Cause of hypopneas?

[geoDoug]

Hi all,

This is a broad enough question that I'm not sure I feel like wading through a search. So here goes:

About a week ago, I got a data-capable machine (M-Series Pro) and started using a Hybrid mask. So in this first week of enjoying the benefit of looking at my nightly information I've noticed a trend I can't explain.

The last four days, I've incrementally gone down in my AHI, to the point that last night I had zero Apneas, the average leak rate the last two nights is close to the intentional leak rate of the mask, and my vibratory snore index last night was close to zero! So I'm getting the Apneas under control. This all makes sense to me. I'm attributing the gradual decline to getting used getting used to the mask and adjusting it just right.

Now for my question. What I don't get: Over those four days, the hypopnea index has risen. My AHI two nights ago was 3.8 and last night was 5.7 (all HI). What causes hypopneas? Why the difference? Is there anything I can do about that rise? Given the low number, do I dare tempt fate?

(And yes, yesterday I was tickled pink about that 3.8 number and psyched about the zero today.)

Doug.

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CPAPopedia Keywords Contained In This Post (Click For Definition): AHI, Hypopnea

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CPAPopedia Keywords Contained In This Post (Click For Definition): AHI, Hypopnea

[Slinky]

I'm gonna take a guess. Some of those hypopneas would have become apneas except the machine pressure stopped them. Like your throat was starting to obstruct and the pressure prevented it from totally obstructing.

[Snoredog]

Hi all,

This is a broad enough question that I'm not sure I feel like wading through a search. So here goes:

About a week ago, I got a data-capable machine (M-Series Pro) and started using a Hybrid mask. So in this first week of enjoying the benefit of looking at my nightly information I've noticed a trend I can't explain.

The last four days, I've incrementally gone down in my AHI, to the point that last night I had zero Apneas, the average leak rate the last two nights is close to the intentional leak rate of the mask, and my vibratory snore index last night was close to zero! So I'm getting the Apneas under control. This all makes sense to me. I'm attributing the gradual decline to getting used getting used to the mask and adjusting it just right.

Now for my question. What I don't get: Over those four days, the hypopnea index has risen. My AHI two nights ago was 3.8 and last night was 5.7 (all HI). What causes hypopneas? Why the difference? Is there anything I can do about that rise? Given the low number, do I dare tempt fate?

(And yes, yesterday I was tickled pink about that 3.8 number and psyched about the zero today.)

Doug.

Maybe I can explain it so you can understand it, look at the graph below, say that represents your airway, for example the space between the back of the throat and the base of your tongue:


As you can see, the Normal airway is shown on the far left, as you fall deeper into sleep, your tongue muscle relaxes. As the tongue loses muscle tone it gets closer to the back of the throat and narrows the airway. The more it relaxes the narrower that air space becomes as seen in Flow Limited graph, that space can narrow from Normal on the left to Flow Limited -> to Snore where the space becomes narrow enough for audible snoring to be heard until it finally slams shut with apnea on the far right.

Note: these events take place as you inhale. Obviously as you inhale it creates a corresponding low pressure area in the esophageal canal. Your airway is more likely to collapse under low pressure of inhale as opposed to higher pressure of exhale. This same low pressure is also thought to contribute to effects of GERD (another discussion).

Flow Limitation: Look at the Flow Limited graph, it is actually the same as a Hypopnea. The only difference here is a Hypopnea lasts >10 seconds or longer in duration, has at least a 50% reduction in Normal air flow associated with a 3% drop in blood oxygen levels. Flow Limitation is anything less than what defines a Hypopnea.

Since most machines don't have Pulse Oximetery, hypopnea is nearly always calculated based upon the drop in airflow volume and time or 2 of the 3 needed characteristics to classify it as such.

Applying CPAP pressure to splint the airway:

Another way to look at it, is drawing of a mountain on a piece of paper. At the base of the mountain is Flow Limitation, as you travel up the mountain towards the summit you encounter Hypopnea as you reach the summit you have Apnea, as you travel over the summit headed down the other side again you encounter more Hypopnea and as you reach the base on the other side you may have more flow limitation.

You would think if you applied enough pressure to eliminate the events on the up side and prevent the apnea at the summit it would be high enough to prevent those same events on the desending side, but it is not. Remember as you fall deeper into sleep you relax more, the more you relax and reach those deeper stages of sleep the more pressure is needed to keep those apneas under control. So it is a uphill and downhill battle for the machine.

Once you apply enough pressure to eliminate the most severe, you travel over the "summit" and are headed downhill on the other side of the mountain.

The thing to do is to continue to increase pressure so those residual Hypopnea/Flow Limitations subside. Once you have increased pressure high enough to eliminate the apnea seen and reduce the residual Hypopnea/Flow Limitation to <5, you have reached your ideal pressure.

But there may be other outside factors which can influence that result from a night to night basis. If your residual AHI >6, I would try increasing pressure by .5cm and see what happens.

[geoDoug]

Snoredog, you rock.

The mountain analogy is where it clicked for me. My pressure is low (6 cm). I was Encore Pro doesn't have a flow limitation graph, and as far as I know I need an apap to get it. (I have an m-series pro.)

I may try upping my pressure a little to see what happens.

Doug.

[georgiaboy]

Thank you Snoredog for the great explanation. You have enlightned me on what was a mystery to me. The illustration made it seem simple.

This board is great.

[geoDoug]

One other thing...

Snoredog, do you feel like submitting that to Our Collective Wisdom?

Doug.

[split_city]

Maybe I can explain it so you can understand it, look at the graph below, say that represents your airway, for example the space between the back of the throat and the base of your tongue:


As you can see, the Normal airway is shown on the far left, as you fall deeper into sleep, your tongue muscle relaxes. As it loses muscle tone it gets closer to the back of the throat and close off the airway. The more it relaxes the closer that air space becomes, from Normal airflow space on the left to Flow Limited -> to Snore where audible snoring may be heard until it finally slams shut with apnea on the far right.

Just to add to the picture above. In addition to the relaxation of the tongue muscle (and other muscles surrounding the airway), the path of snoring --> hypopnea --> apnoea may also be the result of a loss in tension in the airway i.e. the airway moves upwards towards the head and becomes floppier and therefore, much easier to collapse.



Don't worry too much about the box surrounding the airway.

Note: these events all take place as you inhale. Obviously as you inhale it creates a corresponding low pressure area in the esophagus. Your airway is more likely to collapse under low pressure of inhale as opposed to higher pressure of exhale.

This is still debated. The airway can either collapse actively (from negative pressure during inspiration) or passively (end expiration). You have described active collapse so I won't add to that. In terms of passive collapse: The activity of most muscles (including the tongue) is lowest at the end of expiration (no flow in the airway). Increased pressure in the tissues surrounding the airway would increase the chances of passive collapse i.e. pressure of tissues just pushes on the airway and collapses it because there is no tone to prevent it. This is generally the way the airway collapses in mixed apnoea events but it might also be the case in obstructive events. My current study is going to have a look at this in more detail

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CPAPopedia Keywords Contained In This Post (Click For Definition): Hypopnea

[JZ]

snoredog -- Fabulous! Thanks for an excellent explanation that all of us non-technical, non-medical people can understand. I agree that this should go into Our Collective Wisdom. I am definitely going to bookmark it.

Janna

[DreamStalker]

...
This is still debated. The airway can either collapse actively (from negative pressure during inspiration) or passively (end expiration). You have described active collapse so I won't add to that. In terms of passive collapse: The activity of most muscles (including the tongue) is lowest at the end of expiration (no flow in the airway). Increased pressure in the tissues surrounding the airway would increase the chances of passive collapse i.e. pressure of tissues just pushes on the airway and collapses it because there is no tone to prevent it. This is generally the way the airway collapses in mixed apnoea events but it might also be the case in obstructive events. My current study is going to have a look at this in more detail

This sounds a lot like how Al Gonzo might try to spin it (splitting hairs and parsing terminology). The end of exhale is one and the same as begining of inhale.

[split_city]

...
This is still debated. The airway can either collapse actively (from negative pressure during inspiration) or passively (end expiration). You have described active collapse so I won't add to that. In terms of passive collapse: The activity of most muscles (including the tongue) is lowest at the end of expiration (no flow in the airway). Increased pressure in the tissues surrounding the airway would increase the chances of passive collapse i.e. pressure of tissues just pushes on the airway and collapses it because there is no tone to prevent it. This is generally the way the airway collapses in mixed apnoea events but it might also be the case in obstructive events. My current study is going to have a look at this in more detail

This sounds a lot like how Al Gonzo might try to spin it (splitting hairs and parsing terminology). The end of exhale is one and the same as begining of inhale.

Not really. Not all people start inspiration as soon as they reach the end of expiration. There can be a pause (referred to as the expiratory pause).

Anyways, to help distinguish active collapse versus passive collapse:

Active: Occurs when airway collapses due to the generation of negative pressure created by the diaphragm. The collapse doesn't have to occur right at the beginning of inspiration. It might occur a 1/3 of the way through or at the peak of inspiration. It depends when the negative pressure overcomes the the contraction of the muscles surrounding the airway. Airway patency is a balancing act

Passive: Occurs during a time of zero flow or close to zero flow (generally at the end of expiration or towards the end of expiration). Is the result of mass loading of the airway from the increased tissue pressure surrounding the airway

[split_city]

Remember as you fall deeper into sleep you relax more, the more you relax and reach those deeper stages of sleep the more pressure is needed to keep those apneas under control.

Here's a question for you Snoredog (or anybody else out there). If, as you say, you relax more as you reach deeper sleep (and therefore, muscle activity is lower), why do most OSA patients experience less apnea events during slow-wave sleep (stages 3 and 4) compared to stages 1 and 2?

[DreamStalker]

...
This is still debated. The airway can either collapse actively (from negative pressure during inspiration) or passively (end expiration). You have described active collapse so I won't add to that. In terms of passive collapse: The activity of most muscles (including the tongue) is lowest at the end of expiration (no flow in the airway). Increased pressure in the tissues surrounding the airway would increase the chances of passive collapse i.e. pressure of tissues just pushes on the airway and collapses it because there is no tone to prevent it. This is generally the way the airway collapses in mixed apnoea events but it might also be the case in obstructive events. My current study is going to have a look at this in more detail

This sounds a lot like how Al Gonzo might try to spin it (splitting hairs and parsing terminology). The end of exhale is one and the same as begining of inhale.

Not really. Not all people start inspiration as soon as they reach the end of expiration. There can be a pause (referred to as the expiratory pause).

Anyways, to help distinguish active collapse versus passive collapse:

Active: Occurs when airway collapses due to the generation of negative pressure created by the diaphragm. The collapse doesn't have to occur right at the beginning of inspiration. It might occur a 1/3 of the way through or at the peak of inspiration. It depends when the negative pressure overcomes the the contraction of the muscles surrounding the airway. Airway patency is a balancing act

Passive: Occurs during a time of zero flow or close to zero flow (generally at the end of expiration or towards the end of expiration). Is the result of mass loading of the airway from the increased tissue pressure surrounding the airway

No really ... there is no reason to define the pause as an inspiratory pause.

I'm not denying that a pause occurs between the end of exhalation and start of inhalation, I'm pointing out that there is not a logical reason for that pause to be attached to the period of exhalation as opposed to the inhalation.

In other words, "passive collapse" which equals period of no airflow, does not have to be associated wiith either inhale or exhale ... it is what it is ... a period of no airflow between exhalation and inhalation. Now maybe you wish to differentiate it from the other period of no airflow between inhalation and exhalation but there is no reason that should be attached to either inhalation or exhalation.

[split_city]

...
This is still debated. The airway can either collapse actively (from negative pressure during inspiration) or passively (end expiration). You have described active collapse so I won't add to that. In terms of passive collapse: The activity of most muscles (including the tongue) is lowest at the end of expiration (no flow in the airway). Increased pressure in the tissues surrounding the airway would increase the chances of passive collapse i.e. pressure of tissues just pushes on the airway and collapses it because there is no tone to prevent it. This is generally the way the airway collapses in mixed apnoea events but it might also be the case in obstructive events. My current study is going to have a look at this in more detail

This sounds a lot like how Al Gonzo might try to spin it (splitting hairs and parsing terminology). The end of exhale is one and the same as begining of inhale.

Not really. Not all people start inspiration as soon as they reach the end of expiration. There can be a pause (referred to as the expiratory pause).

Anyways, to help distinguish active collapse versus passive collapse:

Active: Occurs when airway collapses due to the generation of negative pressure created by the diaphragm. The collapse doesn't have to occur right at the beginning of inspiration. It might occur a 1/3 of the way through or at the peak of inspiration. It depends when the negative pressure overcomes the the contraction of the muscles surrounding the airway. Airway patency is a balancing act

Passive: Occurs during a time of zero flow or close to zero flow (generally at the end of expiration or towards the end of expiration). Is the result of mass loading of the airway from the increased tissue pressure surrounding the airway

No really ... there is no reason to define the pause as an inspiratory pause.

I'm not denying that a pause occurs between the end of exhalation and start of inhalation, I'm pointing out that there is not a logical reason for that pause to be attached to the period of exhalation as opposed to the inhalation.

In other words, "passive collapse" which equals period of no airflow, does not have to be associated wiith either inhale or exhale ... it is what it is ... a period of no airflow between exhalation and inhalation. Now maybe you wish to differentiate it from the other period of no airflow between inhalation and exhalation but there is no reason that should be attached to either inhalation or exhalation.

Don't shoot the messenger

Ok, lets look at the difference between the pause at the end of expiration to that at the end of inspiration and decide which pause would likely result in airway collapse:

Pause at the end of inspiration:
1) Lung volume higher: Lung volume affects airway patency. The higher the lung volume, the less collapsible the airway
2) The airway is likely to be stretched when at the end of inspiration: Stretching the airway reduces airway collapsibility
3) Tongue muscle is most active at (or around) the end of inspiration: helps to prevent airway collapse
4) Tissue pressure surrounding the airway is lowest at the end of inpiration: less force to close the airway

All these factors are likely to prevent airway collapse

Pause at the end of expiration:
1) Lung volume is lower: airway smaller and more collapsible
2) Airway is likely to be floppier: airway more collapsible
3) Tongue muscle activity at it's lowest: less tone in the airway --> airway more collapsible
4) Tissue pressure surrounding the airway is at it's highest

All these factors are likely to encourage airway collapse

Therefore, passive collapse of the airway would likely only occur during the pause between the end of expiration and the start of inspiration and not at the time when you hold your breath at the end of inspiration.

[roster]

The pause that refreshes and the pause that don't.

Sorry Pepsi. Sorry Mrs. Smith.

[geoDoug]

I didn't realize this was going to create such a lively discussion. As an experiment, rather than increasing the overall pressure, I'm going to reduce the amount of pressure the cflex lets up on to see if that makes a difference. It seems to me, from a purely layman's perspective, that it may.

Doug.