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Facts You Need To Know About Your
CPAP Machine for Sleep Apnea

The Pressure Setting

When you receive your new CPAP machine for sleep apnea you will usually get some training and instructions on how to use it.

Typically, your health care provider will set the pressure to a value that worked best in eliminating apneas during your sleep study and thus was prescribed by your sleep specialist.

For most people it will be anything from 8 to 12 cm of water, sometimes as high as 15 cm, rarely higher.

The unit of pressure "cm of water" has traditionally been used by doctors in this field and it simply represents a pressure exerted by a column of water of that height.

For instance, the atmospheric pressure is equal roughly to 10 m (1000 cm) of water pressure which represents the weight of all the air above us in a 1 cm by 1 cm column. Converted to other units the atmospheric pressure is equal to about 760 mm of mercury or 15 PSI (pounds per square inch).

From these relatively simple physical conversions we can see that 10 or 15 cm of water represents only 1 to 1.5 % of the atmospheric pressure. So, when you put your mask on and turn on the blower the pressure you feel is only slightly higher than the pressure we are so used to and it is absolutely safe for our body.

Although each CPAP machine can be set to generate different pressures, from 3 up to 18 or 20 cm of water, you should not change its setting. If you feel that your prescribed pressure doesn’t work for you any more, contact your doctor.

The Leak

The blower inside your CPAP machine is designed such that it will maintain the same set pressure during inhalation and exhalation, even with some moderate air leak. In fact, some leak (called "intentional") is necessary to keep you from rebreathing the air you just exhaled.

The rate of intentional leak is selected such that the volume of exhaled air that normally would remain trapped in the mask and tubing will leave the system through a small hole(s) in the mask before you start your next inhalation.

You should always make sure that your leak vent is not obstructed. When the leak becomes too large (for instance around a loose mask) the blower cannot supply enough air and the pressure drops. In such a case you may not be receiving the treatment you were expecting, and the air blowing around the mask may cause some other discomfort (dryness in the eyes, hissing sound). That is why selecting the proper mask size and type is so critical to the effectiveness of your CPAP therapy.

A Slow Start

Another feature of your CPAP machine for sleep apnea is its ability to start from a low pressure to give you time to get used to it and fall asleep, while the pressure will be rising slowly to its target level over 5 or 10 minutes.  It may be called a soft start, ramp, delay, or some other name, but refers to the same concept of a gentle beginning.

Some doctors don't want their patients to use this feature because it delays the onset of therapy and if a patient falls asleep quickly he or she may experience quite a few apneic episodes before the prescribed air pressure builds up.

Obviously, those who don't feel uncomfortable and fall asleep easily don't have to use a long ramp or any ramp at all.

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