Continuous Flow vs Pulse

#1 Dee

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31 October 2011 - 02:47 PM

Oxygen - Continuous Flow vs Pulse - Differences &

Nocturnal Oxygen Settings

Hi Mark;

Couple of questions.....
Was talking to a few of my COPD friends about POC, oxygen conserver's and continual flow ( CF) o2. We all have Stage 4 and Fev1s between 16 and 20..

The thing we've noticed is we all use CF (continuous flow) at home but out in the community either use Pulse driven POC's or tanks with Pulse conservers. We all have noticed our breathing rates climb, our sats drop, even if we're on our "standard" CF settings on these portables. Even if we're in wheel chairs and not doing much..... same thing.

My belief is because we're on a pulse system, , either the timing mismatches with us and we get less o2 or our breathing rates are higher and mismatch. Its funny- we all have noticed if we're low sats at the Dr but plug into the wall O2 our sats go up .. which makes me think its a pulse verse CF issue... am I right?

Also.. one of my friends on palliative care for this disease uses 3 1/2 L ( per Dr) during the day but on her own drops her self down at night to 3 L. cause she believes since she's just sleeping she's not using as much energy so she doesn't need as much O2. And no .. she hasn't remembered to tell her Dr this .

I thought we can actually less effectively use O2 at night due to shallow breaths, poor muscle etc. so I'm pretty concerned that she just drops hers. I know for me the first use of O2 was for sleep due to the problems I mentioned... so again... am I on the right track to be concerned for her?

Hi LJ,

You ask some very good questions! And your suspicions are closer to the truth than you might realize. So, let me compliment you saying: "Good thinking!"

There IS a difference between continuous flow (CF) and pulsed oxygen, no matter the source. To make matters yet more confusing, the difference is variable between particular POC's and conserving devices. But, we can't stop with just the device types and their individual differences. When you add rest vs sleep vs activity/exertion into the mix, you throw yet another variable into the blender that increases the magnitude of the other differences and explains why - at comparable settings between devices and CF - you see such disparate results.

Each POC has a maximum capacity for how much oxygen it can produce per unit of time. For example, small POC's - in the 5 pound range - can produce between 750 mls and 1 L of about 90 % pure oxygen per minute. They are calibrated to dispense that oxygen up to so many mls-per-breath and to a maximum rate of ('X') number of breaths-per-minute below which they can guarantee the advertised purity. Often, the maximum number of breaths is below the number to which folks respiratory rate will increase to during exercise/exertion. So, they end up either generating breaths that do not receive oxygen, or more often, a breathing rate that exceeds the maximum for purity of oxygen dispensed which results in decreased purity of delivered oxygen.

Conserving devices tend to operate as advertised up to some maximum response rate, above which they simply don't respond. Further "where" in the inspiratory phase the oxygen is delivered will further determine how much oxygen actually reaches the gas-exchanging units in the lungs and how much stays in the bronchial tubes where it has no opportunity to participate in exchange (That is what we call "dead-space" ventilation.) If the pulse is delivered during the first 2/3 of each inspiratory cycle, it has a better chance of being 'used' than if the pulse is delivered over the whole duration of the inspiration. As well, when you increase your activity and respiratory rate, the volume you take in per-breath can also increase. With CF and pulsed oxygen, the resultant concentration - portion of the total volume of gas taken into the lungs - during each breath varies and becomes lower when larger volumes are taken in against a fixed volume of oxygen delivered during each breath. In comparing CF to pulsed oxygen changes in respiratory rate and volume actually favor pulsed oxygen with regard to which delivers MORE oxygen volume-per-breath, when we look across the spectrum of the many pulsed oxygen devices. Some fall short in comparison, while other will always beat CF. (This is not well understood by MANY doctors, nurses, RT's and consequently, oxygen users who tend to think that CF is ALWAYS better than pulsed, without exception. That is just not true for MANY pulsed devices.

So, the overall result is less oxygen received from the pulsed systems and POC's under conditions of exertion than for CF WHEN IT IS INCREASED appropriately. Therein lies the crux of the matter. No one should assume that the same CF setting is fine for activity - or sleep. It is not in the least unusual to find that one needs several more liters flow to maintain comparable saturations during activity than when they are at rest. It is not surprising to find someone who saturates just fine with, say, 2 L while at rest, but who needs 5, 6, 8 or more liters to stay comparably saturated with exertion. Yet, the common practice is to prescribe only 1 or 2 additional liters flow for activity - clearly NOT enough, as your measurements/observations have borne out.

As well, among conservers ("OCD's", hereafter) and POC's are significant differences in what constitutes equivalencies. Our saying for that is that "2 is NOT equal to 2", to nut-shell it. POC's vary in how much they deliver with each pulse AND how much they deliver with each pulse as settings change. In some cases, as demand increases, the same 'total' volume of oxygen is simply cut up into smaller pieces, since each POC as a maximum production rate capacity. So, even though a setting may be increased, it can either become reduced in pulsed volume as demand increases, or, worse yet, while the volume may stay the same, demand beyond a POC's capacity can cause dilution of purity so that the pulses are of decreasing concentration, effectively lowering oxygen support by lost purity of the delivered oxygen volume.

With conserving devices, some will respond to a maximum breathing rate (20 or 30 or 40 breaths per minute, for instance) if respiratory rate exceeds the maximum rate capability of a device, it simply fails to deliver oxygen on some breaths. Additionally, while each manufacturer sets an "assumed volume of air taken in per breath in determining the volume of each pulse, if the user's actual inspired volume exceeds the 'assumed volume for a certain setting, then the "effective" concentration delivered will be lower than what the manufacturer states.

It is well known by those who understand the 'foibles' of POC's and OCD's and advanced COPD, in particular, that while the user's respiratory 'rate' and 'per-breath volume' may increase as demand for breathing during activity increases, we also know that factors like dynamic hyperinflation, changes in blood flow through the lungs and change in matching of blood flow to areas being ventilated occur, even though more volume may be taken into the lungs at higher demand, less "effective ventilation" often - almost always, and very predictably - DECREASES. Too often, those who have poor understanding of this critical process will attribute decreasing saturation during exertion to a simple explanation that the muscles are demanding more oxygen than the diseased lungs can provide and building up CO2 in the process. While blood gas measurements may seem to suggest that to be the case, the truth is that it is more owing to increased ventilatory disturbances than to significant increase in oxygen demand that cause the observation of decreasing exertional saturations, than to much increase in muscle demand or oxygen.

All that said, CF is not always or inherently better than pulsed flow. Which is true depends upon the individual device in use and what it's capacity to produce oxygen and meet ventilatory demand are AND to what setting the device is being used at. How to know or determine the difference is a complicated process and beyond the scope of what I am trying to explain, here. The rule of thumb for folks who CAN'T ascertain those exact parameters is to acquire a pulse oximeter and closely and frequently monitor their saturation under the various conditions of their lives and adjust the setting to best meet the objective of a minimum allowable saturation where and when possible. AND, if the POC or OCD they have is NOT meeting the challenge, they should do everything in their power to change to another device that CAN meet their needs. If they cannot change devices, then they need to learn how to pace themselves so as to minimize drops in oxygen levels and to avoid the discomfort and potential organ damage that goes with repeated and prolonged inadequacy of blood oxygen levels.

Finally, you are correct to be concerned about your friend's decrease in her oxygen flow at night. AND you surmised correctly that while asleep one does NOT demand as much oxygen based upon less activity, because of changes in breathing pattern (shallower and slower) less 'molecules' of oxygen enter the lungs and therefore less oxygen is available.

Sleep studies of oxygen saturation patterns show that one should almost always use a flow setting that is similar to the setting they need for exercise/exertion. At the least, they should arbitrarily use one liter more for sleep than they use for resting conditions while awake. We do NOT tend to sense dangerous drops in oxygen when we sleep. What we usually observe is decreased energy, changes in thinking/judgment and progressive organ dysfunction, especially where the right side of the heart is concerned. Nocturnal hypoxia can lead to secondary pulmonary hypertension, right heart failure, edema, often seen in the lower extremities and/or weight gain that fluctuates a couple of pounds over the course of a day as one sheds the excess water through urination.

Best Regards,
Mark Mangus, Sr. BSRC, RRT, RPFT, FAARC


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