In a respiratory apparatus for treatment of sleep apnea and other disorders associated with an obstruction of a patient's airway and which uses an airflow signal, an obstruction index is generated which detects the flattening of the inspiratory portion of the airflow. The flattening index serves as an obstruction index used to differentiate normal and obstructed breathing. The obstruction index is based upon comparison of values of airflow in different sectors of the inspiratory peak of the wave function and is particularly suitable for distinguishing M shaped or square shaped respiratory patters indicative of partially obstructed airways.