Pulmonology question

[leviathan]

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We had this on an exam, and as far as I know, there's no way to know the answer from the info given. The question asked what the FiO2 would be for a patient on 10 LPM venturi mask with an inspiratory time of 1 second.

Is this a flawed question, or how would you get to the answer based on only those two variables?

 

[Paseo Del Norte]

That's a pretty...interesting question. We have no idea what the air to oxygen ratio is, we cannot even calculate total flow with the information provided. We have no idea what the patient Vt is nor can we even calculate total cycle time based on the information given.

One thing I can think of is a no brain but tricky question. Is the AEM hooked to oxygen or medical air? All that additional information would simply be irrelevant if we are talking medical air.

 

[dragonark]

FiO2 does not have anything to do with their inspiration time....10 LPM is high flow O2, and the different delivery systems allow for x amount of air to be mixed in with the oxygen flowing in.

NC max FiO2 ~40%

Regular oxygen mask max FiO2 ~60%

Venturi Mask ~80%

Non-rebreather 100% O2

 

[leviathan]

That's a pretty...interesting question. We have no idea what the air to oxygen ratio is, we cannot even calculate total flow with the information provided. We have no idea what the patient Vt is nor can we even calculate total cycle time based on the information given.

One thing I can think of is a no brain but tricky question. Is the AEM hooked to oxygen or medical air? All that additional information would simply be irrelevant if we are talking medical air.

Good, someone agrees with me. 🙂

Although it was not mentioned, we were to assume it was 100% oxygen delivery to the patient. Nothing was told about inspiratory flow rate, Vt, or anything else.

 

[leviathan]

FiO2 does not have anything to do with their inspiration time....10 LPM is high flow O2, and the different delivery systems allow for x amount of air to be mixed in with the oxygen flowing in.

NC max FiO2 ~40%

Regular oxygen mask max FiO2 ~60%

Venturi Mask ~80%

Non-rebreather 100% O2

10 LPM is not high-flow, although nurses and EMTs will often say that. High-flow O2 delivery devices usually require a minimum of 60 LPM to match a reasonable inspiratory flow rate. Inspiration time does matter for this calculation, but it needs to be coupled with more information so you know what the room-air😳xygen mixture was as the patient is inhaling.

I feel like I understand the concept pretty well, but when a trauma surgeon / critical care MD marking my exam tells me that the answer is supposed to be 38% with the only two variables given being litre flow and inspiratory time, I just assume he knows what he's talking about.

Can anyone else provide a possible explanation how you are supposed to get 38% from knowing that 10LPM is provided with inspiration of 1 second, and without being told the patient has a Vt of 1000 mL?

 

[Paseo Del Norte]

Ok, for an FiO2 of 0.38, we need an air to oxygen ratio of about 3.65:1.

Utilising the standard formula and plugging 10 LPM of oxygen into the equation, we will need 36.5 LPM of air entrained and 10 LPM of oxygen. So, theoretically in a vacuum our Ve cannot exceed 46.5 LPM. However, the inspiratory flow demand is going to vary according to the type of patient.

If I have a respiratory rate, I could calculate TCT and figure an I time of one second to come up with an E time and a much better answer. However, without additional information...

In addition, an FiO2 of 0.38 does not appear to be a setting on any of the AEM's that I have ever used, so we are basically placing the port somewhere between the 0.35 & 0.40 mark and guestimating the FiO2?