For those who have limited exposure to airway pressure release ventilation (APRV), it may help to conceptualize the following discussion through two paradigms. One is to consider high pressure (PHigh) similar to a higher pressure CPAP with periodic releases or breaks. When pressure is held over time, the pressure may dissipate over the lung field. As the pressure dissipates and alveoli begin to open, the wall tension of connected (unrecruited) alveoli is changed, facilitating their recruitment. The time over which the CPAP (PHigh) is held is the “Time high” (THigh). The second paradigm is visualizing blowing up a balloon and holding it by the balloon neck. The PHigh could be considered the pressure within the inflated balloon. PLow is the lower pressure outside the balloon. The Thigh could be considered the amount of time the neck of the balloon is held closed. The TLow could be analogous to how long the neck of the balloon is released. Using the inflated balloon analogy, how much air leaves the balloon (and thus how much remains) is a factor of the pressure differential between the inside of the balloon and the outside, how long the neck of the balloon is released, and extrinsic forces being applied to the outside of the balloon. So, if the neck of an inflated balloon were released for 30 seconds, most of the air would leave the balloon as the pressures equilibrated and the “patient would derecruit.” If the neck of the inflated balloon were to be released for 0.5 seconds, there would be some release of air, but a significant amount would still be retained within the balloon. As will be explained later, this is why there is CO2 release without derecruitment and why the traditional setting of PLow is zero. Further, extrinsic factors can impact how much air is released during a given TLow. For example, as abdominal hypertension progresses to abdominal compartment syndrome, the abdominal contents press against the diaphragm, causing more air to be released during a given TLow, similar to the inflated balloon being compressed while releasing the neck of the balloon.
Another concept to consider with APRV is that the pressure (PHigh) administered during the THigh is similar to CPAP in that the patient can breathe during PHigh. As will be discussed more fully, the patients native breaths initially will be ineffectual in releasing CO2 and most of the ventilation will occur during the set TLow phase. However, as the patient gets stronger, more CO2 will be released during the patient's native breaths. This allows for weaning, as the PHigh will be dropped and the THigh will be extended until the patient achieves a traditional CPAP setting and can be extubated. An important note is that TLow is not adjusted as part of ...