Episode 77: Mastering APRV with Rory Spiegel

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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

1. 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.
2. 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.)
3. 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.
4. 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.
5. 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.
6. 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.
7. 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, as it’s still associated with a normal/low driving pressure, since the “PEEP” gradually increases as trapping increases. A large release volume + low driving pressure is felt to be lung protective in APRV thinking.
8. Driving pressure on ARPV can be checked on most vents by performing an expiratory (not inspiratory) hold to determine the effective “PEEP.”
9. Patients can breathe spontaneously on APRV and be comfortable, but this is mostly determined by lung recruitment and how close they are to FRC. When the lungs are still tightly closed, spontaneously breathing will not be either comfortable or safe, so when initially flipped to APRV, patients should NOT be breathing; they will look uncomfortable, require very high minute ventilation, and generate high pressures. (There is also great discomfort here due to the hypercarbia usually unavoidably present.) Use a shorter Thigh and ventilate using the vent releases in this period, while using deep sedation and/or paralysis to suppress breathing.
10. As patients stabilize and recruit, the minute ventilation needed to maintain pCO2 will drop as ventilation becomes more efficient. When MV and the CO2 approach normal physiologic ranges, sedation can be lightened and patients allowed to breathe. Ultimately, severe ARDS patients on APRV require less total sedation and need for paralysis than in other modes.
11. Weaning occurs as thus: CO2 will gradually fall and release volumes naturally increase as the lungs recruit. Eventually they become hypocapnic, so Thigh must be increased to reduce the minute ventilation. As MV reaches normal, stretching the Thigh further causes hypercapnia, so patients should now be allowed to start breathing spontaneously to make up the difference in MV. Breathing should look comfortable, with a benign clinical appearance and gentle inspiratory flows (not sharp peaks); if not, recruitment may not yet be optimal and it may not be time for spontaneous breathing.
12. Rory does not drop the Phigh during the weaning period, although many teach this; he finds it often causes derecruitment. He adjusts Phigh only in response to the perceived disease state; for example, it may need to be weaned as the disease improves and the initial Phigh may start to cause overdistention. He rarely touches it until the patient is ready for breathing. Once the patient is breathing spontaneously, this provides a good feedback tool to adjust Phigh; if you drop Phigh and spontaneous breathing looks worse (like a failed SBT – lower volumes, high rates), you derecruited them and Phigh should go back up. Spontaneous effort is a more sensitive and faster method of feedback than monitoring the release volumes alone.
13. Permissive hypercarbia is okay. But severe hypercarbia before starting APRV is a marker of advanced underlying disease and lung injury which may make it difficult to tolerate APRV, and persistent hypercarbia on APRV is a marker of failure—the lung is not recruiting, and the mode is probably not totally safe as a result (persistent acidosis + persistently high driving pressures and risk of overdistention of ventilated lung).
14. Hypotension is not necessarily a contraindication to APRV. Cardiac output is best when the lungs are at FRC, neither over- or under-distended. However, it’s true that the lungs “overdistend instantly, but recruit over time,” so until the lung recruits, intrathoracic pressure may be elevated, and delicate patients (eg hypovolemic) may not tolerate this well.
15. Pneumothorax should not be a contraindication to APRV. The more recruited the lungs, the less strain on each individual lung unit given the same overall driving pressure.
16. Using APRV is a skill that requires practice. However, it also helps create a general mindset of approaching the lung physiologically with the goal of restoring FRC, as well as appreciating the value of using minute ventilation as a marker of recruitment; these tools probably benefit patients even in other modes. With this approach, APRV can often be avoided, and used mainly as a rescue modality.