Oxygen feed into BiPap system

[dwbenziger]

I just took delivery of a DreamStation BiPAP Auto SV machine.  My doctor prescribed a 1 Liter flow of oxygen so the vendor also provided a Respironics Oxygen concentrator feeding into an adapter between the mask and the heated hose from the Dreamstation.  Now the 1 Liter flow of Oxygen from the concentrator is of such a low pressure that you can barely feel it on your cheek if you hold the host pointed at your face.  The heated hose from the Dreamstation is relatively high pressure (relative to the oxygen hose) varying within the range specified by my doctor.  

I am a retired engineer and everything i know tells me that a low pressure oxygen source feeding into a relatively high pressure Air source from the dreamstation will result in  no flow or flow back into the low pressure oxygen source.  If I am right -  there can be no oxygen flowing into the mask.  I have been wrong many time in my 40 year engineering career - so please help me understand  how this can be working

Thanks in advance

[pholynyk]

As you must well know, there is a difference between flow rate and pressure. The O2 concentrator is obviously flow limited. You could make a manometer by dunking the O2 hose into a tall glass of water (real tall, 10" or so) and measuring how far down you can push it before the bubbles stop. It should be farther than the maximum cm H2O of your CPAP machine - you are correct that a low pressure O2 won't overcome a greater CPAP pressure.
Or maybe the O2 concentrator has a spec sheet that tells you what its maximum pressure is?

[Walla Walla]

I'm no where near being an engineer but I think what happens is instead of blocking the O2 flow it actually pulls the O2 flow into the main stream.

[JesseLee]

I'm going back 20 years to schooling times. If I'm not mistaken, you will experience a drop in hose pressure when flow rate increases. So, during inspiration your oxygen concentrator should overcome pressure during increased flow rate periods in the hose. Essentially, the oxygen concentrator will participate in addition of oxygen concentrate during the time you are inhaling.

[LarryRapp]

I am an engineer but an electrical one and I have had a stroke.  I also get oxygen from a concentrator.  On my machine, the pressure varies so that it always provides 3 liters.  I think yours probably does the same.

[ShaunBlake]

I just took delivery of a DreamStation BiPAP Auto SV machine.  My doctor prescribed a 1 Liter flow of oxygen so the vendor also provided a Respironics Oxygen concentrator feeding into an adapter between the mask and the heated hose from the Dreamstation.  Now the 1 Liter flow of Oxygen from the concentrator is of such a low pressure that you can barely feel it on your cheek if you hold the host pointed at your face.  The heated hose from the Dreamstation is relatively high pressure (relative to the oxygen hose) varying within the range specified by my doctor.  

I am a retired engineer and everything i know tells me that a low pressure oxygen source feeding into a relatively high pressure Air source from the dreamstation will result in  no flow or flow back into the low pressure oxygen source.  If I am right -  there can be no oxygen flowing into the mask.  I have been wrong many time in my 40 year engineering career - so please help me understand  how this can be working

Thanks in advance

I'm in the same situation as you; I'm on supplementary oxygen and using a concentrator, trying to get by on 2lm. As Polywonk... er pholynyk has assured you, the oxi concentrator will have enough pressure to deliver whatever flow is dialed in. In my case, I was comfortable assuming that I was getting enough; my 'problem' was not knowing if it was doing any good. I HAD to have a monitor to check my oxygen saturation!

I've got ResMed equipment, and an oxi monitor for it is horrendously expensive -- 9- to 13 hundred bucks. <choke> I learned from numerous threads and feedback from several users here that a CMS50F would give me the straight skinny on my O2 saturation.

My doctor just ordered a temporary ResMed monitor to verify that I'm okay at 2lm, and I'll bet yours will be doing the same. If you can wait, or if you want to ask him, be assured that you will be getting some extra O2. If, on the other hand, you would like to have your own, I think you will like the comfort and convenience of one of the inexpensive (less than $100) units. (I found the CMS50F for $80.)

[Mogy]

I'm going back 20 years to schooling times. If I'm not mistaken, you will experience a drop in hose pressure when flow rate increases. So, during inspiration your oxygen concentrator should overcome pressure during increased flow rate periods in the hose. Essentially, the oxygen concentrator will participate in addition of oxygen concentrate during the time you are inhaling.

I believe that this is how it is probably done. Look up 'venturi valve'. It is commonly used to create vacuum when you only have compressed air. There are many applications for it.

[dmeRT]

your concentrator should alarm when the flow is blocked.

[JesseLee]

your concentrator should alarm when the flow is blocked.

Good point. There are pressure switches to detect kinked or blocked hoses. If the pressure in the CPAP hose we're too great, it would think the airline was blocked and alarm.

[dwbenziger]

Thanks to all for your responses.  

Collectively, they led to more research and ultimately to the realization that the oxygen is correctly feeding into the mask as prescribed by my doctor.   PHOLYNYK commented that pressure and flowrate are different.  I agree, but in an open system they are directly related - ie. more pressure = more flow.  However, the oxygen concentrator is flow controlled so the direct relationship doesn't hold true.  In fact the spec sheet on the Oxygen concentrator says its output pressure is 5.5 PSI apparently independent of the flow.  Converting this to cm H2O (the unit of measure in the BiPap field) yields a value of 386 cm h20.  This is way above the 25 cm max pressure in my therapy setting so it is clear that oxygen will indeed flow into the DreamStation delivery hose and at the 1L rate prescribed.  
'
BTW I looked into the Venturi effect one of you mentioned - but the adapter doesn't show the physical narrowing the would have generated suction allowing low pressure air to flow into the breathing tube.

So thanks again.  My engineering degree is nearly 50 years old  and you can add this false alarm to the others over my career caused by a curious mind trying to understand how and why things work.