I've reviewed numerous mechanisms of action and functions of High Flow Nasal Cannula (HFNC) but I haven't touch on whether or not it works to help ventilate patients. I have discussed in the mechanisms of action that it does wash out the CO2 from the dead space in the nasopharynx, oropharynx, etc, but does that show a numerical decrease in the PCO2? The studies I had reviewed prior to this one weren't promising.
One of the indirect ways that HFNC can bring down CO2 is by bringing down the patients respiratory rate. There's plenty of data to support the decrease in the respiratory rate. Since the person isn't breathing as hard nor as fast, less CO2 is produced. Less CO2 is produced means the patients needs to be ventilated less. Things get better. Prior to this study, though, the data just wasn't there to show that this actually happened in a statistically significant way. I've said this before and I'll say this again, I will not recommend HFNC to a patient with a COPD patient sucking wind in the ED with an exacerbation that has a gas that looks like 7.06/96/66. That patient either needs some non-invasive ventilation with a very close eye or the endotracheal tube.
In this study they placed COPD patients, not in exacerbation, on HFNC and measured a number of parameters but you and I are here for the CO2. Patients had their PCO2 measured at baseline, on 20L HFNC, and at 30L HFNC. At 20L the PCO2 was at approximately 91 (plus or minus 6.7)% of their baseline and at 30L their PCO2 was at approximately 87.4 (plus or minus 6.2) % of their baseline. That data was statistically significant.
This may be completely out of bounds but if we can (although I probably shouldn't) extrapolate that to a patient with a PCO2 of 60, 20L should bring them down to approximately 54.6 and 30L down to 52.4. Something is better than nothing and if you can hold the patient over while they get their steroids and nebulizations, it may be worth a try in the real world.
- EJ
Bräunlich J, Köhler M, Wirtz H. Nasal highflow improves ventilation in patients with COPD. International Journal of Chronic Obstructive Pulmonary Disease. 2016;1077-1085.
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