We discuss the practicalities of using airway pressure release ventilation (APRV) with Dr. Rory Spiegel (@EMnerd_), emergency physician and intensivist at MedStar Washington Hospital Center (and EMNerd at Emcrit).







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Takeaway lessons









* The most immediate benefit of APRV is to help restore lungs to FRC (functional residual capacity). While this can be achieved with PEEP, most people don’t use enough PEEP. APRV proves a higher mean airway pressure while also reducing sedation requirements, and provides a physiologically automatic titration of “PEEP” based on lung compliance.







* Phigh can usually be set to equal the previous plateau pressure on a conventional mode (assuming reasonably appropriate settings there). This helps match higher Phigh to a more poorly compliant lung and vice versa. The release volume that results should be checked to give a sense of the effects; it should be more or less in the range of normal tidal volumes, although usually lower than your previous tidal volume on a conventional mode due to the intentional air trapping. (If it’s not lower, your Tlow may be set too long, allowing too much release.)







* Thigh can range from 1.5 seconds to infinity. Longer T-high is better for recruitment, shorter is better for ventilation. When initially flipping to APRV, shorter Thigh is usually needed; try to match the patient’s minute ventilation (from the prior mode) fairly closely, although usually you’ll need to accept a small loss of ventilation. A too-long initial T-high is a common error; patients this sick usually cannot tolerate more acidosis. Usually an initial T-high of 2 seconds is about right.







* Tlow should be set to terminate when the expiratory flow drops to 75% of the peak expiratory flow (so if the peak was 100 L/s, terminate Tlow when it drops to 75 L/s). This was about the point in pig models where alveolar derecruitment began to occur. Peak and end expiratory flow can be checked on most modern vents, although it may not be easy to find in the screens. Usually the right T-low is around 0.3–0.6 seconds.







* Occasional patients may need a shorter T-low than this for optimal recruitment. But few need longer; Tlow should rarely be lengthened, even as patients recruit. Although the amount of air trapping will usually increase as the lung becomes more compliant (e.g. the same T-low duration will terminate expiration at 85% instead of 75% of peak expiratory flow), this is usually fine; this is when you’ll start weaning and stretching your Thigh.







* Plow should be set to zero in almost all cases, allowing the fastest expiration (higher Plow reduces the driving pressure and substantially reduces expiratory flow). In a few vents (older Puritan Bennett, older Servos), the machine may attempt to synchronize with patient efforts by allowing the Tlow to “kick out” and extend, creating large release volumes and loss of desired air trapping. Increasing the Plow may provide some safety margin in this case, although switching from APRV altogether is probably the best solution.







* As the patient recruits on APRV, release volume should gradually increase despite a fixed Phigh, as the lung recruits. The expiratory flow curve will flatten and the compliance will increase. Thus, release volumes are initially small—”lung protective” in conventional thinking—and later will increase. This increase should be allowed,