= Positive end-expiratory pressure
{title2=PEEP}
\b[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.<How to measure driving pressure by Hamilton Medical>[Measurement on Hamilton Medical ventilators; End-expiratory hold]{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> $C$,
$$
\frac{dV}{dt}=-\frac{V}{RC},\qquad V(t)=V_0e^{-t/(RC)}.
$$
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 \b[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.<How to measure driving pressure by Hamilton Medical>[Driving-pressure definition and measurement conditions]{ref} Naming the pressure reference is therefore part of defining the reported mechanical quantity.
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