High oxygen concentration in medical air lines - intermittently

  • Wednesday, July 07, 2021 8:36 AM
    Message # 10734698
    Cary Darden (Administrator)

    I had a pretty odd call late yesterday from a customer stating that their ventilators kept failing a pre-use check stating that the gas supply needed to be checked.  When I got on site I noted that the pressure on oxygen was about 53 psig and med air was 50 psig.  The specific failure that the ventilator was giving was O2 cell / sensor check failed.  The gas supply test would pass as would the flow test, so the issue was with concentration.

    Some history / stats on the area involved.  The ICU is fed by a single zone valve serving rooms 1-16.  The initial verification was performed in early 2014, no issues at the time with the piping.  There piping tees such that rooms 12-16 are on a feed with room 16 being an end point and then rooms 1-11 are on a feed with room 1 being an end point.

    When I began testing concentration the oxygen all checked out normally, no issues.  When I tested med air the numbers were going all over, from 21% O2 up to 45 or 50% O2.  This was in room 16 which is one of the end points.  I then went to rooms 12 and 14 which were both open and they also had varying numbers on the med air outlets.  I then went to the only other available room, room 5 and the med air outlet read a constant 21.5-21.9% O2.  Room 9 later became available and it also read a constant 21.5-21.9% O2.  

    See the attached video, this was taken from room 12.  There were several heated high flow devices as well as ventilators in use within the ICU zone.  There was a heated high flow device (these have blenders on them) installed with the patient in room 11 and it was set at 60% O2.  They swapped that out for a different unit and I also raised the med air pressure to be the same as the oxygen pressure so they were both at 53 PSIG.  After some purging of the med air outlets in the impacted rooms (12, 14 & 16) they all began to read the same, 21% O2 for all med air outlets.

    Has anyone seen failures on blending devices for oxygen and med air allowing either gas to work its way back into the pipeline and impact the O2 % concentration of the other line? 


    Last modified: Wednesday, July 07, 2021 8:36 AM | Cary Darden (Administrator)
  • Wednesday, July 07, 2021 8:37 AM
    Reply # 10734702 on 10734698
    Cary Darden (Administrator)

    You will notice that as the flow is increased the med air seems to displace the excess oxygen in the line, then when the flow decreases again the oxygen concentration begins to raise back up.

  • Thursday, July 08, 2021 6:21 AM
    Reply # 10737178 on 10734698


    I have run across this twice in my career. Once was the fault of a ventilator in an ICU the other was in blender in a nursery. I found oxygen percentages in the medical air upwards of 50%. The offending device was located and removed and the lines were purged to get gas % to within specs. I used to have a video of the Nursery incident…..I’ll look through my phone and see if it is still around. 


  • Thursday, July 08, 2021 10:55 AM
    Reply # 10737724 on 10734698

    One of our facilities in Nebraska sent me this, I found it interesting. 

    1 file
  • Thursday, July 08, 2021 12:18 PM
    Reply # 10737947 on 10734698
    Cary Darden (Administrator)

    Thanks Larry, that alert is very helpful!

  • Friday, July 09, 2021 9:29 AM
    Reply # 10740092 on 10734698


    I have encountered this issue a dozen or more times and once while a hospital Director of Respiratory Care Department. It is less common today, due to improved air dryers and filters.


    By way of history and background, I’ve attached a replay to Chis Andersen’s Forum post in 2010.


    The most probable cause of the 02 fluctuation you report is failure of the check valve(s) in an 02-med air blender or ventilator. Blenders generally use rubber type duckbill check vales vs. metal spring/plunger type valves (similar to med gas outlets) in vents. The latter are more robust and tend to fail less often.


    Your reporting of higher 02 concentration in the air line suggests the failure is with the check valve on the 02 inlet of a blender or vent. But air inlet check valves tend to fail more frequently due to water and/or particulate contamination. If the system pressures are equal this issue is generally not seen.


    From your post it appears the respiratory medical device and pressure were changed at the same time, and if so, we do not know if there are still malfunctioning devises in use. It is important the hospital identify all vents and blenders in use in the ICU during your testing and that they test each devise individually. There are different ways to test for this. If interested, I can re-post another response to Chis on one method.


    Your reporting of significant fluctuation in 02 concentration is most likely due to vents in use on the same zone, caused during the high flow inspiratory cycle. Same for the lower 02 concentration as you increased the flow during your testing.


    The root causes of check valve failures are: 1. not performing annual pm on vents and blenders and not replacing duckbill check valves when due in blenders,

    2. contaminated (particulate matter and liquid water) med gas systems.


    Please let me know if anyone has questions related to this issue and I’ll try to answer.


    Hope all is well with everyone. I do miss seeing you guys at the annual meetings, particularly the discussions in the bar – sometimes learned as much there as the meetings ;). Life is good in retirement – highly recommend to all.


    Seems my post is too large, so posting response to Cris Anderson separately.



  • Friday, July 09, 2021 9:38 AM
    Reply # 10740108 on 10734698


    MGPHO Forum Response to Chris Anderson’s post – Dec. 2010



    First let me say that I am not current on all the latest RC equipment, and I did not follow all of the nuances above.

    The most probable cause for the intermittent decrease in the FI02 (fractional concentration of inspired 02) is defective check valve(s) on blenders or vents connected to both piped air and 02. This was first reported back in the 1970's in the Respiratory Care Journal by Hal Zicheck (sp) of Brookdale Medical Center in NY. I have seen this problem repeatedly when we were in the respiratory device service business. Repaired and pm'ed hundreds if not thousands of 02/Air blenders.

    Because the 02% decreased, this leads me to believe the check valve failure is on the air side and the air pressure must be higher than the 02 pressure to get a decreased 02 concentration. The air check valves fail more often than the 02 side (see below).

    The usual way this check valve failure presents is the 02 concentration in one or more RC devices within a critical care unit will increase or decrease (depending on which check valve fails, 02 or air). This happens because the higher pressure gas will back-feed into the lower pressure gas piping system via the defective valve. I should emphasize that the check valve failure is from a different blender/vent than the vent/blender where the 02% change is observed. That is probably why replacing 3 vents as mentioned did not correct the problem. I have never heard of an instance where the pipeline concentration changed outside of the critical care zone where the check valve failure occurred, probably because only so much retrograde flow can go through the valve and the pipelines pressure only vary by a few PSIG if they are within NFPA 99 standards.

    Both the Bird Blender and The Sechrist mixers have wide inlet pressure variation specs., about 10 to 15 PSIG, and the 02% output will still be constant, so normal fluctuations in line pressure would not of itself cause the problems noted above. In fact, most RC devices will work satisfactorily down to 35 to 40 PSIG, which can be verified by looking at the manufacturer's specs. So, I don't think line pressure fluctuation alone (within the 40 to 55 PSIG range) would cause this problem. That is why the Air/02 pressure differential audio alarm on Bird Blenders is (was) set for a difference of 20 PSIG. Within this pressure range the 02 concentration should remain constant. By the way, sometimes with water contamination this alarm does not activate due to water clogging the alarm.

    This brings us to what is the possible root cause of the check valve problem, if in fact the check valves are the cause. First, as stated 02/air blenders and most ventilators do require annual parts replacement (including the check valves) and cleaning per manufactures specs. My experience with this type of failure usually showed residual water or particulate matter contamination on the air side filters and/or check valves. Disassemble the 02 and air inlets on the blenders/vents and look for green and/or blue discoloration of the bronze inlet filters. Also look at the air side check valves for a brown rust colored residue (dried water). If you see blue/green larger particulate matter on the 02 side bronze filters it is usually not a problem, just flux or other particulate matter caused by the high velocity of the 02 flow. If there are indications of residual water the underlying cause of that must be addressed at the air source equipment.

    In my opinion, if the air and 02 pipeline pressures are within the NFPA 99 standard and the critical care low pressure alarms are set OK you will not see this type of a problem without a medical device problem. RC devices are built to allow for pressure swings within this range. No need to change NFPA 99 specs. It is not uncommon for line pressure to drop into the 40's PSIG with high demand on a system as seem with high flow devices used in cc units and the equipment works fine.

    Yes, it is best to disconnect blenders and vents from the air and 02 outlets when not being used, but that can be a pain when you go to use them.


  • Sunday, July 18, 2021 9:40 PM
    Reply # 10760384 on 10734698

    I have seen this happen at least 4 times over the past 31 years. 3 time in a NICU at two different medical centers and once in an ICU at a fourth medical center.

    Each time it was a defective check valve assembly on the blender (3 NICU blenders were separate delivery devices) and the ICU was an internal blender in an Adult Ventilator. Each time the oxygen working pressure was 2 - 3 psig higher than the medical air working pressure.

  • Tuesday, July 20, 2021 8:47 AM
    Reply # 10763340 on 10734698
    Cary Darden (Administrator)


    Thanks so much for your input, it is always highly valued and appreciated!  I had looked up the manual on the impacted blenders and recommended bio-med perform the test procedure described in the blender O&M for making sure the duckbill valves work correctly.

    Last modified: Tuesday, July 20, 2021 8:47 AM | Cary Darden (Administrator)

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