 Ventilator

A 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.^[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.^[ref]

An original circuit example shows why upstream delivered volume need not equal the volume entering the patient. In a simplified volume balance,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 $\Delta V_{\mathrm{circuit}}=C_{\mathrm{circuit}}\Delta p$. 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.