Intrathoracic major airway obstruction produces inspiratory as well as expiratory sounds. The VRG contains both inspiratory and expiratory neurons. It can be inspiratory, expiratory or biphasic, although it is usually heard during inspiration. Distal airway obstruction predominantly produces expiratory sounds. Bronchiolar disease usually causes wheezing that occurs in the expiratory phase of respiration. The term active refers to the ventilator's forced expiratory system. The term passive refers to the ventilator's non-forced expiratory system. Manual abdominal compression is another technique used to increase expiratory flow which later improves coughing. On examination, crepitations and expiratory rhonchi may be heard with auscultation. Positive end expiratory pressure (PEEP) is pressure applied upon expiration. More recently, medical calculators have been developed to calculate predicted values for peak expiratory flow. There are a number of non-equivalent scales used in the interpretation of peak expiratory flow. The Bird Mark 8 added the capabilities of NEEP (Negative End Expiratory Pressure). Expiratory flow is determined by patient factors such as compliance and resistance.