eFigure 25-1.

Relationship between driving pressure (x-axis) and airflow (y-axis) through a series of critical orifices of varying diameters (in mm).

Using this family of curves, it is possible to determine the diameter of a critical orifice in a patient with upper airway obstruction. Assuming a driving pressure of 100 cm H₂O, the diameter is given by the curve closest to the maximum flow observed in the flow plateau obtained from the flow-volume loop obtained from the patient. Curves were constructed by using graded external resistances in a normal subject.