Posted by Greg Harper on May 07, 1998 at 07:27:17:

In Reply to: Re: Problemsexhaling wih C-PAP machine posted by Jeff on May 05, 1998 at 07:41:41:

Dear Jeff,

Let me try to answer your questions in order.

My first question would be; why do you feel that CPAP has been ineffective over the past
two years? Are you having problems sleeping at night? Or, do you have daytime symptoms that lead
you to believe that the therapy is not working for you? The bottom line is if you say that you have a problem, then you have a problem. I'm just not sure what it is.

The above explanation usually pertains to people who are just getting started with CPAP. If you are still having problems "emptying your lungs" then there is a problem and I would suspect your nose and the area between your nose and the back of your throat (nasopharynx). The nose and nasopharynx are relatively rigid structures that do not dilate easily with CPAP therapy. You could have a genetically small nose, problems with nasal
congestion, a deviated septum, allergies, etc....
It is hard to say. I have known people who do well with CPAP during their sleep studies who then go on to discontinue therapy because of problems with increased work-of-breathing (WOB).

Let me explain something about lung volumes since you asked. But first, let me define some terms.
The tidal volume (Vt) is the amount of gas that you breathe in or out during the normal resting breathing process. It is usually 6-7 cc/Kg and varies greatly in size throughout the day and night depending on your body's metabolic needs. This volume usually represents about 5% of your total lung volume. Let's use Vt of 300 cc as an example.

The total lung volume (TLV) is the total amount of gas that is contained in your lungs. This volume varies greatly among individuals but for an adult male let's say that it is about 6000 cc.

The inspiratory lung volume (ILV) is the maximal amount of gas that we can breathe in at the end of a Vt breath. The ILV is usually 3X the size of the Vt. This is the volume of gas that you breathe in when you go to blow out your birthday candles.

The expiratory lung volume (ELV) is the maximal amount of gas that you can actively push out of your lungs with maximal effort at the end of a Vt breath. This is usually about 25% of the total amount of the biggest breath that you can exhale all the way.

Finally, there is the residule volume (RV). This is the amount of gas that cannot be removed from the lungs. It is always there, even at the end of a maximal expiration. However, this does not mean that the same gas stays in the lungs forever. Your body moves around, the size of the breaths that you take change, blood flow thoughout the lungs is redistributed, etc... so that the gas is used up or exhaled but the volume remains the same.

Here is an example how lung volumes change and are redistributed when some uses CPAP:

normal CPAP

Vt+ILV+ELV 4800 cc 3000 cc

ELV+RV 2400 cc 3600 cc

RV 1200 cc 3000 cc

TLV 6000 cc 6000 cc

As you can see, there is a redistribution of the amount of "usable" volumes that a person
has available to them when their lungs are hyperinflated. Notice how the TLV stays the
same in both instances but the amount that you have control over changes. These volumes will also change depending on the therapeutic pressures of CPAP that are used. The higher the pressure, the greater the change in usable lung volume, the harder it is to breathe.

In conclusion, it is hard to say at this point whether Bi-Level therapy would help the
quality of your sleep because I'm not sure why therapy is ineffective for you.

I hope that some of this information has helped.

Sincerely,

Greg Harper, RRT, RCP

Respiratory Care Practitioner

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