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Mechanical ventilation assists or replaces the movement of gas into and out of the lungs using a machine. A ventilator can deliver that assistance through a mask or a breathing tube.^[ref] This supports pulmonary ventilation; it does not replace every function of the lungs, circulation or gas exchange.

In positive-pressure ventilation, pressure applied at the airway opening helps drive inhalation. The resulting tidal volume depends on both that pressure history and respiratory mechanics. For a passive single-compartment model, a useful equation isHere $V$ is volume above the selected end-expiratory reference, $C$ is respiratory-system compliance, $R$ is airway resistance, and $p_0$ represents the baseline positive end-expiratory pressure. The model neglects active respiratory muscle pressure and changes in these coefficients.

For constructed $C=0.05$ L/cm H${}_2$O, $R=5$ cm H${}_2$O·s/L, $V=0.5$ L, $\dot{V}=0.5$ L/s and $p_0=5$ cm H${}_2$O, the predicted airway pressure is $5+10+2.5=17.5$ cm H${}_2$O. At zero flow with the same volume, the resistive term vanishes and the model gives 15 cm H${}_2$O. Thus an airway-pressure peak contains different information from a no-flow pressure measurement. These numbers illustrate the equation, not patient settings.

The importance of testing ventilation strategies was demonstrated by a 2000 randomized trial in acute respiratory distress syndrome and acute lung injury. It compared lower and traditional tidal volume strategies indexed to predicted body weight, with different plateau-pressure limits. In 861 participants, reported mortality was 31.0% versus 39.8%.^[ref] The experiment compared complete specified protocols in a defined population; it does not establish one universal setting from tidal volume alone. Physical models explain how pressure and flow interact, while controlled clinical evidence tests consequences that those models cannot supply by themselves.

The oxygenation ratio, often called the P/F ratio, divides arterial oxygen partial pressure by inspired oxygen fraction:The denominator is a fraction, not a percentage number. Since it is dimensionless, the ratio retains the pressure units used in the numerator. An arterial blood gas analysis provides the partial-pressure measurement; its measurement target differs from oxygen saturation.^[ref]

For a constructed example, $P_{\mathrm{a}{O}_2}=75\mathrm{mmHg}$ and $F_{\mathrm{I}{O}_2}=0.50$ give $R=150\mathrm{mmHg}$, approximately $20.0\mathrm{kPa}$. Dividing by $50$ instead would give a result 100 times too small. Values expressed in kilopascals cannot be compared numerically with values expressed in millimeters of mercury without unit conversion.

The 2012 Berlin definition used this ratio in characterizing acute respiratory distress syndrome. That historical consensus also considered other syndrome criteria; the ratio alone does not establish the syndrome.^[ref] The inspired fraction and respiratory-support conditions must be recorded alongside the arterial measurement. Changing the denominator is not an experiment showing that arterial partial pressure must change proportionally.

A first-order uncertainty propagation calculation shows why both measurements matter:If independent standard uncertainties are stipulated as $3\mathrm{mmHg}$ and $0.02$ for the example above, each relative uncertainty is $0.04$. Their quadratic combination gives a relative standard uncertainty $\sqrt{0.04^2+0.04^2}\approx 0.0566$, or about $8.5\mathrm{mmHg}$ in the ratio. This example concerns measurement arithmetic, not a diagnostic threshold.