Dear colleagues!

Best wishes in Easter Day!

Well, I have a technical question regarding the use of home CPAP equipment in clinics. During these day we studied CPAP machines for treating apnoea patients in case of ARDS caused by COVID19.

The basic idea lies in generation of precise amount of oxygen in the system by measuring FiO2.

The issue is that we know the pressure, generated by the home CPAP - 15 cmH2O. We also can measure the flow of oxygen by using flowmeter in hospital. The main problem is how to measure total flow to solve the equation:

FiO2 = (21 *Flow(air) + 100*Flow(oxygen))/ Flow(total).

We have some solutions and hope you can comment and scientifically criticise the idea to help modify these accessories for CPAP machines asap.

1) First method is to use a T-cylinder (including valves), that regulates the flow of CPAP.

https://grabcad.com/library/respirator-free-reanimation-venturi-s-valve-1

N.B. - these valves are used without ventilators (as I understood - for venturi masks), but we want to adjust them to the CPAP.

An oxygen and air is mixed together in the system creating:

P(total)=P(air)+P(oxygen),

FLOW(total)=FLOW(air)+FLOW(oxygen).

During inspiratory phase endogenous changes in lung volumes promotes pressure gradient (P(cpap) - P(lungs) = dP)

Will dP generates proportionally dP(air) and dP(oxygen) in a same ratio as it was in a non-isolated system? It means that will dFLOW(air) and dFLOW(oxygen) be in the same ratio:

FiO2 = (21 *dFLOW(air) + 100*dFLOW(oxygen))/dFLOW(total),

- In case of expiratory phase valves protect air and oxygen outflow as well as promote better exhale.

And the main question is:

How to make validation of the FiO2 based flow?