 Static versus dynamic lung compliance

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 $C_{L,\mathrm{stat}}=dV/d{P}_L$ 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.^[ref]

An original one-compartment model isolates flow contamination. Relative to a chosen baseline, let mouth-to-pleural driving pressure satisfyWith stipulated $C_L=0.20\mathrm{L}(\mathrm{cm}{\mathrm{H}}_2\mathrm{O}{)}^{-1}$, $R=5\mathrm{cm}{\mathrm{H}}_2\mathrm{O}\mathrm{s}{\mathrm{L}}^{-1}$, $\Delta V=0.50$ L and $\dot{V}=0.50\mathrm{L}{\mathrm{s}}^{-1}$, 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 ${\tau }_i=R_i{C}_i$, thenFor $C_1=C_2=0.10$ 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.^[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.^[ref]