EM resident and aspiring intensivist with a passion for all things critical care and an unhealthy obsession with airway management. Mediocre mountain-biker and wannabe polyglot in his free time. Occasionally tweets at @HappyDays_EM.
Academic emergency medicine and surgical intensive care in Washington, DC. Small k_m for echocardiography, cars, and diet coke.
Emergency Medicine Resident and MedED Enthusiast. Learning and teaching medicine one doodle at a time!
The Pre-brief
We all know PEEP. Airway Pressure Release Ventilation (APRV) is an alternative to traditional mechanical ventilation, traditionally used as a “rescue” mode for refractory hypoxemia, which takes the idea of PEEP and runs away with it.
Positive end expiratory pressure is applied in order to open up (i.e. “recruit”) collapsed alveoli and to prevent them from collapsing at the end of expiration (also known as cyclic derecruitment, a major player in ventilator-induced lung injury, VILI). By opening alveoli, intrapulmonary shunting is decreased thereby increasing oxygenation. (Read more about PEEP HERE)
APRV
APRV is an inverse ratio pressure-controlled mode of ventilation that applies a high constant pressure (Phigh) with intermittent, short releases to a lower pressure (Plow). Phigh is the principal component which maintains the mean airway pressure. Using a high, constant pressure increases mean airway pressure and maximizes alveolar recruitment, thereby increasing oxygenation. Like CPAP, patients are able to (and should!) spontaneously breathe while on APRV which increases patient comfort.
Ventilation (exhalation of CO2) occurs during time-cycled “releases” and during the patient’s spontaneous ventilation. Releases are brief drops in pressure which allow for ventilation but are “short” enough to prevent collapse of the recruited alveoli. In addition to Phigh, there are three other settings that determine the releases: Thigh, Tlow, and Plow.
- Plow is the low pressure the ventilator “releases” to
- Thigh is the time spent at Phigh
Tlow is the time spent at Plow
APRV uses a long Thigh in order to maximize alveolar recruitment and a short Tlow in order to prevent alveolar collapse. Ideally, Tlow is short enough that the pressure in the alveoli never reaches Plow. This creates a functional air trap or “auto-PEEP.”
Typical initial settings vary, but the following are reasonable:
- FiO2 set at 1 (titrate to SpO2 88-94% or PaO2 > 55)
- Phigh 25-35 cm H2O (typically set to plateau pressure on conventional settings)
- Plow 0 cm H2O
- Thigh 5 seconds
- Tlow 0.5 seconds (Editor’s note: in the TCAV approach to APRV, Tlow is set so that flow terminates at 75% of the peak expiratory flow rate)
As mentioned before, APRV allows patients to spontaneously breathe which augments ventilation in addition to the timed releases. While studies have shown APRV to improve oxygenation compared to conventional lung-protective ventilation, there has yet to be a proven mortality benefit.
The Debrief
- APRV is an additional form of mechanical ventilation that applies a high constant pressure (Phigh) in order to increase alveolar recruitment and oxygenation
- APRV has timed releases to a low pressure (Plow) for a brief period in order to allow for ventilation while preventing alveolar collapse
- The four settings unique to APRV are Phigh, Plow, Thigh, and Tlow
- Patients can (and should!) spontaneously breathe while on APRV, which increases comfort and contributes to ventilation
- While APRV improves oxygenation, it has not been shown to lower mortality; Further studies are needed to determine how and when to optimally apply APRV
- Look out for a post in the future on how to set APRV more precisely!
References
- Swindin J, Sampson C, Howatson A. Airway pressure release ventilation. BJA Educ. 2020;20(3):80-88. doi:10.1016/j.bjae.2019.12.001
- Zhou Y, Jin X, Lv Y, et al. Early application of airway pressure release ventilation may reduce the duration of mechanical ventilation in acute respiratory distress syndrome. Intensive Care Med. 2017;43(11):1648-1659. doi:10.1007/s00134-017-4912-z
