The ROX Index, and “How Much Flow?”

I love high flow nasal cannula (HFNC) oxygenation. It’s a great and simple tool to improve oxygenation, ventilation, and work of breathing in people who are struggling but may not need intubation. It’s also a great tool to use when you extubate someone who is a little marginal. We’ve all that experience of the patient who technically meets the criteria for extubation but your gut tells you they may have a hard time. Extubation to HFNC can help them fly following removal of the tube. It can also be used to prevent intubation in someone who is on the fence. But, how do you know who it will work for?

Dr Eddy Joe Gutierrez has a nice post regarding the ROX (Respiratory rate-OXygenation) Index. It’s a simple calculation that can be used to predict who will benefit from HFNC to reduce work of breathing and impending respiratory failure vs who just needs to get intubated. To get the ROX Index, just divide the ratio of SpO₂/FiO₂ by the respiratory rate. The SpO₂/FiO₂ ratio is similar to the PF ratio, but using the pulseox instead of having to grab and ABG. Eddy Joe points out that you’ll want to actually count the respiratory rate and not use the number that all to often gets erroneously charted. MDCalc has a calculator here to make it easier. ROX ≥ 4.88 is a good indicator of HFNC success. Lower than that, there is risk that HFNC will fail and you’ll need to intubate anyway. See Eddy Joe’s whole post for more details.

Another thing that this post addresses is the question, “how much flow should I start my patient on?” This is often an issue when providers have little experience with HFNC. When we first started using it, I would frequently find patients on 100% FiO₂ and 10lpm of flow. This is the exact wrong way to use HFNC. The real benefit is the flow, not the FiO₂. Eddy Joe points out that he typically starts with 50lpm, and this is roughly where I usually start as well. Some people won’t tolerate that much flow, but starting high gives you some wiggle room. If they are uncomfortable with 50, drop down to 40. Odds are, by comparison, this will be more pleasant and you’ll have a little great success than if you start low and go up. Eddy Joe shared a study on Twitter the other day that found that 30-40lpm is the optimal flow rate to use. So, it’s nice to have data to back up what my experience/gut had showed me. Read the entire study here.

So, if you’re not using HFNC, you should. If you are, try the ROX index to guide you. Also, start your flow high. Remember, the real benefit is in the flow and for that you need high rates, at least 30lpm.