains quicklime) mixed to a watery consistency, through which the gases are bubbled. A second collection over water is likely to be needed to remove any clay particles from the first collection (lots of clay in cement).
I don’t see any easy way to monitor the pp-O2 and pp-CO2 which is arguably more important. The fall back is to rely on the patient’s oximeter.
In practice I imagine this would be run as a semi-closed circuit, with a decided excess of O2 added. With an ideal rebreather (no O2 wasted), the O2 supply would be extended approx 7x (spitballing here). If only 2x is possible I would call it a win.