How does it all even work?

[W_HAMILTON]

I've been thinking up a lot of questions about how my CPAP machine even works. These may sound like stupid questions, but I just don't understand.

(1) How can it record intricate data such as apneas and hypopneas? My mask has no sensors, the tube has no sensors. How can it even record these?

(2) If all masks have vents to let out the CO2 you expel, and different masks have different "leak" patterns that reflect this...how can a set pressure be the same for all types of masks? If one mask has a higher normal "leak" rate than another mask, wouldn't the pressure have to be increased to make up for this additional loss of air?

I think I had some other questions like these but I forgot them (dang OSA brain )

[archangle]

The CPAP machine has sensors that detect air flow and pressure at the machine. Unless there's a leak somewhere in the machine or hose, if you know the airflow rate and pressure at the machine, you know the airflow rate and pressure at the air inlet on the hose.

The assumption is that the leak rate is mostly the same over short periods of time. The machine calculates the average leak rate over the past minute or two and subtracts it. The machine "watches" the airflow and looks for the rate of change. When you breathe in and out, the rate of airflow caused by your breathing goes up and down. The machine subtracts out the average airflow for the past minute or two and thereby knows when you're inhaling, exhaling, or stopped breathing. For instance, if the average air flow for the past minute is 50, and the air flow is currently 75, the machine assumes you're inhaling 25. Similarly, if the flow drops to 25, you're exhaling 25. If it's 50, you're not breathing.

The actual practice is probably more complicated, but that's the idea.

If you have a large leak rate, or if the mask leak rate is changing rapidly, yes, it will screw up the results. On the average, it does seem to work well enough to provide useful data.

[JDS74]

I've been thinking up a lot of questions about how my CPAP machine even works. These may sound like stupid questions, but I just don't understand.

(1) How can it record intricate data such as apneas and hypopneas? My mask has no sensors, the tube has no sensors. How can it even record these?

I think I had some other questions like these but I forgot them (dang OSA brain )

The apnea data is recorded when there is no airflow. For OSA's, the hose and your initial airway fill to pressure and then airflow stops. On Respironics machines ( and I assume others as well ), the computer in the machine notices the no-flow condition and creates a short pressure pulse to test for the type of apnea. If it is an obstructive event, there will still be no flow. For a central event, because the path to your lungs is open ( and the volume is much greater ) there will be some airflow. If you have an auto machine, the pressure is increased for the OSA events to try to open the airway, for CSA events, the machine just records them.

Hypopnea events are a reduction in air-flow marked by a lower than your current average breathing pattern. I don't know the exact trigger that the machine uses to make this determination.

[archangle]

The apnea data is recorded when there is no airflow. For OSA's, the hose and your initial airway fill to pressure and then airflow stops.

There's always airflow at the machine. Put your hand over the vents on your mask and feel the airflow, even when you hold your breath. The machine has to estimate the leakage of the mask and try to estimate the airflow into your nose and mouth.

On average, it works amazingly well considering things like changing pressure with FLEX, EPR, BiPAP, mask leaks, etc.

[TalonNYC]

I have noticed that you do have to slightly increase the pressure when using a Full Face Mask vs. a Nasal Mask or Nasal Pillows.

As for the rest, since an OSA causes different changes in flow rate than a Central does, which is how the machine can tell one from the other.