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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.

Respiratory driving pressure, $\Delta P$, is the difference between plateau airway pressure and total end-expiratory pressure when measured under suitable passive conditions:The total baseline includes intrinsic positive end-expiratory pressure when present; substituting only the externally imposed baseline can overestimate this tidal pressure difference.^[ref]

For the linear static model, respiratory-system compliance is $C_{\mathrm{rs}}=V_T/\Delta P$. An original calculation with tidal volume 0.45 L, plateau pressure 23 cm H${}_2$O and total baseline 8 cm H${}_2$O gives $\Delta P=15$ cm H${}_2$O and $C_{\mathrm{rs}}=0.030$ L/cm H${}_2$O. If the external baseline alone were 5 cm H${}_2$O, incorrectly subtracting it would give 18 cm H${}_2$O and a different apparent compliance.

This pressure difference loads both the lung and chest wall. At static endpoints, transpulmonary pressure gives the partitionThus a stipulated airway increment of 12 cm H${}_2$O corresponds to a lung increment of 4 when the pleural increment is 8, but to a lung increment of 10 when the pleural increment is 2. Equal airway driving pressures do not establish equal lung distension without information about the chest-wall contribution.

The measurement method matters as much as the subtraction. A dynamic estimate during flow can include resistance; an end-inspiratory hold and an end-expiratory occlusion address different endpoints. The primary pilot study explicitly compared static and dynamic ways of obtaining whole-respiratory-system compliance and driving pressure.^[ref] These distinctions explain the quantity's mechanical meaning without turning an illustrative number into an individual ventilation recommendation.