There is no mystery behind the Airway Pressure Release Ventilation (APRV) mode. In fact, there is no mystery behind any mode. Your patient doesn’t know what mode they are ventilated with. They simply respond to physical parameters – pressure, volume, time, flow, etc. Like in all other modes, in APRV, air movement is facilitated by a pressure difference (gradient) between the airway and the alveoli. During inhalation, the airway pressure rises to P high. During exhalation, the pressure falls to P low. APRV uses an extremely long inhalation time (T high) during which P high is applied. That maximizes the surface available for gas transfer by recruiting collapsed alveoli and pushes more oxygen through the alveolo-capillary membrane by increasing the mean airway pressure (MAP). Exhalation takes place during T low when the pressure drops. P low value is mostly irrelevant as the exhalation time is so short that the pressure usually does not fall to the set level. The mechanics of APRV is very similar to that of pressure control ventilation (PCV) with an inverse I:E ratio. Because of the extended breathing cycle (long T high), the number of mechanical breaths is usually low (~ 10 / minute). To avoid hypoventilation and hypercarbia, patients are allowed to take spontaneous breaths around P high. This is the reason why neuromuscular blockade should be avoided during APRV.
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