Many doctors I came had this reasoning that HFNC reduces anatomical dead space and they believed this make a therapeutic difference. They had picked up this belief from literature which I traced to a single paper.
In case of cost of therapy it boils down on whether to use 60LPM of oxygen through a cannula or an average 15LPM through a mask. The correct answer saves on oxygen utilisation.
Let us go into a simple estimate
- Tidal volume : 1000ml
- Anatomical dead space (Residual lung volume + airway volume) 500ml
- Nasopharynx: 150ml
- Anatomical dead space saving by HFNC: 150/500 = 30%
- At 100% FiO2 that works out to:
- Decrease in alveolar CO2 concentration from 5% to 3.5%
- Increase in oxygen concentration from 95% to 96.5%
I.e 40% decrease in CO2 and 1% improvement in Oxygen concentration
Is 60LPM worth this 1% improvement in Oxygen? Does this Anatomical dead space reduction really make sense in Type 1 respiratory failure of COVID?
Clinically we have got feedback that NIV R20http://www.xrossfusion.com/NIV-R20.html maintains SpO2 efficiently. But a scientific paper is yet to be published. If so this would be a sort of myth buster.
If Anatomical dead space reduction (or residual capacity improvement) does not make sense that what makes sense in HFNC? Well, prevention of entrainment of room air is the one and only mechanism that makes HFNC work. If so there should be other ingenious ways of preventing room air entrainment without wasting oxygen. NIV R20 is one such innovation.