The normal elastic work of breathing (W el,n) represented by the triangular area is not excessive. A tidal breath of 500 ml delivered by the ventilator will increase the volume of the respiratory system to its end-inspiratory lung volume (EILV Normal). To trigger the ventilator, the patient’s inspiratory muscles have to develop an inspiratory effort ≥ the trigger threshold set on the ventilator (2 cmH 2O in the example).
In normal subjects, the end-expiratory lung volume (EELV Normal) is the relaxation volume of the respiratory system or functional residual capacity (FRC), where no inward or outward recoil pressure exists (the pressure of the respiratory system is 0 cmH 2O relative to the atmosphere).
Pressure–volume (P–V) relationship of the respiratory system when pressure is measured at the airway opening.
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