Understanding Airway Pressure Release Ventilation

APRV or Airway pressure release ventilation
by Stock and Downs in 1987

Provides CPAP for a prolonged time with a time-cycled release phase to a lower set of pressure for short period of time

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  1. P high – this is the CPAP pressure
  2. T high – this is the time period on the higher pressure
  3. P low – this is the lower set of pressure
  4. T low – this is the time period on the lower pressure

Time on P high maintains a certain lung volume for alveolar recruitment
Time on P low allows ventilation and CO2 removal
Patient able to breath spontaneously at any time regardless of the ventilator cycle
If there is no spontaneous respiratory effort, APRV becomes inverse ratio ventilation

  • How does APRV work in ARDS?
    1. Mean airway pressure increased for lung recruitment
    2. Avoids repetitive inflation and deflation thereby preventing VILI or the need for recruitment maneuvers

Mean airway pressure on APRV formula:
(P High × T High) + (P Low × T Low)(T High + T Low)

Other Notes:

  • Spontaneous breathing allowed, not confined to an arbitrary I:E ratio, improves patient comfort and synchrony
  • Adding PSV to P high is feasible but contradicts limiting the airway pressure and may cause lung distention; alters normal sinusoidal flow of spontaneous breathing – ultimately PSV+APRV defeats its purpose and is not recommended
  • APRV + automatic tube compensation (ATC) – helps overcome artificial airway resistance during SBP without causing lung distension and preserving sinusoidal flow pattern

Advantages of APRV:

  1. Improved oxygenation parameters (PF ratio, lung compliance), better VQ matching,
  2. Improved oxygenation, better V/Q match, lesser dead space
  3. Improved hemodynamics: Decrease RA pressure, increased venous return, improves preload, increases CO; cf inverse ratio PCV in ARDS – higher CI, O2 delivery, SVO2%, UO and lower vasopressor/inotrope usage, lactate, CVP
  4. Effects on regional blood flow: improves blood flow to respiratory muscle, GI tract, UO, GFR
  5. Decreases need for NM blockade and sedation
  • Indications
    1. ARDS
    2. Atelectasis after major surgery
  • Contraindications
    1. Requiring deep sedation (cerebral edema with inc ICP, status epilepticus)
    2. No data on asthma in exacerbation or COPD (short release time not beneficial if prolonged expiration required)
    3. No data on patients with neuromuscular disease

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  • Set up
    1. pressures and TV
      1. P high should be below the high inflection point (HIP) on static volume-pressure curve
      2. P low should be above the low inflection point on the curve
    2. time
      1. T high should allow complete inflation (indicated by end-resp phase of no flow when not breathing)
      2. T low should allow complete exhalation with no gas flow at its end
    3. Recommend setting ATC to 100%, avoid over sedation

Initial set up

  1. P high 20-30cm H20
  2. P low 0-5 cm H20
  3. T high 4-6s
  4. T low 0.2-0.8s

Troubleshooting

  1. Poor oxygenation
    1. inc P high, T high or both to increase mean airway pressure
    2. prone position
  2. Poor ventilation
    1. inc P high and decrease T high to increase minute ventilation
    2. increase T low by 0.05-0.10 s increments
    3. Decrease sedation to increase patients contribution to minute ventilation

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Below is a 14-minute youtube video by Allan Prost that explains the basics of APRV.

 

References

Ann Thorac Med. 2007 Oct-Dec; 2(4): 176–179. doi:  10.4103/1817-1737.36556 PMCID: PMC2732103 Airway pressure release ventilation by Ehab G. Daoud

YouTube,. “Airway Pressure Release Mode Of Mechanical Ventilation.Avi”. N.p., 2016. Web. 19 Jan. 2016.

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