= Static versus dynamic lung compliance
\b[Static versus dynamic lung compliance] distinguishes an equilibrated elastic pressure-volume relation from a breathing-cycle measurement. <Lung compliance> uses <transpulmonary pressure>, the difference between <alveolar pressure> and <pleural pressure>. A static local value is $C_{L,\rm stat}=dV/dP_L$ after flow and relevant transients have subsided; a finite interval instead gives $\Delta V/\Delta P_L$ over a stated pressure range.
The timing convention for dynamic measurements must be explicit. Conventional dynamic lung compliance uses volume and transpulmonary-pressure changes between the zero-flow endpoints of a breath. It is not automatically the same quantity as a ratio using peak airway pressure. The respiratory-function treatise distinguishes these definitions.<Pulmonary Function Testing by Warren M. Gold and Laura L. Koth>[Static and Dynamic Compliance: dynamic lung measurements use zero-flow endpoints of a tidal breath]{ref}
An original one-compartment model isolates flow contamination. Relative to a chosen baseline, let mouth-to-pleural driving pressure satisfy
$$
P_d=\frac{\Delta V}{C_L}+R\dot V.
$$
With stipulated $C_L=0.20\,\mathrm{L\,(cm\,H_2O)^{-1}}$, $R=5\,\mathrm{cm\,H_2O\,s\,L^{-1}}$, $\Delta V=0.50$ L and $\dot V=0.50\,\mathrm{L\,s^{-1}}$, the pressure is $2.5+2.5=5$ cm water. Dividing volume by that pressure gives 0.10 rather than the actual elastic compliance 0.20. Setting flow to zero removes the resistive term in this ideal model. This is a demonstration of a flow-contaminated pressure ratio, not a redefinition of the zero-flow dynamic measurement.
Even zero flow at the mouth need not establish full internal equilibration. <Unequal filling times in apparent lung compliance> can be illustrated with two independent elastic compartments receiving the same imposed pressure step. If their <time constants> are $\tau_i=R_iC_i$, then
$$
\frac{\Delta V(T)}{\Delta P}=\sum_i C_i\left(1-e^{-T/\tau_i}\right).
$$
For $C_1=C_2=0.10$ in compliance units and $\tau_1=0.2$ s, $\tau_2=2$ s, the effective response after 0.5 s is about 0.114, whereas its long-time limit is 0.20. The shorter observation captures less filling of the slow compartment. These are constructed passive model values, not clinical reference ranges.
A primary 2022 pilot study compared four static and dynamic calculation methods during passive mechanical ventilation. Its methods distinguish plateau pressure, pressure at zero flow, constant-flow slopes and expiration-derived estimates.<Static and Dynamic Measurements of Compliance and Driving Pressure: A Pilot Study by Pierre Tawfik and others>[Figure 1 and Introduction, Methods A–D; Materials and Methods, passive ventilation]{ref} Those airway-pressure measurements describe the respiratory system, including the <chest wall>. Separating lung mechanics requires an additional internal pressure reference, such as the esophageal-balloon approach used in the 1959 experiment.<Airway Resistance and Lung Compliance by Ole Storstein and others>[Methods, intra-esophageal balloon and differential pressure recording]{ref}
Back to article page
