I received a timely MGPHO Forum private message and because other verifiers may be in a similar situation, I’m posting my reply here as well.

His situation is: “Due to the COVID-19 situation we are assisting many facilities and consulting engineers with the repurposing of non-critical areas of hospitals into temporary critical care units.”

His questions are:

“What are the O2 and Med Air flow demands for a typical ventilator?

What is the minimum pressure requirement for vents?

What is the max pressure differential between O2 and Air for vents?”

 “Typical” Adult Mechanical Ventilator Specs - Adult ventilator specifications can vary somewhat from manufacturer and model, so I’ll speak in general terms.

-Inlet maximum flow requirements for oxygen and med air – 180 LPM each – actual patient flow varies, see discussion below

-Maximum patient peak inspiratory flow setting – 180 LPM

-Vent minimum inlet pressure requirements are generally 35 psig – a few may be 40 psig

-Maximum pressure differential between O2 and Air for vents and 02/air blenders is: +/-20 psig. Primarily to assure the set 02 concentration is accurate

Consult the hospital’s Respiratory Care Department for the brand and specific model of vents they own/rent/will use. With that information, Google the inlet specs for each on the manufacture’s website.

“Typical” Adult Male Patient Parameters – can vary considerably

-Peak inspiratory flowrate – a short of breath (SOB – got to love medical abbreviations) patient can have – 300 LPM or more

-Patient minute volume – Volume of gas inhaled or exhaled in 1 minute (resting)– 8 Liters. May be higher in patients in respiratory distress

-Inspiratory/Expiratory Ratio (I:E Ratio) – portion of ventilatory cycle is each phase of respiration – 1 part inspiration to 2 to 4 parts exhalation. Ventilators can also be set for inverse I:E ratio ventilation (another discussion). We’ll simply use an I:E Ratio of 1:3

So, it is important to note that many adult ventilators can function as designed with an inlet pressure as low as 35 psig.

Patient ventilator inspiratory flow wave patterns can be set in numerous ways (high or lower flows for all or part of the inspiratory cycle), but generally will follow a patient’s spontaneously breathing patients’ pattern unless it is too high or exceeds the maximum vent inspiratory flow, then if not contraindicated, a patient can be medically sedated.  In controlled mechanical ventilation (patient not breathing, the inspiratory flowrates and I:E Ratios can be better controlled.  

With an I:E Ratio of only one third of each breath, when the outlet gases may be actually flowing (a few vent models may function differently), varying inspiratory flow patterns and varying 02% setting divided between outlet 02 and air the inlet gases, it is not possible to accurately calculate the gas outlet flow draw of even a “typical” vent patient.  

So, what are some possible ways of assessing whether non-critical care 02 and air outlets have adequate flow/pressure requirements? The obvious first choice is to test each outlet per NFPA 99 critical care outlet flow/pressure drop standard.

If the test results are borderline, another possible way to help hospitals decide if  non-critical care 02 and air outlets may be used follows. Please remember, these are not normal times, and just as hospitals are taking extraordinary measures to cope with an unprecedented patient surge, we should help them succeed to the best of our ability.

Ask the hospital Respiratory Care Department to set up a few vents on high inspiratory flow settings, with a simple test lung attached, in the med gas zone(s) they are considering. Then, connect a test gauge in another room in the same zone (not in same room) or watch the zone valve gauge (second choice) and watch the maximum pressure drop on the gauge during the inhalation phase of the vents. You want to see a pressure drop to no more than 38 to 40 psig.

If vents are not available to do the following test. It may take a few people to perform. Set up 2 or more test flowmeters with a small ball valve between the outlet and flowmeter venting to the room – no back pressure. Then open and close the flowmeter test valve rapidly to 180 LPM for approximately second 1 second on and 2 seconds off, while someone is observing the pressure gauge as noted above. This may take a bit of practice.  

Report the written methodology and quantitative findings to the Director of Respiratory Care, facilities engineering and others as appropriate. Under no circumstances report that the zone is suitable for vent use. This is a hospital clinical and administrative decision based upon their unique situation. It is not a verifier’s place (legally or technically) to recommend verbally or in writing one way or another as to use of that med gas zone for vent use.  

These are very different and difficult times for hospitals, clinical professionals and the patients they care for, and in my view, verifiers need to work closely with hospitals to assist them where at all possible.

Please excuse the long post, but this is an important topic to understand in these times. What we are seeing in the greater NYC area, and growing in other areas, may be coming to your area soon. Please stay safe, follow CDC guidelines and when in hospitals use all PPE per that hospital’s COVID-19 protocols. If you have your own N95 respirators (masks) use them.

Stay Safe and Well,

George Scott, Registered Respiratory Therapist – long retired, former hospital Director of Respiratory Care departments and instructor in hospital and university schools of respiratory care

Thanks for the good questions Mark. If you are confused, others may be too.

I’ll try to address each individually. I’m trying to express everything in general or typical terms, and of course when doing so, there are always exceptions.

Yes,180 LPM is the maximum peak inspiratory flow of most adult vents and usually lasts a fraction of the inspiratory vent cycle. Most flow wave patterns are bell shaped and can be wide of narrow depending upon the rate of flow. Square wave patterns can  are also used.

No one needs to know this – just to explain where minute volume comes from. I tried to keep it basic in my previous posts but…. Respiratory Care 101 is now in session. The 8 liters (volume) is the approximate minute volume (gas inhaled/exhaled over 1 minute) - is not the tidal volume. The tidal volume (named after tides - goes in and out) is the volume of gas a patient inhales or exhales each breath and roughly varies by patient weight - body wt. x 3 - so a 200 pound man would have a normal tidal volume of +/-600 ml or 0.6 liters. Here is the relationship: tidal volume x respiratory rate = minute volume (600ml * 12 breaths/min = 8 liters). Normal respiratory rate is usually 12 to 16 per min. but can vary significantly. Minute volume is sometimes expressed as liters per minute, which can look like flow, but is actually a volume measurement. This can lead to confusion. Minute volume is what determines the arterial C02, so the respiratory therapist would control the ventilator volume to keep C02 at that patient’s normal level. Minute volume can vary considerably between patients. Minute volume can range from 6 to 12 liters. No exam, I promise :)  

Try to stay away from sizing and leave that to the experts. Your math of 8 L*60 mins = 480 L consumption is correct. From above we see a range of minute volume between 6 to 12 L. Yes, that volume of consumption would be divided between both 02 and med air.

Hope this helps. If not please ask.

George

Hopefully everyone has been able to stay healthy lately.  As a result of the recent COVID-19 pandemic and as indicated by this thread of related posts, I’m sure that you have all been fielding a several questions regarding medical gas systems and their ability to accommodate additional demand, specifically from ventilators. Obviously several variables come into play that might impact your answers.  

The Board of Directors would like to help.  We’re planning on hosting a webinar soon to discuss some common questions and attempt to address the primary concerns that our health care communities are facing today.  However, we need your input.  Over the next couple of days, we would like to collect a list of the most frequently asked questions that panel of subject matter experts could discuss.  If you would please reply with a few questions or specific topics related to this issue, we would appreciate it.  

Kyle,

Brilliant idea.  let me know if I can help. 

The question above is the one I am getting the most.  I am also seeing a lot of responses like the one Cary posted - some are good, some ... mehhh.

I’d like to preface my comments in the context of an interview with a critical care nurse practitioner, in the metro NYC area, on PBS News last evening. The hospital she works at is good sized, but not one of the largest in the area. Yesterday, the hospital opened its 15^th ICU, filled it and then opened their 16^th. All patients were on vents. Each unit was staffed by at least one trained critical care nurse and assisted the other nurses. I have no idea how large each new ICU was. Could this be what we see rolling out across American cities? The peak of the patient surge in the NY area is currently forecast at 2 to 3 weeks out.

These are extraordinary times; hospitals are seeing patient surges that are way beyond emergency surge protocols most hospitals envisioned, and they are adapting in unconventional ways.  Med gas systems could be stressed to capacity and beyond, so we may need to think outside the box.

Cary, my guess is the 40 to 60 LPM flow quoted is the inspiratory flow, which of course only flows on inspiratory phase of the vent - maybe a portion of inspiration, maybe the entire inspiratory cycle. This is not a continuous flow demand on the med gas system – only intermittent.  When patients are sedated, which many are, they may require less inspiratory flow, and if typical, is good news.

Re how many vents can the non-critical care zone support. Simple answer is we don’t know for sure. But first testing each pressurized gas outlet per NFPA 99 critical care flow/pressure drop criteria and if it passes, is a good sign and we have a standard to reference.

Cary, failing the NFPA 99 testing, or maybe in addition to it, your flow/pressure relationship testing can give good info to the hospitals. A couple of thoughts regarding testing. In addition to the pressure drop you are using; you can also document the max flow at a pressure drop to 40 and 35 PSIG. This would give the hospital more information for a decision. Do the testing with multiple flowmeters positioned around the zone and document the accumulative flow. Position the test gauge within the zone distant from the test flowmeters, so you get a better zone pressure drop. Most full function critical care vents have low inlet pressure alarms and will alert the hospital if the zone draw exceeds the vents capabilities. Noncritical care vents may not have this feature.

I realize the hospitals are looking to verifiers for definitive answers, but…  Again, for your liability, it may be best to give the hospital the facts and let the hospital decide if they wish to use the zone for vents. That said, many hospitals may not be aware of meaning of the results. Each verifying company needs to make decisions based upon their unique situation.

A final thought. Do the NFPA 99 “white cloth test” on each pressurized gas outlet the hospital is considering using. I’ve repeatedly seen existing noncritical care outlets, that are used only for low flow applications, discharge large amounts of particular matter during periodic inspection particulate testing and on consultations. Ventilator peak inspiratory flowrates can temporally aerosolize old particulate matter, working its way down the pipeline until it lodges in the vent inlet filters. This is generally not an issue with low flow use. Be particularly alert for this in older air systems that may have contained liquid water in the pipelines and could now contain significant quantities of particulate matter. My “personal best” air outlet was over 200mg/M3 at an Ohio hospital and required extensive purging. Maybe suggest the hospital stock extra vent inlet filters. Some may have 2 filter types at the air inlet.

Stay safe and well.