Airway pressure release ventilation (APRV) mode of mechanical ventilation is an elevated CPAP level with timed pressure releases. This mode allows for spontaneous breathing. These breaths can be unsupported, pressure supported, or supported by automatic tube compensation. They key is a dynamic expiratory valve in the circuit which allows spontaneous breathing at high lung volumes. While any patient can be adequately supported using APRV, it is generally used for patients the require recruitment of alveoli to maintain oxygenation, such as in ARDS (along with other treatments such as inhaled prostacyclin, neuromuscular blockade, PEEP, and prone position).

Indications for APRV ventilation

  • Acute lung injury (ALI/ARDS)
  • Diffuse pneumonia
  • Atelectasis requiring greater than 50% FiO2
  • Tracheo-esophageal fistual

Initial APRV Settings

  • PHigh at the PPlateau (or desired PMean + 3 cmH2O). If your are switching to APRV from a different mode, then PHigh can be set at the previous mean airway pressure.  A good starting level would be 28 cmH2O. Higher transalveolar pressures recruit additional alveoli, but, try to keep PHigh below 35 cmH2O.
  • THigh at 4.5-6.0 seconds. This is the inspiratory time. The respiratory rate should be 8 to 12 breaths per minute — never more.
  • PLow at 0 cmH2O to optimize expiratory flow. The large pressure ramp allows for tidal ventilation in very short expiratory times.
  • TLow at 0.5-0.8 seconds. The expiratory time should be short enough to prevent derecruitment and long enough to obtain a suitable tidal volume. A tidal volume target is between 4 and 6ml/kg. If the tidal volume is inadequate, the expiratory time is lengthened; if it is too high (>6ml/kg) the the expiratory time is shortened.
  • Automatic tube compensation (ATC) should be on if spontaneously breathing.

APRV_Pressure_Flow_Timing

Like Pressure Control-Inverse Ratio ventilation (PC-IRV), APRV utilizes a long “inspiratory time” (THigh) to recruit alveoli and optimize gas exchange. The open exhalation valve allows for spontaneous breathing during THigh.

Demonstration of APRV using live pig lungs.

Monitoring a patient on APRV

APRV should help rest the inspiratory muscles and utilize the diaphragm. Once the initial settings are applied, look for anterior chest muscles to be used much less and the diaphragm to be doing the majority of the work. This should occur within hours after placement on APRV. The patient should be breathing more comfortably as recruitment occurs.

The earlier APRV is used, the more effective it is in recruiting the lung and the more likely it is to be tolerated. if initiating APRV late in the course of ARDS, patients sometimes will not look comfortable despite optimal aPRV settings, and they may need an alternative mode.

APRV Flow-Time Waveforms

It is important to observe the Flow-Time waveform to optimize the settings. During the pressure release phase, the patient will exhale passively. Adjust the TLow to cut off the expiratory flow during a release at about 50% (25-75%) of peek expiratory flow rate (PEFR).

APRV_Flow_Time_Waveform

  • Never allow the termination of expiratory flow to go <25% of PEFR. This intentional intrinsic PEEP allows PLow to be set at 0 cmH2O without causing derecruitment.
  • Generally, the TLow can be as short as 0.3 seconds (closer to 75% of PEFR) in restrictive disease and as long a 1.5 seconds (closer to 25% of the PEFR) in obstructive states.

Making Adjustments to the APRV Settings

Oxygenation Options

  1. When possible wean FiO2 to <50% for a SpO2 >90% or a PaO2 >60 torr.
  2. To improve oxygenation via higher PMean:
    • Increase PHigh in increments of 2 cmH2O.
    • Decrease TLow to be closer to 75% PEFR.

Respiratory Acidosis

  1. Increase PHigh (up to 40 cmH2O).
  2. Increase THigh (if spontaneously breathing) in increments of 0.5 seconds up to 8 seconds. If PaCO2 increases with this change, it may reflect inadequate lung volume. If this is the case, try increasing PHigh to re-establish an adequate FRC.
  3. Increase TLow to allow more time for alveolar emptying, but only if the expiratory flow of a realease doesn’t drop below 25% of the PEFR.
  4. If further increases in THigh fail to drop PaCO2, you may need to do the opposite: Decrease THigh to increase the rate of releases. This will decrease the PMean and oxygenation. This will then also increase PHigh to maintain the PMean. Maximize PHigh and release rate up to 30 (more like PC-IRV).
  5. If una