Airway Pressure Release Ventilation

Ventilation mode that targets a high mean airway pressure in order to maximise lung recruitment, whilst still allowing safe spontaneous ventilation. APRV provides:

APRV can be thought of simply as inverse-ratio PCV with an open inspiratory valve, so the patient can breath spontaneously across the mandatory respiratory cycle.

Note the P-levels are sometimes called PEEP, but this is a bit of a misnomer because these pressures are supplied throughout the respiratory cycle - more akin to CPAP.

Advantages and Disadvantages of APRV
Advantages Disadvantages
  • ↑ Alveolar recruitment
  • Improved oxygenation
  • ↑ Secretion clearance
  • ↓ Volutrauma
  • ↓ LV afterload
  • ↓ Sedation requirement
  • ↑ Volutrauma
    Often ↑ VT with pressure release breaths.
  • ↑ RV afterload
    ↑ Volume requirements.
  • ↓ VR

Indications

  • Refractory hypoxaemia due to bilateral lung pathology
    Classically ARDS.

Technique

Settings

Parameters:

  • P-High
    Major determinate of both the final mean airway pressure and the mandatory tidal volume.
    • ↑ May lead to over-distension, volutrauma, and haemodynamic instability
    • ↓ May lead to hypoxaemia, derecruitment, and ↑ PaCO2
  • P-Low
    Note this is not the PEEP, as expiration is (or should be) terminated before flow ceases and so P-low is not reached. P-low therefore determines the gradient of release flow.
    • A lower P-low causes a steeper gradient and more rapid exhalation, so in general P-low is set as low as possible (usually 0)
    • ↑ May be more protective if VT is excessively large
      ↓ Atelectasis, ↓ volutrauma.
    • ↓ Will ↓ tidal volume
  • T-low
    Time spent at P-low. Determines the degree of exhalation and derecruitment that occurs, so setting this correctly is critical. Should be adjusted to 75% of PEFR.
    • ↑ Will ↑ volume of the release breath
      Hyperventilation, volutrauma, derecruitment.
    • ↓ Will ↓ volume of release breath
      Ineffective ventilation.
  • T-high
    Time spent at P-high. Initially at ~9× the T-low, which should provide 10-14 release breaths/minute.
    • Should be ↑↑↑ than T-low to prevent derecruitment
    • ↓ Will ↑ release frequency, which may cause derecruitment
    • ↑ Will ↓ release frequency, which may ↑ PaCO2
  • Release frequency
    Number of release breaths per minute, determined by T-high and T-low.
  • ATC
    Required to provide the patient pressure support for spontaneous breaths.

The key difference between APRV and other inverse-ratio methods of ventilation is that the expiratory phase is titrated to the patients lung compliance.

Initial Settings:

  • P-High
    25cmH2O (<30) for lung protection.
  • P-Low
    0-5cmH2O.
  • T-low
    0.5-0.7s; adjust to achieve end-expiratory flow of 75% of PEFR and VT <8mL/kg.
  • T-high
    5s.
  • FiO2
    As required.
  • ATC
    On.

Weaning

  • Readiness
    • Spontaneous ventilation achieved
    • Gas exchange improved
      FiO2 <50%, CO2 reasonable.
  • Process
    • ↓ P-high by ~2cm and ↑ T-high by 0.5-2s Q4-8H
    • Can change to PSV when P-high is ~16cmH2O and T-high is ~15s

Weaning from APRV essentially aims to transition the patient from APRV to “almost”CPAP, as the patient will end up spending most of the time at (a lower) P-high, with only a couple of mandatory (release) breaths. Most of the work of ventilation is therefore done by the patient.

Troubleshooting

Hypoxaemia:

  • Usually indicates under-recruitment
    • ↓ T-low if end-expiratory flow < 75% PEFR
    • ↑ P-high 1-2cmH2O
    • ↑ T-high 0.5-1s
    • ↑ P-low 1-2cmH2O

Hypercapnoea:

  • Encourage spontaneous ventilation
  • ↑ P-high 1-2cmH2O
  • Adjust T-High
    ↑ May improve recruitment and V/Q matching; ↓ will ↑ release frequency.
  • ↓ P-low

Complications

  • Barotrauma
  • Haemodynamic instability
    High airway pressures ↓ VR and ↑ RV afterload.

Further Reading

References

  1. Spiegel, Rory, and Max Hockstein. Airway Pressure Release Ventilation. Emergency Medicine Clinics of North America 40, no. 3 (August 2022): 489–501. https://doi.org/10.1016/j.emc.2022.05.004.