 Positive end-expiratory pressure

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.