Spontaneously breathing individuals with a disease that leads to early airway closure (e.g., emphysema) have a prolonged expiratory phase and use pursed-lip breathing. However, a patient cannot use pursed lip breathing with an endotracheal tube in place.
To mimic pursed lip breathing earlier ICU ventilators provided a method of exhalation called Expiratory retard, which added a degree of resistance to exhalation. Expiratory retard prevented early airway closure and improved ventilation.
Ventilator circuits, expiratory valves, and bacterial filters placed on the expiratorv side of the patient circuit produce a certain amount of expiratory retardation because they cause some resistance to flow.
The clinician must check for expiratory resistance by observing the pressure manometer and the ventilator pressure-time and flow-time graphics. Increased resistant is present if pressure and flow return to baseline very slowly during exhalation .