Fig. 1.

A conceptual model describing the generation of tidal volume (Vt) as a function of assistance in pressure support ventilation. As shown, the pattern of interaction comprises three phases—under-assistance, adequate assistance and over-assistance—which may or may not all be evident in a specific patient, depending on the range of PS applied and her/his capability of producing inspiratory effort (Pmus). The latter is proportionally modulated by the patient in the range of adequate assistance from the ventilator–included in between the thresholds of under-assistance (PS_under) and over-assistance (PS_over) to achieve a target Vt (Vt_target). The patient will compensate any change in the level of assistance by an opposite change of Pmus: under-assistance (incapability of keeping Vt_target) will happen whenever the patient is required to exert a Pmus greater than what is sustainable over time (Pmus_lim). When PS is zeroed Vt will depend on the product of Crs and Pmus; on the contrary when PS_over is overcome and inspiratory effort almost zeroed except from the trigger activity (i.e. over-assistance), Vt depends solely on PS and Crs