Flow - Pressure Question

cchase · Post by **cchase** » Sat Dec 09, 2006 1:37 pm

I'm trying to gain a better understanding of the relationship between mask leaks and treatment pressure. I use CL2 with a RemStar M-Series APAP treatment around 9-10 cm/H2O. The CL2 mask is rated with a normal flow of about 30 liters per minute (LPM) at this pressure. Of course, I don't get a 30 LPM air flow at night, but something higher due to mask leaks. If my leak rate averages, say 45 LPM, I have increased my flow rate by 50%. Understanding the relationship between pressure and mask flow rate is important because for xPAP to work I need a steady flow rate (no mask leaks) to maintain treatment pressure; otherwise the pressure will drop. I have two questions:

First, does the RemStar M-series or any other xPAP machine have a flow sensor that calibrates a base flow rate during the first few minutes of treatment each night, monitor for higher rates during the night, and then increase my treatment pressure to adjust? This scenario seems unlikely, but I thought I'd ask.

Second, if mask leakage is not compensated for by the APAP machine, then can we calculate the pressure drop based on the increased flow or rate of mask leak? I've found a few caculators on-line that are used to estimate pressure drop, but they are designed for engineering problems in construction and a bit difficult to use without the background.

I got interested in this topic thanks to James Skinner's work on Encore Pro Analyzer. Using his program, I examined my first six-months of treatment data and found a correlation between my leak rate and events. The higher my leak rate during the night, the more likely I would increase the number of hypopneas and apneas. I'd like to figure out how careful I need to be about leaks to minimize the risk of increasing events.

Thanks for any thoughts.

Chris

curtcurt46 · Post by **curtcurt46** » Sat Dec 09, 2006 1:50 pm

XPAP's are supposed to compensate for small leaks, as far as the Respironics newer machines are concerned. I have no idea about other machines. Having said that, just as you have found, the performance is better with little or no leaks. I am sure you could calculate the pressure increase needed for a particular leak but you may not always leak at the same rate. A higher pressure will also create a situation for the possibility of greater amounts of leaks. The simplest method is to control the leaks and use the lowest pressure that gives you the best stats.

Snoredog · Post by **Snoredog** » Sat Dec 09, 2006 2:12 pm

these machines are equipped with a MASS airflow sensor, similar to what your fuel injected automobile has. It measures the flow pressure on each side of the MASS airflow sensor, then there is a pressure transducer which monitors the final delivery pressure. The latter is the same function the 420E has with the separate center tube running down the hose to the mask.

The mass airflow sensor is on the right with the 2 tubes connected below, honeycomb grid is on the inside (hose on each side of the grid), to the left is the 3rd hose that goes to the pressure transducer which maintains wanted delivery pressure. If a leak develops the pressure transducer will sense it, the MASS airflow sensor would detect increased flow (and trip large leak alarms etc.). A malfunction of the pressure transducer circuit causes all kinds of weird things to happen like full RPM.

In a perfect world if your mask is leaking 30L/m at 9cm pressure, the Remstar will show 30L/m on the LCD, if it shows 45L/m then you have 15L/m of external leak between the machine and the mask or between mask cushion and your face. So if the LCD matches your mask flow chart you are good to go, if it goes higher well...

You only have to worry if the machine can keep up with a large leak if your pressure is over say 15cm or if it is close to the delivery limit of the machine, then it won't be able to compensate for the leak. This is the reason when I see someone with a pap pressure of 18 or 19cm and are given a standard auto with a limit of 20cm, the machine won't have enough room to compensate for any leak when you get close to the limit.

cchase · Post by **cchase** » Sat Dec 09, 2006 2:54 pm

Thanks for the information and terrific figure, Snoredog. I thought you might know something about this topic. Sounds like the airflow sensor in the M-series will compensate for leaks up to a point, depending upon the treatment pressure and size of leak.

So, if I take the positive correlations seriously that i found between 90% leak rates and events, OA, etc. it may mean that the airflow sensor is not able to respond rapidly enough on some occasions. To confirm, I'd have to look very carefully at each event and the leak rate that was occurring at the time - these are hard to data to come by.

I guess the simple answer here is that even with air flow sensors to compensate for leaks, it is best to seal up the mask.

Chris

Snoredog · Post by **Snoredog** » Sat Dec 09, 2006 3:15 pm

[quote="cchase"]Thanks for the information and terrific figure, Snoredog. I thought you might know something about this topic. Sounds like the airflow sensor in the M-series will compensate for leaks up to a point, depending upon the treatment pressure and size of leak.

So, if I take the positive correlations seriously that i found between 90% leak rates and events, OA, etc. it may mean that the airflow sensor is not able to respond rapidly enough on some occasions. To confirm, I'd have to look very carefully at each event and the leak rate that was occurring at the time - these are hard to data to come by.

I guess the simple answer here is that even with air flow sensors to compensate for leaks, it is best to seal up the mask.

Chris