Airway Pressure Release Ventilation (APRV)

general concept

  • APRV is a dual level positive pressure airway mode which allows for spontaneous breathing that may be supported or not 

  • Oxygenation is achieved through increasing the mean airway pressure (MAP) by remaining a prolonged time on the higher pressure level called Phigh

  • Ventilation is achieved through time cycled "release"of pressure to the lower level called Plow and the patient's own respirator efforts during Phigh

  • The thinking behind this mode includes

    • Lung recruitment through increase in mean airway pressure (MAP)

    • Limiting peak pressures

    • Avoiding repeated alveolar collapse/atelecttrauma

    • Allow spontaneous respiration (reducing need for sedation and neuromuscular blockade)

  • Primary Indications

    • ARDS with low lung compliance

    • Diffuse pneumonia

  • Relative Contraindications

    • Elevated Intracranial Pressure

    • Large bronchopleural fistula

    • Significant obstructive lung disease

PC-IMVs,s

Chatburn taxonomy

Initial Settings for APRV

  • P high at previous Pplat on conventional ventilation. Keep below 32cmH2O

  • T high at 4-6 seconds

  • P low at 0 - 5 cmH2O. Use lower values to optimise expiratory flow

  • T low at 0.5 to 0.8 seconds

  • PS for spontaneous breaths if desired and patient triggering

  • There is no need to set a respiratory rate as it results from 60 seconds divided by the sum of Thigh and Tlow

Pressure curve during APRV

  • Paw = Airway Pressure

  • Pexp = Expiratory Pressure = Plow

  • Pinsp = Inspirator Pressure = Phigh

  • Texp = Expiratory Pressure = Tlow

  • Tinsp = Inspiratory Time = Thigh

  • The area under the curve (shaded) represents the mean airway pressure over time (MAP)

  • Changes to the ventilator setting resulting in an increased shaded area are consistent with an increase in MAP and therefore oxygenation

  • Note that the patient can breathe spontaneously during Phigh

Oxygenation during aprv

  • Oxygenation depends on two factors

    • Mean Airway Pressure (MAP)​

    • FiO2

  • Factors increasing the MAP will increase the area under the pressure curve and therefore oxygenation

    • Increasing Phigh

    • Increasing Thigh

    • Increasing Plow

    • Reducing Tlow ​

  • FiO2 is the simplest way of increasing oxygenation while a high FiO2 is related to increased oxygen toxicity and free radical formation

Given the pressure curve during APRV resembles a square the MAP can approximated like this:

Ventilation during APRV

  • Ventilation (CO2 removal) occurs

    • during Tlow 

    • during spontaneous breathing on Phigh (see graph above) which can be supported with PS

  • Ventilation can be increased ​through

    • Reducing Thigh​ (at the cost of oxygenation as more time will be spent in expiration)

    • Increasing Tlow (until expiratory flow reaches zero. No further CO2 removal occurs after this)

    • Decreasing Plow to optimise expiratory flow by maximising the pressure gradient between Phigh and Plow

  • Adjusting Tlow​

    • Aim to cut off the peak expiratory flow during a release at about 50% of peak expiratory flow. Do not allow the termination of expiratory flow to go <25% of the peak expiratory flow rate as it would cause collapse.

    • The above strategy causes intrinsic PEEP and allows P low to be set at 0 without causing derecruitment while maximising the pressure gradient that drives the expiratory flow. 

    • Adjust Tlow to the underlying disease. Low compliance (such as in pARDS) will need short Tlow to avoid derecruitment while obstructive disease with high resistance will work with longer Ti to allow for a longer expiratory phase  

Hypercarbia

  • Decrease Thigh in 0.5 second increments. Shorter T high means more releases/min, less mean airway pressure but more ventilation

  • Increase P high to increase delta P

  • Check T low. If possible, increase T low to allow more time for alveolar emptying while keeping expiratory flow greater than 25% of peak expiratory flow rate

Hypocarbia:

  • Increase T high in increments of 0.5 seconds, this will improve oxygenation (increase in MAP) and reduce the rate of releases per minute

  • Decrease P high while monitoring oxygenation and avoiding derecruitment

Hypoxia:

  • Increase P high in increments of 2 cmH20 to improve mean airway pressure

  • Decrease T low to be closer to 75% peak expiratory flow rate to improve mean airway pressure

  • Increase FiO2

Weaning:

  • Decrease P high by 1-2 cmH2O while increasing T high by 0.5 seconds for every 1 cm H20 drop in p high

  • Drop and stretch every 2 hours as tolerated. Monitor SpO2 as mean airway pressure gradually lowered

  • When P high reaches 10 cmH20 and Thigh reaches 12-15 seconds, change the mode to CPAP with PEEP at 10 cmH2O plus minus PS

Adjusting APRV settings

Contact/Suggestions

Juerg Burren, MBBS MD FMH (CH) EPIC FCICM

Paediatric Intensivist and Ventilation Enthusiast

​​

Tel: +61 (0) 481 751 073

info@pivares.com

© 2019-2020 by Juerg Burren for PIVARES

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