= Ventilator
A \b[ventilator] is a machine that assists or replaces <pulmonary ventilation> by supplying a controlled gas-delivery path to the <lungs>. The interface can be a mask or a breathing tube; the machine and interface together support <mechanical ventilation>.<What Is a Ventilator? by NHLBI>[Opening explanation, machine and interfaces]{ref} Gas delivery, <oxygen> concentration and respiratory mechanics remain distinct parts of that support.
A useful engineering description separates the commanded quantity from the measured response. A machine can regulate an applied pressure or a flow history while monitoring pressure and gas volume. Its displayed or reconstructed quantities depend on sensors, timing and the selected model. The manufacturer's treatment of <plateau airway pressure>, for example, distinguishes a hold measurement from computed monitoring estimates and states conditions under which they are available.<Measured values for Ppeak and Pplateau by Hamilton Medical>[Principles of measuring plateau pressures; Monitoring parameter Pplateau]{ref}
An original circuit example shows why upstream delivered volume need not equal the volume entering the patient. In a simplified volume balance,
$$
V_{\rm source}=V_{\rm patient}+\Delta V_{\rm circuit}+V_{\rm leak}.
$$
If a stipulated source supplies 0.50 L, <ventilator circuit compliance> stores 0.04 L during pressurization, and 0.06 L escapes through a leak, only 0.40 L reaches the modeled patient compartment. This is an illustrative uncompensated circuit, not a description of every ventilator's displayed <tidal volume>.
The stored term can be approximated as $\Delta V_{\rm circuit}=C_{\rm circuit}\Delta p$. A stipulated compliance of 2 mL/cm H$_2$O with a 20 cm H$_2$O pressure rise gives the stated 40 mL. Measuring volume at another location, or compensating for known circuit properties, changes what is being reported.
A <ventilator> consequently supplies an experimentally testable physical system: gas path, actuation, sensors and control logic. A pressure or volume setting alone is not a complete observation of alveolar gas renewal, especially when flow, leakage or circuit storage changes.
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