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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.

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.

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.

Respiratory-system compliance is the change in volume per change in elastic pressure across the passive lung and chest wall together. For a local static relation,Lung compliance alone instead uses transpulmonary pressure, $P_L=P_{\mathrm{alv}}-P_{\mathrm{pl}}$. The whole-system and lung-only quantities are different even when they refer to the same volume change.

At that shared volume, the elastic pressure increments add. With component compliances $C_L$ and $C_W$,The reciprocals are component stiffnesses; respiratory-system elastance is $E_{\mathrm{rs}}=1/C_{\mathrm{rs}}$. For positive component compliances, the combined value is smaller than either component. OpenStax describes opposing lung and wall recoil and their pressure references.^[ref]

An original example uses $C_L=0.20$ and $C_W=0.10\mathrm{L}(\mathrm{cm}{\mathrm{H}}_2\mathrm{O}{)}^{-1}$. The combined compliance is about 0.0667 in the same units. An added 0.50 L requires 2.5 cm water of lung elastic pressure and 5 cm water of wall pressure, totaling 7.5 cm water. Attributing that total entirely to the lung would underestimate lung-only compliance threefold.

In a passive ventilated-breath measurement, a zero-flow plateau airway pressure can be compared with the appropriately measured end-expiratory pressure. Under the stated equilibrium and no-effort assumptions, an interval estimate isFor stipulated $V_T=0.5$ L, $P_{\mathrm{plat}}=12.5$ and $P_{\mathrm{end}}=5$ cm water, this is again about 0.0667. The baseline must account for the relevant positive end-expiratory pressure; a pressure measured during nonzero flow also contains a resistive contribution. The primary pilot study explicitly compares pressure-based respiratory-system estimates under passive ventilation.^[ref]

State whether the number is a local slope or an interval ratio, which pressure references were used, and whether muscle activity and flow were excluded. Separating lung and wall contributions requires internal pressure information beyond the whole-system volume response.