# EMT with pressure and volume question that might save lives

## Main Question or Discussion Point

I am an EMT and have a concern. Some patients require high volumes of oxygen (as high as 15 liters per minute sometimes). Many times oxygen concentrators only go up to 5 lpm. It may be misleading to say 5 lpm because the machines have to operate based upon pressure rather than volume. 5 lpm is only what you'll get directly out of the nozzle. With a long hose, patients will receive about 1 lpm less per 5 or 6 feet. This is not exact, but I have witnessed this first hand. At nursing homes where they have these limited O2 concentrators, it is common practice to run a line out of each concentrator. These three lines are connected to two lines by three prong adapters. The two lines are connected to a single line nasal cannula using one three prong adapter. In this situation, is the patient actually receiving 15 lpm of O2? If a patient had three nasal cannulas in their nose simultaneously from three separate O2 concentrators at 5 lpm, would they be receiving 15 lpm of O2?

Diagram
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__ --- ___
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Any help on this matter is greatly appreciated, as I am not convinced the daisy chaining method is actually getting patients the oxygen they need.

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Doug Huffman
Gold Member
I do not believe 15 l/m O2 can be delivered via canula for the dilution. If you are concerned with the flow restriction of small gauge tubing then larger tubing is needed for the low delivery pressure.

IIRC, flow restriction is proportional to D/L

Q_Goest
Homework Helper
Gold Member
I'm not an expert on medical oxygen concentrators but I see they are based on pressure swing absorption (PSA) technology. It sounds like you're eluding to the concentrator flow being affected by the pressure on the outlet of the machine which could very well be the case for a PSA. The higher the discharge pressure of the PSA, the lower the flow capacity. If that's the case, then as Doug mentions, increasing the diameter of the tubing might help. This assumes the primary restriction is through that tubing of course, and not through the device at the nasal cannula.

It should be easy enough to measure the output of these oxygen concentrators using a rotameter (flowmeter). You can purchase one from McMaster Carr for about \$100. Click on "For Use With: Oxygen" and "Flow Measured In: Liters".
http://www.mcmaster.com/#flow-rate-meters/=velohg

Doug Huffman
Gold Member
The cannula and tubing lumen are similar. At significant flow velocities the restrictions at the joints may be significant. When I built piping systems the total restriction was of concern and might militate to larger diameters.

Thanks for your replies! There is actually a flowmeter on the outside of the oxygen concentrator that looks just like some of the ones you linked me to. Doug, I think you're right about not being able to deliver 15 lpm via nasal cannula. We typically switch to a non-rebreather over 6 lpm. I certainly believe the tubing and joints to be limiting. Maybe there are tubes with higher diameter. If 5 lpm is coming out each of the three concentrators (as in my diagram), will it add up to 15 lpm going to the patient(assuming the cannula, non-rebreather, connectors, or tubes aren't choke points)? It seems that, according to Boyle's law, it should add up to 15 lpm. The issue is trying to put all that volume through a small diameter. Pressure might be forced to increase due to choke points, decreasing volume at the final output location. Do I have this right?