Starting pressure recommendation

Having been diagnosed with the SNAP test, I'm about to get fitted and start treatment (titration to be done using the APAP machine as I go along), and the recommended initial pressure is 4. I learned that there is an algorithm used to calculate an initial pressure, and I plugged in my BMI, neck circumference and AHI into it, and came up with 3.1. My question is, since most machines do not go lower than 4, what happens if 4 is too high? Has anyone had this happen? Is it not a big deal? Thanks.

Whatever they have it set for, I wouldn't touch it. Its their testing.

napnean wrote:Having been diagnosed with the SNAP test, I'm about to get fitted and start treatment (titration to be done using the APAP machine as I go along), and the recommended initial pressure is 4. I learned that there is an algorithm used to calculate an initial pressure, and I plugged in my BMI, neck circumference and AHI into it, and came up with 3.1. My question is, since most machines do not go lower than 4, what happens if 4 is too high? Has anyone had this happen? Is it not a big deal? Thanks.

Hi napnean -- I'm going to say at this point it is not a big deal.

That algorithm sounds a little funny anyway, for several reasons: First, as you mentioned most machines don't go lower than 4 (I actually don't know if there is any machine that goes below 4, though I haven't researched it) so an algorithm that produces a unrealistic pressure recommendation seems kind of questionable. Second, it doesn't make sense to me that AHI would factor into your pressure recommendation -- AHI is about the frequency of breathing events, but it does not predict the pressure a person will need. Your pressure setting is about how much pressure is needed to hold your airway open. That is going to depend on anatomical characteristics of your airway, not how many times a breathing event happened during your test.* So I can't understand why AHI would be part of the algorithm to determine your pressure requirement. And third, most people would not even be able to tolerate a pressure as low as 3.1 -- Pugsy would say it would feel like trying to breathe by sucking air through a straw.

In any case, you're looking at a less than one cm pressure difference between 3.1 and 4, and I really think it is not a big deal. If you're concerned, though, you can check your AHI after using your machine to see how it's looking.

Forum members with more experience and knowledge than I have will be along to shed more light on your questions but I think I'm in the ball park.

* A person can have a high AHI (severe apnea) and need a low pressure to keep their airway open. Similarlly a person with a very low AHI (mild apnea) might need a high pressure. There isn't a significant correlation between AHI and pressure requirement.

That makes a lot of sense. Thanks for your replies. I have read about a machine that goes down to a 3, but it was not one of the major ones. I guess my concern generally was not having any leeway on one end of the range; if 4 was too high, there'd be nowhere to go. I could always get more pressure, but never less. Well, I'll find out soon enough, anyway. My RT comes tomorrow.

napnean wrote:That makes a lot of sense. Thanks for your replies. I have read about a machine that goes down to a 3, but it was not one of the major ones. I guess my concern generally was not having any leeway on one end of the range; if 4 was too high, there'd be nowhere to go. I could always get more pressure, but never less. Well, I'll find out soon enough, anyway. My RT comes tomorrow.

Your RT will likely be able to reassure you about your questions. I'd be shocked if anyone here knows of anyone who actually had to get that oddball machine that goes down to 3 because 4 was just too high! Good luck, let us know how it goes.

napnean wrote:Having been diagnosed with the SNAP test, I'm about to get fitted and start treatment (titration to be done using the APAP machine as I go along), and the recommended initial pressure is 4. I learned that there is an algorithm used to calculate an initial pressure, and I plugged in my BMI, neck circumference and AHI into it, and came up with 3.1. My question is, since most machines do not go lower than 4, what happens if 4 is too high? Has anyone had this happen? Is it not a big deal? Thanks.

I've heard that the average pressure for the population of CPAP users is around 10 cm. So if anyone who has not had a sleep study wanted a CPAP pressure to start off with, 10 cm would be a good choice.

A setting of 4 cm is very rare and most find it difficult to get enough air to compensate for exhaled CO2 into the mask interface. The lowest settings for patients needing low pressures begin around 6 to 8 cm so I don't think you have to worry about needing less than 4 cm. If anything you could raise your low setting to 6 cm to start your APAP off at since it will self adjust as soon as you begin to have apenas.

My in lab titration sleep study started out with a pressure of 5. That was their standard starting pressure.
Don't worry about the 4 cm being too high..it won't be. In fact it may not even feel like enough air is moving and feel rather stifling.
There is one machine (might be others but I don't know much about the off brands) that I know of that does have a 3 cm starting pressure. That would be the Devilbiss brand.

While there are a few people whose OSA is well treated at 4 cm pressure the majority of people will need quite a bit more.
If playing the odds...your machine won't spend much time at 4 cm...it will spend more time higher.

Some studies have used prediction equations to determine CPAP pressures while others have used them as an aid in manual sleep laboratory titrations. The equations have some utility but they are not as accurate as a lab titration or an auto titration. Lab values are usually within plus or minus 3 cm H2O of the predicted values. Note the following abbreviations were used in the equations:

ODI = oxygen desaturation index
NC = neck circumference in centimeters
BMI = body mass index
AHI = apnea-hyponea index

Eq 1. Pressure = 2.1 + (0.048 x ODI) + (0.128 * NC)

Eq 2. Pressure = -0.611 + (0.913 x BMI) + (0.02 x AHI)

Eq 3. Pressure = -5.12 + (0.16 x BMI) + (0.13 X NC) + (0.04 x AHI)

Perhaps the original poster used one of the above equations to predict starting pressure? Note, AHI is used in two of the equations....

MyIdaho wrote:Some studies have used prediction equations to determine CPAP pressures while others have used them as an aid in manual sleep laboratory titrations. The equations have some utility but they are not as accurate as a lab titration or an auto titration. Note the following abbreviations were used in the equations:

ODI = oxygen desaturation index
NC = neck circumference in centimeters
BMI = body mass index
AHI = apnea-hyponea index

Eq 1. Pressure = 2.1 + (0.048 x ODI) + (0.128 * NC)

Eq 2. Pressure = -0.611 + (0.913 x BMI) + (0.02 x AHI)

Eq 3. Pressure = -5.12 + (0.16 x BMI) + (0.13 X NC) + (0.04 x AHI)

Perhaps the original poster used one of the above equations to predict starting pressure? Note, AHI is used in two of the equations....

Hi,

I have a couple of questions RE: the calculation "terms" Specifically:

ODI is this the % O2 Saturation in the blood?

AHI is this the desired or measured AHI? (and if measured how?? From a sleep study?)

What was the "source" for these equations? I would be interested in more reading on the subject.

Also,

The reason that I ask is that I am getting a new machine and I was curious as to how these might predict a starting pressure. I had a sleep study and I have since increased my pressure (with my doctors participation and "blessing" lol) from 12 to 13.5. My new machine is an auto machine and so I am planning to set it up for a range of 8-15 as a start.

I was just wondering how the equations would "predict" my pressure.

Thanks in advance,

Dave

Edited for typo

park_ridge_dave wrote:ODI is this the % O2 Saturation in the blood?

AHI is this the desired or measured AHI? (and if measured how?? From a sleep study?)

What was the "source" for these equations? I would be interested in more reading on the subject.

Also,

The reason that I ask is that I am getting a new machine and I was curious as to how these might predict a starting pressure. I had a sleep study and I have since increased my pressure (with my doctors participation and "blessing" lol) from 12 to 13.5. My new machine is an auto machine and so I am planning to set it up for a range of 8-15 as a start.

I was just wondering how the equations would "predict" my pressure.

Thanks in advance,

Dave

An oximetry study or a sleep study provide the number of oxygen desaturation events per hour (ODI). Enter that number in the equation.

The referenced studies were using their equations to predict pressures for previously untreated patients so you need AHI and/or ODI results from a pre-treatment sleep study. If you have one of the wrist oximeters that provide a data summary, one could calculate starting pressure without a sleep study. Please note, the equations are not very accurate. The laboratory titrated pressures are usually within plus or minus 3 cm H2O of the predicted pressures. In my case, my doctor recommended a pressure 3 cm higher than the equation and with the S9 in apap mode, the normal pressure is 6 cm higher than the predicted value.

I have the files saved in pdf format on my home computer but no longer have the links... Send me a PM with your email address and I'll send them to you.

I'm the original poster. My SNAP test report contained the pressure recommendation of 4, followed by "(after Miljeteig and Hoffstein). When I looked them up, I found this algorithm:

(0.16 x BMI) + (0.13 x NC) + (0.04 x AHI) - 5.12

My BMI is 24, neck 28cm, AHI 18.5. I hope this helps. It may be that I copied something incorrectly, or left something out, but that is my best effort, and it does, if correct, explain why the doctor called for the pressure of 4.

napnean wrote:I'm the original poster. My SNAP test report contained the pressure recommendation of 4, followed by "(after Miljeteig and Hoffstein). When I looked them up, I found this algorithm:

(0.16 x BMI) + (0.13 x NC) + (0.04 x AHI) - 5.12

My BMI is 24, neck 28cm, AHI 18.5. I hope this helps. It may be that I copied something incorrectly, or left something out, but that is my best effort, and it does, if correct, explain why the doctor called for the pressure of 4.

Your doctor used the Hoffstein equation (#3 in my list above). Using your data, the predicted starting pressure is 3.1 cm H2O. However, as I understand it, 4 cm H2O is the minimum setting for most (all?) cpap machines so the doctor would thus need to specify 4 as the initial setting. I wouldn't be concerned about this slightly higher pressure, many of us are many multiples higher without problems. (For example, my median pressure is 16.5 and max pressure is 18.) Good luck!

napnean wrote:Having been diagnosed with the SNAP test, I'm about to get fitted and start treatment (titration to be done using the APAP machine as I go along), and the recommended initial pressure is 4. I learned that there is an algorithm used to calculate an initial pressure, and I plugged in my BMI, neck circumference and AHI into it, and came up with 3.1.

Hi,

If your doctor is not planning to have you do a titration sleep study, then I would find a new doctor. Using equations to guess at a number is the medical equivalent of a drive by shooting (in my opinion). While that might work for routine cases, there is a reason that 50% of OSA patients give up within the first year. If this really needs to work for you, then you really need to find a better treatment plan that involves a sleep study to determine your pressure levels. For example, at this point, your doctor doesn't really have a valid diagnosis for you, nor does he or she know what type of machine you need.

-john-

@John: The plan is to have the APAP machine (Respironics System One) do the titration. The expense of the lab titration was daunting, plus I felt that I was extremely unlikely to fall asleep (my usual time to fall asleep being around 3 am). If things don't work out well, the lab titration is still on offer.

It seems pertinent if I give my example for your info.
1) The pressure in my machine starts from 3 cm.
2) I have slept at 3-15 cm range with no difficulty. In the past I also had a range of 4-15 cm and now I am sleeping in the range of 5-12 cm. So, the comfort-level varies from person to person.
3) I have tried for sleep study, twice. Failed in both as I could not sleep in the lab setting and timing. So, you are not alone.
Good luck.