The evidence around high flow humidified nasal oxygen in adult patients is developing, although the physiological mechanisms remain poorly understood. Instead of replicating information relating to the performance and summary of mechanisms, I’d encourage you to check out this very succinct and thorough overview from the LITFL Critical Care Compendium.
Although I am the first to acknowledge that in an adult population, the beneficial effects (CPAP/PEEP) of flow are demonstrated at flow rates of 50-70L/min, many wards/departments/units/regional hospitals don’t have sufficient equipment (gas blender, etc.) to produce/regulate these types of flows. So the purpose of this post is to demonstrate how to achieve up to 30L/min flow at up to 0.60 FiO2 with a standard humidifier, two-pronged plastic connector (Batman connector) and a humidified oxygen circuit with High Flow Nasal Cannula such as the Fisher Paykel Optiflow system.
Below is a set-up diagram of the components in the system, which I have been referring to as Ward “High” Flow.
So, why bother you ask? There is a very apparent gap in the admitted inpatient medical emergency setting when it comes to the adult patient in respiratory distress (high work of breathing) requiring high concentration oxygen. The generic process (from experience in Medical Emergency Teams at 3 different hospital I have worked in) is:
Patient dyspnoeic with low SpO2 = 15L/min Oxygen delivered via Non-Rebreather Mask or Partial Rebreather achieving an FiO2 somewhere in the range of 0.65 to 0.90. If Therapy is instituted,the underlying cause is addressed and the patient responds to treatment (read: bronchodilators, ventilators, diuretics and repositioning), the oxygen is weaned and the patient is returned onto simple nasal cannula or room air. There is limited insult to mucocilliary mechanisms and everyone breathes a collective sigh of relief and the MERT is over.
But what about those patients that just don;t get better that quickly. I have seen many situations when the patient is left at the end of our swag of usual interventions, improved somewhat, yet still dyspnoeic with high oxygen demands and limited respiratory reserve. Should this 97 year old man, who is not a candidate for ICU, nor would he wish for that, be left on 10-15L/min face mask oxygen languishing with moving goalposts of ‘aim for SpO2 of….insert declining number below 90%’? Or is there something relatively cheap, easy and evidence based we can do? This is not an isolated case. Nor is it the only useful application for a simple ward based “High” flow solution.
The key benefits are that, in the acute phase, this setup can achieve 30L/min flow. This flow more comfortably meets increased respiratory demand, provides a moderate degree of CPAP effect (depending largely on Nasal Cannula fit) and aids with nasopharyngeal washout of expired carbon-dioxide. The setup also provides heated humidified gas – facilitating secretion clearance. All these factors can bridge that uncomfortable distance between the ward and the ICU.
When have I suggested this option? Many times.
If a patient is describing feeling flow starved – ‘I’m not getting any air’ and there is ongoing respiratory distress, with increased work of breathing past the immediate treatment phase and, for whatever reason, this person is not going down the path to intensive care, consider this option.
As a crude guide, my experience (almost unequivocally) has been that when applied as indicated, transition from 15L/min face mask oxygen results in improvement in SpO2 within 2-5 minutes, decreased work of breathing, respiratory rate and increased patient reported comfort within 5-10 minutes and all with a reduction in FiO2.
This is not a common therapy in any hospital I have worked in. Beyond the set-up of the circuit, the major issue is the continuity of this therapy as the flow rates don’t easily calibrate with our common experiences and mental computation of FiO2 delivered by different oxygen devices. This is why I have written this post. The below table is a correlation of fraction of inspired oxygen with the dialled-up flow rate from Air and Oxygen. I hope this can aid the ongoing management and remove some of the hesitation around utilising this very easy and beneficial therapy.
I have no conflicts of interest to disclose and have referred to Fisher Paykel only because these are the products with which I am familiar.
For more information, indications and rationale please check out the links within this post. If you use this set-up, please get in touch and let me know your experiences.
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