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Microgram - Is Flow Additive?

I posed the above poll on Twitter to see what people thought. The reservoir answer, while the most popular, was actually a distractor item - that option doesn't actually have anything to do with liter flow / speed. The basic question here is: If you add more devices to a patient, does it increase the overall speed of oxygen deliver? Why does this even matter? 

If a patient has an inspirtory flow demand that exceeds the flow of gas we are delivering to them, they will entrain room air in inhalation, causing the fraction of oxygen we are delivering to them to be diluted. Therefore, we want to do our best to meet or exceed their inspiratory flow demands. I know a lot of people are using flush with these devices, but I wanted to use the standard 15 Lpm in this example to drive the point home. 

 

Check out this illustration:

You have 30 gallons per minute hitting the bottom of the tank. Behind the mystery door, would you be able to tell me how many hoses are feeding that 30 gallons per minute flow? No. I could have 10 hoses running at 3 gallons per minute, or 2 hoses running at 15 gallons per minute. What really matters is the total amount of volume that is hitting the bottom of the tank over a set time limit (in this case one minute). 

One hose is running at 5 gallons per minute, another at 15, another at 10. We can see that none of these hoses are running at 30 gallons per minute, but their combined flow does cause 30 gallons per minute to be delivered to the tank. The hole in the tank (representing our patient taking in air) is irrelevent to the speed coming in - though it does have other clinical implications. Our goal would be to create a reservoir for the patient. What would that look like in our illustration? The tank filling up faster than it is draining - this would be exceeding the inspiratory demands of gas flow. 

 

Another illustration: 

You may think, but if you're looking at two cars driving down the road at 15 MPH that there speed is not additive. In this instance, you would be correct. However, we have to look at this problem differently to get the answer we are looking for. 

 The amount of cars delivered to the pile is an accurate representation of the amount of volume delivered, not how many MPH it is traveling on the road. 

 In conclusion, yes, the flow we are delivering with two devices is additive. This is important information when we consider strategies to rid the patient of nitrogen, do our best to meet or exceed their inspiratory demand, and reduce anatomical dead space. 

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