Dead space and its measurement

Dead space is the volume of ventilated air that does not participate in gas exchange.

Anatomical dead space is the volume of air that fills the conducting zone, about 150 mL (~30% of normal tidal volume). It is measured by the nitrogen washout (Fowler) technique:


 * 1) Subject breathes in pure O2
 * 2) Most of the O2 mixes with the air already in the alveoli (75% N2)
 * 3) The gas in the anatomical dead space remains as pure O2 because it remains up in the conducting zone
 * 4) When the gas is expired, the pure O2 is expired first. This volume is equal to the dead space.  Note that in practice, O2 is replaced by N2 more gradually, as a sigmoid curve. However, it is quick enough that it can be approximated (averaged out) as an abrupt transition. 

Alveolar dead space includes those parts of the respiratory zon e that do not participate in gas exchange. In the ideal healthy adult, this is zero.

Also can use Bohr equation (actually less important), which is what you have below

Alveolar part of tidal volume + dead space volume --mixes--> total tidal volume. CO2 is measured various ways)

CO2 is constant everywhere --> derive equation

Physiological dead space is the sum of the anatomical dead space and the alveolar dead space. It cannot be measured directly but is calculated by