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Discussion

Low flow vs high flow nasal cannula

Hi! I'm a new grad working on a peds med surg floor. Respiratory season has started strong here and we've been getting lots of kids on high flow oxygen. I have a question that nobody on my floor (nurses, RT's, residents) has been able to answer for me, so I was hoping someone here might know! Unfortunately we do not receive any formal training on how exactly the high flow equipment works so this may be part of the issue for me.

I understand that with high flow flow you can control the FiO2 where with low flow NC you cannot as accurately. But with our high flow set up, we wean kids down to 3L and then once they can wean more we remove the high flow cannula and replace it with a low flow NC at 3 L. I don't understand the difference between 3L high flow and 3L low flow oxygen... They both hook up to the same oxygen flow meter and are at the same rate. High flow is humidified and heated and has the ability to attach in-line nebs, but aren't they getting the same concentration of oxygen? On our equipment I haven't been able to find anything that states the FiO2 or how to increase or decrease the FiO2 for the high flow, so I've been thinking the FiO2 correlates with the LPM of oxygen that the flow meter is adjusted to (like with low flow). But maybe this is where I'm getting stuck?

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    • Experts

    The high-flow set-up has both a flowmeter and a valve that allows for adjustment of the FiO2 more precisely. If you look at the circuit, you'll see that it's connected to the wall oxygen source with the same high-pressure hose that is used for mechanical ventilators, both invasive and non-invasive. There should also be an oxygen analyzer inline that measures the actual FiO2. On the pole holding the humidifier, there will be a knob for adjusting the FiO2, with the oxygen flowmeter either behind or to one side.

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    Before the development of Optiflow and the other high-flow circuits, we didn't run oxygen at high flow rates because there was no way to heat it enough to make it comfortable, and the bubble chamber arrangement didn't really provide much humidification either. In some ways, high flow oxygen acts as a form of CPAP as the flow rate splints open airways. As the patient's lung function improves, the flow may be weaned to 1L/kg first, then the FiO2 titrated to maintain sats, before the flow is weaned again. Once they get down to 3LPM, which is comfortable for most patients, regardless of the FiO2, the switch to low-flow cannulae is made, with the FiO2 at 100%. The patient is actually getting somewhere around 40% at the alveoli due to some blow-off during expiration and some leakage at the nares.

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    Thank you so much for your response!! I went back to work last night and looked everywhere for any kind of knob for FiO2 for my patient and there wasn't one! However I checked in another high flow pt's room, and there was one just like in your picture. But it wasn't connected to anything and was labeled "return to PICU" so I can only assume we had to borrow more set ups from the PICU. So now I know what they look like but we still aren't utilizing them! I triple checked our set ups on multiple pts and it is simply connected from the wall, to the humidifier, to the pt! I sent an email to our educator asking about this, but she said she'd have to ask around and get back to me.

    But if this set up without FiO2 meter is actually how RT is supposed to set up high flow on our floor, isn't it basically the same as just giving the pt heated and humidified regular flow oxygen? Our floor is med surg and our policy is only to keep kiddos up to 6L high flow. So they don't get higher flow than they potentially could on a low flow nasal cannula, it's just probably more comfortable and has the ability for in line nebs. Can you think of any other reason to do it like this?

    Thanks again for your help!!

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    • Experts

    Hmm. If you're only going up to 6L/min regardless, then you're not really delivering high-flow O2. There's a growing body of evidence that high-flow oxygen is safe and effective for all children in the treatment of bronchiolitis (and some other respiratory conditions) and in a significant number of patients prevents intubation +/- PICU admission. Standard protocol is to start with 2L/kg/min (40 kg adolescent with asthma exacerbation = 80 L/min flow) then titrate as able. At our hospital, only the ED and PICU are permitted to initiate HFNC, and patients are admitted to PICU until they're down to low-flow. Other hospitals, particularly in Australia, don't require an ICU admission for HFNC and are saving a lot of money by managing bronchiolitics on the wards. Some of the older kids I've admitted to our PICU on HFNC have been stable and not really ICU candidates - sitting up playing video games and whining about being hungry - but for that hospital policy.

    Now, having said all that, once the cannulae and circuit are in use, it makes poor financial sense to then swap the cannulae out for a low-flow set. And the HFNC are much more comfortable, so especially in younger kids and those with developmental disabilities, we just leave them as is, swapping out only the actual connection to the wall O2. Having the capability of using the Aeroneb is a huge benefit too. But then again, the cannulae are a LOT more expensive than those old low-flow sets, so I doubt they're just being used for low-flow for no reason. I hope your educator is able to illuminate this for you. I'm interested in what she has to say.

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    As far as the FiO2, I would think you really should have those knobs (called blenders). We hook up all of our kids (NICU) no matter what to O2 on a blender so that I can control/wean the FiO2 they are getting at any moment w/o changing the flow. It sounds like all of your kids are hooked up to the wall at 100% O2, which might be okay in general peds but I really don't know. The only time we hook kids up to the wall at 100% is for home going purposes (say if they are going home on 1/2L of 100% then that's what we put them on immediately prior to discharge). You can wean the FiO2 just the same w/ both HFNC and a regular NC, but of course you'd need a blender on either one.

    As far as the difference between high flow and low flow cannulas, it simply is the ability to heat and humidify through the high flow circuit. The kid could be on a high flow nasal cannula and only be getting "low flow" at 1L (or even less), and the air would just be humidified and heated. That would be okay and sometimes we do that. But they are getting the same flow in liters/min as they would on the regular cannula set at the same level on the flowmeter. On the flip side, you technically could deliver high flow through a regular cannula but we simply don't because it is damaging and uncomfortable and therefore requires the high flow cannula. So basically, many times the term high flow refers to the method of O2 delivery (regular cannula vs HFNC) and not the actual L/min flow volume. Long story short, you need the HFNC on the way up in order to safely deliver high flow volumes, but you don't really need to switch to the regular cannula on the way down as heat/humidity doesn't really have the potential to hurt the patient.

    And again, I don't work in PICU/gen peds, but we consider high flow basically anything 3L+. We max out somewhere around 6-8L before we usually go to CPAP. When weaning, we don't switch to a regular cannula until 2L, but sometimes we just don't until later because there really is no rush.

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    Our main reason for switching to high flow is for the PEEP. So we'll often do it for a patient satting well on the regular nasal cannula but still having respiratory distress. So sometimes they end up getting less oxygen but more pressure through the cannula than from the regular nasal cannula.I've had patients effectively on room air but on the high flow set up to give them just a little boost of pressure when they don't quite need CPAP.

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