Cycle variables determine how a breath is ended. A positive pressure breath will always end because a variable has reached a set value; this can be pressure, volume, flow or time. When a ventilator is set to pressure cycle, it delivers the flow until a preset pressure is reached, at which time inspiratory flow ends and expiratory flow begins. When a ventilator is set to volume cycle, it delivers the flow until a preset volume has passed through the control valve, at which time inspiratory flow stops and expiratory flow begins. The flow may be used to terminate the inspiratory phase when it reaches a preset level usually 25% of the maximum inspiratory flow, at which time the inspiration ends. A pressure-cycled ventilator is one that stops pushing gas into a patient’s lungs when a preset airway pressure is reached. An example of a pressure-cycled ventilator is a Bird ventilator, which is commonly used to give intermittent positive pressure breathing (IPPB) treatments. The major disadvantage of pressure-cycled ventilators is that they will not deliver the desired tidal volume if pulmonary compliance is decreased. Time cycling means that expiratory flow start because a preset inspiratory time has elapsed.