Alveolar dead space

Alveolar dead space is the part of ventilated alveolar gas that does not provide the effective blood–gas exchange represented by a given dead-space estimate. An alveolus can receive gas but have little or no perfusion; it then differs from a conducting airway contributing to anatomical dead space.^[ref] A regional ventilation measurement alone does not establish regional exchange.

A constructed two-region model makes the distinction quantitative. Let tidal volume be 0.50 L and conducting-airway dead space 0.15 L. Of the remaining 0.35 L, suppose 0.28 L participates in the modeled exchange and 0.07 L comes from a ventilated, nonperfused region. Alveolar dead space is then 0.07 L, while total effective respiratory dead space is $0.15+0.07=0.22$ L.

The denominator changes the fraction. The nonexchanging fraction of the modeled alveolar contribution is $0.07/0.35=0.20$, whereas total dead space as a fraction of tidal volume is $0.22/0.50=0.44$. Neither percentage can replace the other without changing what is being compared.

If exchanging gas has carbon dioxide fraction 0.05 and the other two compartments contribute negligible carbon dioxide, mixed expired fraction isThe two-compartment dilution expression then gives $1-F_E/F_A=1-0.028/0.05=0.44$, consistent with the total dead-space fraction. This is a deliberately idealized construction; real regions can have graded ventilation-to-perfusion relationships rather than a binary exchanging/nonexchanging classification.

A whole-lung gas-balance estimate consequently does not identify the anatomical location of every ineffective contribution. The primary technical chapter distinguishes conducting space from gas-exchange-based wasted ventilation and discusses the assumptions behind arterial carbon-dioxide substitution.^[ref] Alveolar dead space makes that compartment distinction explicit.