Dead Space and CPAP

In looking for information on dead space in CPAP masks and the effect on treatment, I found this interesting article.
"Dynamic dead space in face masks used with noninvasive ventilators: a lung model study"
http://erj.ersjournals.com/content/23/1/129.full

Dead space is the volume within the breathing system that contains air that does not get into the lungs.
For example, the volume of your nasal passage down through the trachea. The air going into your lungs goes through this area but the last bit that is in there doesn't make it. The higher the dead space, the more rebreathing occurs for a given minute tidal volume.

A "normal" minute tidal volume is about 6000 mL and with a breathing rate of 15 breaths per minute, about 400 mL of air are sucked in. If the dead space is 100 mL, then only 300 mL of new air actually get to your lungs along with 100 mL of the previously exhaled air. The tables in the article indicate that with CPAP treatment, this number can get as low as 40 mL and so improve the efficacy of the treatment - you get more oxygen.

Anyway, I found it interesting if quite technical.

For some masks, the dynamic dead space is higher in single pressure treatment and lower in BiPap treatment.