How does ASV "deliver a breath"?

[idlewire]

This is probably a dumb question but: how does ASV deliver a breath in a way that is different than what continuous pressurized air would do?

I understand that it detects when your airway is open and you're not breathing, and that the timing of it is meant to match your breathing. But at that point it simply raises the pressure of the air its putting out? And that makes you breathe?

I think the essence of my question is: if you had a CPAP continuously running at whatever that pressure is that the ASV switches to, how would it be any different? Both of these machines are just pushing out air aren't they? Is it the waveform of the pressure variance the ASV puts out that somehow gets that air into the lungs better or something?

Thanks for any insights.

[Gideon]

Look at your minute volume. That stat pretty much defines your breathing. There are 2 algorithms running at the same time the first is similar to what runs in the VAuto managing obstructive events.
The second is the ASV algorithm which monitors minute volume and alters pressure support to match your recent minute volume while your breath is in progress

[DeepBreathing]

The ASV change its pressure all the time, responding to your airflow and other parameters. It gives you a higher pressure on inhale (IPAP) and a lower pressure on exhale (EPAP). The difference between these is called pressure support (PS).  IPAP = EPAP + PS

If you experience any sort of obstructive event the EPAP will increase. If you experience a central event or hypopnea the PS will increase, giving you a higher inhale pressure. By contrast an ordinary CPAP or APAP basically runs a constant pressure which an APAP can adjust up and down over a period of time. Not to confuse things too much, but an ordinary machine with EPR turned on will also drop the pressure when you exhale, but that's always by a fixed amount.

The attachments below show how the pressure works during normal sleep, then how it responds to an hypopnea. You can see that the ASV machine is trying to balance your airflow by adjusting the pressure on a breath-by-breath basis. And they really do work very well.

   

   

[Sleeprider]

Both of the above are very good explanations. I'm going to try for the simple one. The ASV can treat a person with central apnea and any respiration problem that causes the "normal" respiratory volume to drop. When the ASV flow sensors detect a drop in respiratory volume, higher IPAP pressure is supplied until the normal flow rate is restored. The ASV is "adaptive", meaning it can supply as much pressure support as necessary to cause a breath in an open airway, and when that pressure is not needed during normal spontaneous breathing, the machine provides less pressure support or none depending on the settings.

Pressure support (PS) is the difference between IPAP and EPAP. Default settings for ASV auto mode are EPAP minimum pressure 4.0, PS min 3.0, PS max 15.0 and a maximum IPAP pressure up to 25 cm. The ASVauto mode increases EPAP pressure to resolve obstructive events, and uses pressure support when needed, as needed to treat flow limitation, hypopnea and central apnea. The ASV monitors the rate of breathing (breaths per minute) and volume of breathing (tidal volume), and maintains both by adaptively supplying pressure support. By comparison, the Vauto increases EPAP pressure for obstructive events, but uses a fixed PS for comfort and to treat hypopnea and flow limitation. Because the Vauto relies on spontaneous breathing effort, it does nothing but supply EPAP pressure during any apnea, and it supplies the set pressure support, or part of it during hypopnea. The pressure support in Vauto and VPAP-S follows the patient breathing, while ASV can take the lead.

[SarcasticDave94]

All the above answers are correct. I don't have anything else to add. G'day y'all.

[idlewire]

Thanks guys for the helpful information. There is still one point which I am not getting, so maybe I can express it better.

Let's say you've stopped breathing. The machine detects open airway. Raises pressure. How does this "force a breath" compared to if there were continuous pressure at that amount?

In other words, at some point you've stopped breathing. Machine raises pressure, say, to 20 cm. What about this forces a breath than if the pressure all along was 20 cm through CPAP? In both cases, at some time t, your airway is open and the machine is putting out 20cm of pressure. Is it something about the *increase* in pressure that forces a breath? Why isn't the air at continuous 20 cm pressure filling your lungs? Or does it have something to do with the timed drop in pressure afterwards?

[DeepBreathing]

Why isn't the air at continuous 20 cm pressure filling your lungs? Or does it have something to do with the timed drop in pressure afterwards?

Yes, you've hit the nail on the head. Just inflating your lungs with air isn't sufficient to cause breathing - what is required is a means to get fresh air into the lungs, and get the "used" air out again. This requires a variation in pressure - higher pressure to get the air in, and lower pressure to let it out again. Have a look at the charts I posted above. You can see that when the flow is reduced the pressure support (and hence the inhale pressure) increases but the exhale pressure does not increase. It's the difference between IPAP and EPAP that causes the air to flow in and out.

[Sleeprider]

My perspective recalling my lifeguard experience of many years ago...with an open airway, the lungs inflate with remarkably little pressure. In fact during artificial mouth-to-mouth respiration it can be hard not to use too much pressure, especially with the adrenalin of the emergency and rescue situation. Expiration occurs passively. When a person does not spontaneously inhale by contracting the diaphragm, expanding the rib cage, the pressure from the ASV (up to 25 cm) can easily inflate the lungs. The ASV logic detects the volume and shuts down the pressure once a normal breath is completed, and allows exhale to occur.

Edit to say, breathing for someone else that would otherwise not survive a near-drowning, is an unforgettable experience.

[sheepless]

I thought I was following the gist of this discussion but now I'm confused. if we're talking about the asv's method of treating ca events (whether by algorithm in asv or back up rate setting in vauto), I've never had the sensation that my asv machine is breathing for me. my sense is that it 'triggers' rather than 'delivers' a breath. by raising pressure it forces me to initiate my own breath or face partial suffocation by pressure run amok.

can it actually complete an inhale and exhale without my participation? (to get the air in and let it out again?).

edit: if it's not apparent, this is response to DeepBreathing's post.

[ajack]

Just put the mask on for a few minutes and hold your breath, throat open and then another cycle closed. The machine will quickly trigger a breath and then quickly deliver more, increasing the pressure each time. It does this till it sees that the target MV is reached and will wind down. So start breathing and see it settle with a few deep breaths. Then look at both in Oscar or ResScan

edit
When you are asleep, you don't have a conscious awareness of reacting to the machine. With a CA, the machine will deliver enough air pressure and breaths during the event to try and maintain target MV. With your throat open, it will breathe for you without any effort on your part. The same would happen with an obstructive event, or your throat closed. It will raise pressure till it clears the obstruction, or it reaches its maximum.