First described by Stock and coworkers in 1987, airway pressure release ventilation allows spontaneous breathing at two different levels of continuous positive airway pressure (CPAP), with airway pressure periodically decreasing from the upper level to the lower level (pressure release). It can be patient or time triggered (in the latter it works exactly as pressure-controlled ventilation with PEEP), and is pressure limited and time cycled.
Airway pressure release ventilation allows spontaneous breathing at any time, either during the inspiratory or the expiratory phase. The intermittent liberation of positive pressure produces a decrease in functional residual capacity, thus assisting alveolar ventilation. The usual settings which should be adjusted in this mode are trigger sensitivity, upper and lower levels of CPAP, and the frequency and duration of pressure liberation. Typical tracings are shown in Fig 3.
Fig. 3 Tracings of tidal volume (\/T), airflow (V), and airway pressure (PaJ obtained in a patient ventilated with airway pressure release ventilation. The high-pressure level was set at 25 cmH2O and a 4-s duration, and the low-pressure level was set at Z cmH2O and a 2-s duration. These settings indicate a back-up rate of 10 cycles/min with an I:E ratio of 2:1.
Another very similar method, intermittent mandatory pressure release ventilation, was developed to provide better synchronization between the patient's expiration and pressure release, and also to provide a certain degree of assistance during each inspiration by means of pressure support ( Rouby 1994). Settings to be adjusted are trigger sensitivity, upper and lower levels of CPAP, the frequency of pressure liberation (adjusted according to the spontaneous respiratory rate, i.e. pressure will be released every two, three, four, or more spontaneous inspiratory cycles), and the level of pressure support. In this mode, ventilatory assistance depends on the patient's spontaneous respiratory rate; the higher the respiratory rate, the higher will be the ventilatory assistance.
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