= End-expiratory hold
An \b[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.<How to measure driving pressure by Hamilton Medical>[End-expiratory hold and measurement conditions]{ref}
The mechanical reason is easiest to see in one communicating compartment. With <airway resistance> $R$, let $Q=(P_{\rm alv}-P_{\rm aw})/R$ 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 $P_{\rm aw}=P_{\rm alv}$. 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 $8\,\mathrm{cm\,H_2O}$ differ by $3\,\mathrm{cm\,H_2O}$. If <plateau airway pressure> is $23\,\mathrm{cm\,H_2O}$, <respiratory driving pressure> is $23-8=15\,\mathrm{cm\,H_2O}$, 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.
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