At least one respiratory parameter of a patient can be monitored by means of a devi ce (ASD) that has the following characteristics. An inlet (IL) receives an airflow of breathable air. An outlet (OL) applies the airflow to a mask worn by the patient. An airflow path (AFP) is arranged between the inlet (IL) and the outlet (OL). The airflow path (AFP) has an axis (AX) and exhibits a decrease(-”D) in diameter followed by an increase (+”D) in di ameter in a direction from the inlet (IL) to the outlet (OL). The increase (+”D) in di ameter comprises an initial portion (WI) in whi ch the di ameter increase accordi ng to a slope (Õ) of less than 10°with respect to the axis (AX). The initial portion (WI) accounts f or at least two thirds of the increase (+”D) in diameter. A pressure measurement arrangement (PS1, PS2) provi des an indication (SO) of a pressure difference in the airflow between two sections (IN,T) of the airflow path that have different diameters.
