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An end-expiratory hold is an interruption of airflow at the end of exhalation during which airway pressure is observed while the respiratory system is occluded. In an appropriate passive, leak-free measurement it can assess total positive end-expiratory pressure, including intrinsic positive end-expiratory pressure. The set external pressure alone need not equal this measured total.^[ref]

The mechanical reason is easiest to see in one communicating compartment. With airway resistance $R$, let $Q=(P_{\mathrm{alv}}-P_{\mathrm{aw}})/R$ relate airflow to the difference between alveolar pressure and measured airway pressure. Once flow vanishes and pressure has equilibrated through the communicating airway, this model gives $P_{\mathrm{aw}}=P_{\mathrm{alv}}$. Zero flow removes the resistive pressure drop; it does not mean that either pressure is zero.

For stipulated pressures, an external setting of $5$ and an equilibrated total of $8\mathrm{cm}{\mathrm{H}}_2\mathrm{O}$ differ by $3\mathrm{cm}{\mathrm{H}}_2\mathrm{O}$. If plateau airway pressure is $23\mathrm{cm}{\mathrm{H}}_2\mathrm{O}$, respiratory driving pressure is $23-8=15\mathrm{cm}{\mathrm{H}}_2\mathrm{O}$, whereas subtracting the external setting would give 18. These constructed values illustrate reference selection, not patient settings.

Occlusion changes the boundary condition rather than directly opening every lung region to the sensor. A compartment isolated by airway closure cannot be assumed to equilibrate with the measured opening. Active muscular effort, a leak or a pressure trace that has not settled also defeats the simple passive-compartment inference. The observed airway-pressure plateau and the inferred regional alveolar pressure must therefore remain distinct.

An end-inspiratory hold is a pause after inflation during which the airway is occluded and external flow is interrupted. Under passive, leak-free conditions, the resulting pressure trace helps distinguish flow-related airway resistance from the pressure remaining at approximately fixed lung volume. A stable endpoint provides a plateau airway pressure for interpreting respiratory-system compliance.^[ref]

Stopping flow does not imply that every compartment instantaneously reaches a common pressure. Stress relaxation, redistribution between lung regions and the apparatus response can produce a continuing pressure change after the rapid resistive drop. The measured value should therefore identify the observation interval, rather than treating the first post-interruption sample as universally equilibrated.

For an original illustrative trace, suppose flowing peak pressure is 26 cm H${}_2$O and stopping flow immediately removes 4 cm H${}_2$O. Let the subsequent relaxation beThe initial hold value is 22 cm H${}_2$O, about 19.47 remains after 0.5 s, and about 18.20 after 1.5 s. The model's long-time value is 18. The change from 26 to 22 and the later change from 22 toward 18 represent different mechanisms in this constructed example.

With total end-expiratory pressure 8 cm H${}_2$O, using the initial hold value gives a nominal respiratory driving pressure of 14, whereas using the asymptotic value gives 10 cm H${}_2$O. The difference demonstrates the importance of time and model assumptions; it does not prescribe a hold duration. Continued muscle activity, leakage or airway closure can invalidate the simple static interpretation. The experimental pilot study compares static and dynamic measurement methods rather than assuming all displayed pressure estimates are interchangeable.^[ref]

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 end-expiratory pressure, PEEP, is airway pressure maintained above atmospheric pressure at the end of exhalation. It specifies an end-expiratory pressure reference, not a tidal volume or a pressure rise measured from the beginning to the end of inspiration. A set external pressure and the total pressure assessed during an appropriate end-expiratory hold need not be identical.^[ref]

One reason is incomplete emptying. In an original passive single-compartment model, let $V$ be excess volume above equilibrium with the prescribed outlet pressure. With airway resistance $R$ and respiratory-system compliance $C$,The remaining elastic pressure above the outlet is $V(t)/C$. Stipulated $R=20$ cm H${}_2$O·s/L and $C=0.05$ L/cm H${}_2$O give a time constant of 1 s. Starting with 0.50 L excess volume, after 0.5 s the model retains about 0.303 L, corresponding to 6.07 cm H${}_2$O above the outlet pressure. These are constructed mechanics, not ventilator settings for a person.

If the prescribed outlet reference is 5 cm H${}_2$O, the modeled retained elastic pressure raises the equilibrated total to about 11.07 cm H${}_2$O. This illustrates intrinsic positive end-expiratory pressure: a pressure contribution that cannot be identified from the external setting alone. The result assumes a communicating passive compartment; active effort, leakage and heterogeneous time constants require further interpretation.

Respiratory driving pressure compares plateau airway pressure with total PEEP, rather than silently subtracting an external setting that may omit retained pressure. The manufacturer's primary measurement account states that distinction and the required passive, leak-free conditions.^[ref] Naming the pressure reference is therefore part of defining the reported mechanical quantity.