Auto or Fixed Pressure?

Once you have decided that CPAP is improving your health, tiredness, lifestyle or just improving the sleep of your partner, there is a decision to make.

Auto or Fixed pressure?

Auto pressure is usually trialled at the beginning of the trial period and  the pressure is set between certain values - for example 5- 18. This means that the CPAP machine will respond to your breathing patterns and if you are not breathing as you started, then the pressure will gradually build up to the pressure needed to keep your airways open. Once this is achieved, the machine will continue to respond to your breathing, increasing or decreasing as required.  The pressure delivered by the machine could be anywhere between the set values at any time during the night. It changes depending on the type of sleep (REM/NonREM) and also body position.

Once you have trialled an Auto, (or if you have a CPAP titration sleep study in a hospital) the machine works out a figure that 90 or 95% of the time, the pressure is under. This is called your set pressure figure, and a fixed pressure machine will be set to this pressure. Resmed fix to the 95%ile and Philips and F&P fix to 90%. 

Both machines can still have a RAMP - where the pressure starts at a lower pressure and builds up over a predetermined time (for example 5 - 45 minutes) And now the newer machines have an AUTO RAMP - where the machine does not increase in pressure until it detects 3 even breaths - an indication that you are asleep. Pressing a button, or turning the machine off and on again allows the ramp to be used depending on the brand.

Some people find that the fixed pressure is fine and they do not notice the difference, while others decide the Auto is more comfortable. Auto is more expensive (up to about $300-$500 more) which is a major factor.

Long term, the Auto machine will adjust every night to various factors including changes in weight, alcohol and sedative consumption, and aging. Most fixed pressure machines will not adjust to these factors, but if you notice tiredness increasing again, come in, have a download, and if you are having more apnoeas, we can increase the pressure slightly to see if that helps. Another way to determine this is to have another sleep study, in a hospital, to decide if you need to change either pressures, or machines. (there are machines for Central Sleep Apnoea, which cannot be successfully treated with a normal CPAP machine.)

We prefer our patients to trial both Auto and Fixed pressure and then decide for themselves, or with the assistance of their sleep specialist, which one to purchase.

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