 Respiratory-system elastance

Respiratory-system elastance is the local increase in passive elastic pressure per increase in respiratory volume. It is the reciprocal of local respiratory-system compliance:Its units are pressure per volume, such as $\mathrm{Pa}{\mathrm{m}}^{-3}$ or $\mathrm{cm}{\mathrm{H}}_2\mathrm{O}{\mathrm{L}}^{-1}$. A larger elastance means that the same small volume increment needs a larger elastic pressure increment.

The lung and chest wall share a volume increment while their pressure increments add. Thus their elastances add,whereas their compliances do not. For stipulated $C_L=0.20$ and $C_W=0.10\mathrm{L}(\mathrm{cm}{\mathrm{H}}_2\mathrm{O}{)}^{-1}$, the elastances are 5 and 10, totaling 15 in reciprocal units. Combined compliance is $1/15\approx 0.0667$. The underlying pressure partition is distinct from attributing all measured stiffness to the lung.^[ref]

Elastic work in a pressure-volume model gives another meaning to stiffness. Around a passive zero-recoil equilibrium, let added volume be $v$ and pressure $P=Ev$ with constant elastance. The reversible work needed to move from zero to $v$ isFor an original SI example, $E=10^6\mathrm{Pa}{\mathrm{m}}^{-3}$ and $v=5\times 10^{-4}{\mathrm{m}}^3$ give final pressure 500 Pa and elastic work 0.125 J. Doubling elastance at the same volume doubles this work; doubling volume at fixed elastance quadruples it. Resistive losses and active muscle work are outside this ideal elastic calculation.

If the pressure-volume curve is nonlinear, local elastance $dP/dV$ can differ from the interval ratio $\Delta P/\Delta V$. Its reciprocal corresponds to the local compliance at that state, not an average across an arbitrary range. A primary ventilator study compared different pressure-volume methods under passive conditions, making this measurement definition operational.^[ref] Report volume range, passive conditions and whether the value is a derivative or a secant.