A system for measuring the mechanical impedance of a patient's respiratory system during spontaneous respiratory activity, characterised by comprising a fan; a motor which operates said fan; said motor and said fan being positioned within a cavity; said cavity comprising an initial end and a final end, both providing access to the outside; said cavity presenting an impedance between said initial and said final end of less than 1 cm H2O/L/s; said fan withdrawing air from said final end and providing pressure variations of small amplitude and of frequency >2 Hz to said initial end; said initial end comprising air pressure and air flow measurement means and a connection directly connected to the airway opening.