= Alveolar dead space
\b[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>.<Pulmonary Function Testing by Warren M. Gold and Laura L. Koth>[Physiologic 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 is
$$
F_E=\frac{0.28(0.05)}{0.50}=0.028.
$$
The 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.<Pulmonary Function Testing by Warren M. Gold and Laura L. Koth>[The Bohr Equation for Respiratory Dead Space; Physiologic Dead Space]{ref} <Alveolar dead space> makes that compartment distinction explicit.
Back to article page
