= Ventilator circuit compliance
{title2=$C_{\rm circuit}$}
\b[Ventilator circuit compliance], $C_{\rm circuit}$, quantifies gas volume temporarily stored in the breathing circuit as its <pressure> rises. Storage includes expansion of tubing and compression of gas already in the circuit. With a stated volume-reference convention, a local linear approximation is
$$
\Delta V_{\rm stored}\simeq C_{\rm circuit}\Delta p.
$$
It describes the delivery apparatus, rather than the <respiratory-system compliance> of the patient.
The <ideal gas law> explains why even rigid tubing can contribute. For a rigid circuit of volume $V_c$ at fixed temperature, $n=PV_c/(RT)$. Expressing the extra gas as volume at the initial absolute pressure $P_0$ gives
$$
\Delta V_0=\frac{\Delta n RT}{P_0}=\frac{V_c}{P_0}\Delta p.
$$
For an original illustration, let $V_c=2$ L and $P_0=1000$ cm H$_2$O. The gas-storage contribution is 2 mL/cm H$_2$O. If tubing expansion adds 0.5 mL/cm H$_2$O, a 20 cm H$_2$O rise stores about 50 mL in this linear model. Absolute pressure belongs in the gas-law denominator; the 20 cm H$_2$O pressure increment is not that denominator.
Under the same gas-reference convention, integrated source flow must supply both patient volume and the change in circuit storage, plus any leaked volume. A remote flow sensor therefore need not measure only <tidal volume>. A sensor near the airway opening moves the measurement boundary past much of the circuit storage. The manufacturer's technical account explicitly motivates proximal measurement by this distinction.<Flow sensors for mechanical ventilators by Hamilton Medical>[Proximal flow and pressure measurement; Clinical evidence]{ref}
Correcting storage is still a model-based operation. A single compliance coefficient does not identify an unknown leak, changing tubing properties or every dynamic pressure difference. The technical source's reproduced experimental abstracts show why circuit correction did not eliminate every discrepancy between ventilator-side and airway-side measurements. Those observations motivate checking the measurement location and gas conditions rather than equating a displayed source volume with gas entering the lung.
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