irway 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.
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).
- 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
- When possible wean FiO2 to <50% for a SpO2 >90% or a PaO2 >60 torr.
- To improve oxygenation via higher PMean:
- Increase PHigh in increments of 2 cmH2O.
- Decrease TLow to be closer to 75% PEFR.
- Increase PHigh (up to 40 cmH2O).
- 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.
- 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.
- 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).
- If unable to manage the acidosis with APRV, the mode may be changed to PCV attempting to maintain the same rate and PMean.
- Decrease PHigh (SpO2 may decrease).
- Increase THigh to decrease the release state.
- Turn ATC off if no spontaneous respirations.
Increased Respiratory Efforts
- If the patient is consistently inhaling forcefully with accessory muscles, they may need alveolar recruitment. Options are:
- Increase PHigh. This will elevate the PMean and encourage recruitment.
- Decrease TLow only if you can maintain the flow during the release phase <75% of PEFR and the PaCO2 and pH are acceptable.
- If the patient seems to be exhaling forcefully, over-inflation may be present. Options are:
- Decrease the PHigh in 1-2 cmH2O increments and increase THigh (to maintain the same PMean).
- Increase the TLow. Allowing more time to exhale only if you can maintain the flow during the release phase >25% of PEFR.
- CXR should be monitored for lung over-inflation.
When FiO2 is titrated below 50%, recruitment is maximized and the patient is breathing spontaneously, a continuous gradual wean can begin by:
- Decreasing the PHigh by 1-2 cmH2O and increasing the THigh by 0.5 seconds for every 1 cmH2O drop in PHigh. This is referred to as the “drop and stretch”.
- “Drop and stretch” should be done every two hours or more if tolerated. As you “drop and stretch” the PMean is gradually lowered, so you will need to monitor SpO2. Changing the CPAP or dropping PMean too quickly will possibly de-recruit alveoli.
- Throughout the weaning process, the patient should be closely monitored for increasing work of breathing, tachypnea, or a drop in SpO2. If this occurs, return to the previous settings.
- When the PHigh reaches 10 cmH2O and the THigh reaches 12-15 seconds, change the mode to CPAP with PEEP at 10 cmH2O and pressure support at 5-10 cmH2O. ATC should be off. Slowly wean CPAP was tolerated.
- The patient should be spontaneously breathing throughout this process, therefore it is not necessary to do spontaneous breathing trials.
- CritCareMed. 2005;33:S228 Other Approaches to Open-Lung Ventilation–Airway Pressure Release Ventilation.
- CleveClinJMed 2011;78:101 Airway Pressure Release Ventilation–Alternative Mode of Mechanical Ventilation in Acute Respiratory Distress Syndrome.
- AnnThoracMed 2007;2:178 Airway pressure release ventilation.
- AACNClinicalIssues 2001;12:234 Airway Pressure Release Ventilation–Theory and Practice.