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Peak airway pressure, $P_{\mathrm{peak}}$, is the largest measured airway pressure over the specified breath or inspiratory interval. Its meaning includes the measurement location, reference pressure and temporal response. At the airway opening during positive-pressure inflation, the peak can include the pressure needed for flow through resistance as well as the pressure distending the respiratory system.^[ref]

For a passive single-compartment model with constant airway resistance $R$ and respiratory-system compliance $C$,Here $V$ is volume above the end-expiratory state and $Q$ is inspiratory flow. If constant flow persists to the end of inspiration, the largest volume and the resistive pressure occur together, givingWith varying flow, the peak is instead the maximum of the time-dependent sum; separate maxima cannot automatically be added.

An original example takes total end-expiratory pressure 5 cm H${}_2$O, tidal volume 0.50 L, $C=0.05$ L/cm H${}_2$O and $Q=0.50$ L/s. For $R=10$ cm H${}_2$O·s/L, peak pressure is $5+10+5=20$ cm H${}_2$O. Doubling resistance raises the calculated peak to 25 cm H${}_2$O while the equilibrated plateau airway pressure stays 15 cm H${}_2$O. A larger peak can therefore occur without a larger static distending increment.

An end-inspiratory hold removes ongoing airway flow under appropriate conditions and helps separate these contributions. Active respiratory effort, leaks, nonlinear resistance or delayed redistribution complicate that separation. Peak pressure alone neither partitions lung and chest-wall loading nor establishes a unique cause of a pressure change.

Plateau airway pressure, $P_{\mathrm{plat}}$, is the airway pressure assessed during an end-inspiratory interval with no flow and sufficiently stable conditions. An end-inspiratory hold can allow the airway and communicating alveolar gas pressures to equilibrate when the respiratory system is passive and the circuit is not leaking.^[ref][ref]

The distinction from peak airway pressure follows a simple single-compartment model:During the modeled hold, $\dot{V}=0$, leavingHere $C$ is respiratory-system compliance, $R$ is airway resistance, and total positive end-expiratory pressure supplies the starting reference. The measured difference $P_{\mathrm{plat}}-P_{\mathrm{EEP},\mathrm{total}}$ is the corresponding respiratory driving pressure.

In an original example, total PEEP 8 cm H${}_2$O, tidal volume 0.40 L and compliance 0.04 L/cm H${}_2$O give $P_{\mathrm{plat}}=18$ cm H${}_2$O. If flow immediately beforehand is 0.5 L/s and resistance is 12 cm H${}_2$O·s/L, the model's airway pressure during that flow is 24 cm H${}_2$O. The 6 cm H${}_2$O difference is the modeled resistive contribution, not an extra static distending pressure.

A plateau does not directly measure pleural pressure or isolate the lung from the chest wall. For the same stipulated alveolar pressure 18 cm H${}_2$O, pleural pressures of 12 and 2 cm H${}_2$O imply transpulmonary pressures of 6 and 16 cm H${}_2$O. The airway value alone cannot distinguish those states.

Finally, a stable displayed number is not automatically the same operation as an actual hold. The manufacturer distinguishes measured holds, rapid occlusion and calculated monitoring estimates.^[ref] Reporting the method and its conditions connects plateau airway pressure to the mechanical question it can answer.

Positive-pressure ventilation assists inhalation by raising pressure at the airway opening, driving gas into the lungs when that pressure exceeds alveolar pressure. It differs mechanically from spontaneous expansion of the thorax or negative-pressure ventilation, which lowers pressure around the lungs to produce the relevant distending difference. A ventilator's mask or tube supplies the applied airway connection.^[ref]

The direction of flow still depends on a pressure difference, not on whether either pressure is above atmospheric. For a simple resistive path,with positive $Q$ directed inward and $R$ the airway resistance. In an original example, pressures 10 and 8 cm H${}_2$O and $R=4$ cm H${}_2$O·s/L give inward flow 0.5 L/s. Pressures 8 and 10 cm H${}_2$O give outward flow 0.5 L/s. Both pairs are positive relative to atmosphere.

This explains why positive end-expiratory pressure can coexist with exhalation. The applied pressure can remain above atmospheric while elastic recoil produces a still higher alveolar pressure and drives gas outward. At equal opening and alveolar pressures, the same ideal resistive model gives zero flow.

The resulting tidal volume also depends on respiratory-system compliance, airway resistance, duration and any active respiratory muscle effort. The no-flow interval used to interpret plateau airway pressure removes one resistive contribution only under the required measurement conditions.^[ref] Positive pressure is therefore a mode of applying mechanical assistance, not a guarantee of a particular inward flow, volume or gas-exchange result.

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.