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Intrinsic positive end-expiratory pressure is the residual alveolar pressure above the externally imposed end-expiratory pressure, associated with incomplete lung emptying. In a communicating passive compartment,The manufacturer's technical account makes this distinction and explains why a fitted whole-breath estimate can differ from the pressure obtained during an end-expiratory hold.^[ref]

An original repeated-breath model isolates the timing mechanism. Let $V_n$ be excess volume above equilibrium with the outlet just before inspiration, and let each inspiration add tidal volume $V_T$. During exhalation, assume a passive linear compartment with time constant $\tau =RC$. After expiratory time $T_E$,The steady excess volume satisfies $V_{\ast }=a(V_{\ast }+V_T)$, henceFor stipulated $V_T=0.40$ L, respiratory-system compliance $C=0.05$ L/cm H${}_2$O, $\tau =1$ s and $T_E=1$ s, $a\simeq 0.368$, $V_{\ast }\simeq 0.233$ L and intrinsic pressure is about 4.66 cm H${}_2$O. Giving this same ideal compartment two seconds to empty yields about 0.063 L and 1.25 cm H${}_2$O. These are consequences of a model, not patient-specific timing instructions.

A nonzero external baseline does not by itself imply incomplete emptying. Conversely, setting an external baseline to zero does not prove that alveolar pressure has returned to zero. Measurements require attention to communication between regions, leakage and active muscle pressure; a single airway value need not reveal every trapped regional pressure. The recurrence explains why resistance, compliance and available time matter together rather than assigning the phenomenon to one pressure setting alone.