general philosophy 

Let's admit it, nobody knows exactly how to set APRV.  Numerous protocols and articles recommend different approaches.  There has never been a human trial comparing two different APRV protocols, so it's impossible to dogmatically say whether any protocol is better than any other protocol.  The most successful clinical trial of APRV to date is Zhou 2017, which has influenced the guidelines below.

These guidelines are intended as a basic scaffold for how to approach APRV.  Each patient is different, so ventilator settings will need to be titrated to the individual patient.  When in doubt, different settings may need to be trialed to determine which works the best.

A review of small APRV trials dating back to 1988 found that none of them caused harm, although many used protocols that are horrific by modern standards (Jain 2016).  This is evidence that APRV is a relatively forgiving ventilator mode.  Slightly suboptimal settings are unlikely to cause harm.

The guideline below is designed for patients with predominantly hypoxemic respiratory failure (e.g. ARDS, atelectasis).  Patients with severe obstructive lung disease aren't ideal candidates for APRV (e.g., they were excluded by Zhou 2017).

intro: discussion of parameters

P-High

• Ideally keep at 30 cm or below.  However, for patients with severe obesity, may need to increase higher.
• Problems with excessive P-High:
  • Lung over-distension
  • Excess driving pressure and large release breaths may cause lung injury.
  • May impair hemodynamics by causing cor pulmonale.
• Problems with inadequate P-High:
  • Derecruitment & hypoxemia.
  • Low minute ventilation and hypercapnia.
  • P-high is what provides the driving pressure for the release breath, which is the mechanism whereby APRV provides mechanical support to the work of breathing.   Inadequate P-high may cause inadequate ventilator support, causing increased work of breathing.

P-Low

• Most APRV protocols in use set P-low to zero, but some do use positive P-low of ~5 cm (Zhou 2017).
• Benefit of setting P-Low above zero.
  • Should make APRV more lung-protective, by avoiding atelectasis and wide fluctuations in airway pressure which drive large tidal volumes (Mireles-Cabodevila 2016; Chatburn 2016).
  • May allow for management of severe hypoxemia without increasing the P-High to dangerous levels.
• Drawbacks of setting P-low above zero:
  • Will reduce tidal volume and thereby promote hypercapnia.
  • Reduction in driving pressure and expiratory flow velocity may reduce secretion clearance.

T-High

• T-high is the main driver of the release frequency.
  • Release frequency = 60/(T-High + T-Low)
  • Initially a frequency of 10-14 releases/minute is reasonable.
  • As patients wean off ventilator support, T-High will be increased and the release frequency will decrease.
• Ideally the T-High should be greater than nine times the T-Low, so that >90% of the overall time is spent at T-High.
  • If you're increasing T-Low above 0.7 seconds, consider increasing T-high as well.
• Problems with inadequate T-high:
  • Too many releases may lead to de-recruitment (by lowering the mean airway pressure).
  • APRV stops being lung-protective, starts resembling generic pressure-control ventilation.
  • Insufficient time for CO2 in blood to diffuse into gas within the airways (“diffusive” CO2 clearance).
• Problems with excessive T-high:
  • Too few releases may cause hypercapnia.
  • Release breaths are what provides mechanical support to assist the patient's work of breathing.  So if the T-High is too long, the vent isn't doing much to support the patient's work of breathing (this starts to resemble CPAP).

T-Low

• Trickiest setting, requires ongoing attention if the lung physiology changes.
• Among a patient who is exhaling passively, T-Low may be gauged by comparing the end-expiratory flow rate to the peak expiratory flow rate.   The usual target is an end-expiratory flow rate of 75% the peak expiratory flow rate.
• Problems with inadequate T-low (will tend to increase the end-expiratory flow rate >75%)
  • Release breath is too small, leading to ineffective ventilation.  Ventilator does little to support the work of breathing.
• Problems with excessive T-low (will decrease the end-expiratory flow rate <50-75%)
  • Release breath is too large, causing derecruitment (leading to hypoxemia and also hypercapnia).
  • Repetitive closing/opening of alveoli during release breath causes trauma to lungs (atelectotrauma).
  • End-expiratory flow rate <50% the peak flow rate suggests that APRV isn't functioning in a lung-protective manner.

#1:  initial settings

• P-High
  • Transitioning from volume-cycled ventilation:  set equal to plateau pressure.
  • Transitioning from pressure-cycled ventilation:  set equal to peak pressure.
  • Often start ~25 cm.
• P-Low
  • Set to zero.
• T-High
  • Set to 5 seconds.
• T-Low
  • Set to 0.5 seconds initially.
  • Adjust to achieve an end-expiratory flow equal to 75% of the peak expiratory flow rate (Jain 2016).
• FiO2
  • Start high, titrate down rapidly based on oxygen saturation.
• Automatic Tube Compensation (ATC) must be turned on.

#2:  adjustment based on oxygenation & ventilation

hypoxemia

• Hypoxemia generally indicates under-recruitment, although other possibilities exist (e.g. right-to-left shunt due to cor pulmonale, cardiogenic shock).
•  Interventions to improve recruitment:
  • Reduce T-Low by 0.05-0.1 second if end-expiratory flow rate is <75% of peak expiratory flow rate or if release breath is >8 cc/kg.
  • Increase P-high by 1-2 cm if <30 cm (or <35cm in morbid obesity).
  • Increase T-High by 0.5-1 second.
  • Last resort:  Increase P-Low by 1-2 cm.

hypercapnia

• Tolerate permissive hypercapnia down to pH of ~7.15 (or even lower if hemodynamically tolerated).  Before trying to reduce the PaCO2, think very carefully about whether you truly need to do this.
  • If the patient is tolerating a high PaCO2 and doing well clinically, the best thing is often to leave this alone.  The interventions listed below will tend to reduce the PaCO2, but may do this at the cost of being less lung-protective.
  • The best way to improve pH may be to manipulate the metabolic acid/base status (e.g. IV bicarbonate for non-anion-gap metabolic acidosis).
• Interventions
  • If patient isn't breathing spontaneously, wean sedation to encourage this (see “patient effort” below).
  • Increase P-High by 1-2 cm (up to ~30 cm or ~35cm in obesity).
  • T-High manipulation here is a double-edged sword:
    • Increasing T-high may improve recruitment and thereby improve CO2 clearance… if patient is de-recruited.
    • Reducing the T-high will increase the frequency of releases, thereby increasing the minute ventilation.
    • Judgement (regarding how well recruited the patient is) & trial-and-error may be needed.
  • Decrease P-low by 1-2 cm (if positive).
  • Last resort:  May increase T-Low by 0.05-0.1 second if end-expiratory flow rate is >>50% of the peak expiratory flow rate.  However this is generally undesirable because it may cause derecruitment.

hypocapnia

• Hypocapnia may be an indication that the patient is ready for weaning – more detail on this below.
• Interventions
  • Decrease P-High if able from an oxygenation standpoint.
  • Increase T-High.
  • Decrease T-low.

#3:  additional parameters to monitor

expiratory tidal volumes during pressure release

• Optimal volumes might be ~6-8 cc/kg (1).
• Undesirable to have volumes >>8 cc/kg IBW.
  • May be caused by lung overdistension –> decrease P-high.
  • May be caused by alveolar de-recruitment during release –> reduce T-low.

patient effort

• Ideally patient should breathe spontaneously over APRV, without dyspnea.
  • Severe ARDS:  ideally patient performs ~10-30% of total minute ventilation
  • Mild-moderate ARDS:  ideally patient performs ~30-60% of total minute ventilation
• If patient isn't performing enough effort:
  • Consider weaning sedation if over-sedated.
  • Consider weaning support level (e.g. reduce P-high and increasing T-high).
• If patient is performing too much effort:
  • Consider adding sedation if patient is anxious/agitated.
  • Consider increasing support level (e.g. increasing P-high and reducing T-high) if concern that patient may fatigue.

expiratory flow deceleration angle

• Perfect angle in a normal lung is ~45 degrees.
• Low angle (“steep” flow curve) may reflect de-recruitment or restriction (e.g. ARDS, abdominal compartment syndrome – example above on the right).
• High angle (“flat” flow curve) may reflect over-distension or obstruction (e.g. bronchospasm or secretions in large airways).
• Changes in angle may be more useful than the absolute value.  For example, sudden increase in angle could signal acute airway obstruction.

weaning off APRV

readiness for weaning

• Patient should be spontaneously breathing at a reasonable rate (e.g. 10-25 breaths/minute).  Goal is a patient who is calm but conscious.  This may be facilitated by:
  • Anxiolysis:  consider adding dexmedetomidine while weaning down propofol (OK to use combination of low doses of both simultaneously).
  • Pain control: consider adding ketamine infusion to allow weaning down opioids.
• T-High is fairly long (e.g. ~8-10 seconds).
• FiO2 50% or lower.
• P-Low of zero.
• Not significantly hypercapneic.
• Ready to assume more of the work of breathing (e.g. not in severe shock).

weaning: “drop and stretch”

• Decrease P-high in increments of 2 cm and prolong the T-high by increments of 0.5-2 seconds.
• May be done every 4-8 hours as tolerated.
• Monitor for desaturation, increased work of breathing, or tachypnea.

liberation from the ventilator

• Once you reach P-high of 20 cm and stretch the T-high to ~20-30 seconds, the patient is doing nearly all the work of breathing (you're very close to being on CPAP).
• At this point you can rapidly wean settings to CPAP at 15 cm (e.g. over ~6hr).  Make sure automatic tube compensation is on throughout this period (it should always be on during APRV).
• Patient may be extubated directly from CPAP of 10-15 cm.
  • Extubate to BiPAP or HFNC to maintain ongoing support.
  • Do not transition to standard nasal cannula for 24 hours.

Acknowledgments:  Thanks to Emily Parent RRT for help writing this guideline.

More on APRV here.

References

• Zhou Y et al.  Early application of airway pressure release ventilation may reduce the duration of mechanical ventilation in acute respiratory distress syndrome.  Intensive Care Medicine 2017.
• Daoud EG et al.  Airway pressure release ventilation:  What do we know?  Respiratory Care 2012.
• Habashi NM.  Other approaches to open-lung ventilation:  airway pressure release ventilation.  Critical Care Medicine 2005.

Notes

1. Some authors have previously suggested that very large tidal volumes might be lung-injurious even in APRV mode (Miller 2017).  Zhou 2017 achieved average lung volumes of ~7.4 +/- 1 ml/kg, suggesting that the achievement of lung-protective tidal volumes using APRV is quite feasible.  In fact, within this study there was no significant difference in tidal volume between the APRV and the low-tidal-volume group.