Airway Pressure Release Ventilation (APRV) is a recruitment mode that provides a high mean airway pressure (MAP) by setting a prolonged inspiratory time, an adequate inspiratory pressure, and a short expiratory time. If adequately set, the mode provides a good recruitment and maintains adequate lung protection. The mode has been used initially to rescue patients with severe ARDS who fail on conventional modes of ventilation. However, If started late, it may be difficult to recruit the lung, and ventilation is often a problem. This is why multiple studies have shown conflicting results. This mode, if used, should be started early in the management of ARDS cases, as shown in a study published in the Intensive Care Medicine Journal by Zhou et al [1]. The study included 138 patients who were randomized to APRV (71 patients) or low tidal ventilation (67 patients). Despite some limitations in the study, it showed that there is no statistical difference in ICU mortality, but patients on APRV had more ventilator-free days, a higher rate of successful extubation, and a lower rate of tracheostomy. More studies are certainly needed to verify its benefits.
APRV SETTINGS
When starting APRV, the settings need to be maximized according to protective lung strategy. There are four main settings on APRV: P High, P Low, T High, and T Low. APRV is a pressure control mode. Each individual breath is time-triggered, pressure-targeted, and time-cycled. Inspiration time (and thus cycling) is determined by setting the T-High. Expiration time is determined by setting the T Low. P High and T High levels help in lung recruitment and improve oxygenation. P Low and T low help in maintaining lung recruitment in expiratory phase. The P Low is usually set at 0 cm H2O and lung recruitment is ensured by setting the end-expiratory flow rate at 50-75% of the peak expiratory flow (PEF) rate. When the flow is cut by 50-75% of PEF, the lung will not completely empty and and auto-PEEP will be build up, similar to the dynamic hyperinflation syndrome in asthmatic patients. Lastly, spontaneous breaths must be preserved with APRV as they participate in a good percentage of minute ventilation.
P High
Selecting the best P High is vital in APRV. P High increases the mean airway pressure on APRV, helps to recruit the lung, and improves oxygenation. It also helps in ventilation as more alveoli are available to participate in gas exchange when the lung is recruited.
If the patient is already on the conventional mode of ventilation, P High is set at 5 cm H2O higher than the mean airway pressure on the conventional mode of ventilation. For example, if the MAP was 19 on CMV, the P High should be set at 24 cm H2O. Alternatively, you can set the P High at the plateau pressure in volume-targeted modes or the pressure control in pressure-targeted modes.
P High can be gradually increased to improve oxygenation but should not exceed 30 cm H2O (35 cm H2O in severe obesity) to stay within the limitation of protective lung strategy. A larger P High causes large release volume, overdistention of the lung, excess driving pressure, and possible impairment of hemodynamics. On the other hand, an inadequate P High causes derecruitment of the lung, hypoxemia, low minute ventilation, and hypercapnia.
P Low
In most protocols, P Low is set at 0 cm H2O. However, some prefer to set it at value of no more than 5 cm H2O. The advantages of P Low above zero are to provide more lung-protection by avoiding atelectasis and wide fluctuations in airway pressure in the expiratory phase. In this way, P Low helps to improve oxygenation without increasing the P High to dangerous levels. On the other hand, P Low above zero may reduce the tidal volume and thereby promote hypercapnia and elevation of CO2. It may also reduce driving pressure and expiratory flow velocity which could impair secretion clearances.
T High
Inspiratory time is determined by setting the T High. The longer the T High, the higher the MAP and thus the better the lung recruitment and oxygenation. The ratio between T High and T Low determines the release frequency per minute which affects ventilation and CO2 removal. The release frequency is calculated with this formula:
Release frequency = 60 /(THigh + TLaw)
T High is usually set at 4 seconds and gradually increased to improve oxygenation. It is important to pay attention to the number of releases, as a higher T High will decrease the release frequency per minute and thus decrease ventilation and CO2 removal. Depending on the T High and T low, the release frequency may vary between 8-14 releases/minute.
A longer T High may improve ventilation as more recruited alveoli participate in gas exchange. However, if the releases are decreased significantly, ventilation and CO2 removal is decreased. In many situations, the outcome must be assessed. Eventually, it may be necessary to decrease the T High to improve ventilation at the expense of oxygenation.
T Low
T low is set to target 50-75% of Peak Expiratory Flow (PEF) to ensure adequate auto-PEEP. Auto-PEEP is responsible for keeping the lung recruited during expiration and preventing collapse. Setting T Low at >75% of PEF may cause small releases and ineffective ventilation. On the other hand, setting T Low at <50% of PFE may cause large releases, loss of auto-PEEP, and lung decruitment.
T Low is the most critical setting on APRV as it needs to be adjusted according to changes in lung physiology. T Low may be optimal for a certain lung compliance. However, if the lung compliance is worsened, the lung will be emptied faster as the Time Constant is now shorter. Thus, the same setting of the T Low will cut the expiratory flow at a lower value and cause derecruitment of the lung. On the other hand, improvement in lung compliance causes a longer Time Constant and will cut the expiratory flow at a higher value, causing worsening in the auto-PEEP and possible ventilation.
The ratio between T High and T Low determines release frequency per minute and affects ventilation and CO2 removal, as mentioned above.
Spontaneous breathing
Spontaneous breathing plays a major role in maintaining adequate ventilation in APRV. Most of the reasons for failing APRV are due to inadequate ventilation and CO2 removal. Contrary to the belief that patients may not tolerate APRV and should be kept heavily sedated, those patients can do very well and maintain adequate spontaneous ventilation on APRV with the appropriate level of sedation. In general, spontaneous breathing should provide ~10-30% of minute ventilation in severe ARDS and ~30-60% of minute ventilation in mid to moderate ARDS without dyspnea. In addition, spontaneous breathing improves alveolar recruitment to the dorsal caudal regions of the lungs.
ADJUSTMENTS After the initial settings of APRV, oxygenation and ventilation can be assessed through arterial blood gases and adjustments can be made as necessary.
What If the patient has Hypercapnia?
First, the level of permissive hypercapnia must be determined. Most patients can tolerate permissive hypercapnia with a pH of ~7.15 if they are hemodynamically stable. Adequate spontaneous breathing must also be ensured. It may be necessary to decrease sedation to encourage spontaneous breathing. Adjustments on APRV to improve ventilation include the following:
Increase P High by 1-2 cm H2O (up to ~30 cmH2O or ~35 in obese patients).
Increase T High by 0.5-1 second to improve lung recruitment and thereby improve CO2 clearance (if the patient is derecruited). On the other hand, reducing T High increases the frequency of releases, thereby increasing the minute ventilation and improving CO2 clearance.
Decrease P Low by 1.2 cm H2O (if positive) to increase the release volumes.
Last resort: may increase T Low by 0.05-0.1 second if end-expiratory flow rate is >50% of the PEF to allow more time for the release volume..
What if the patient is still hypoxemic?
Remember that APRV improves oxygenation by providing a higher mean airway pressure (area under the pressure-time curve). Therefore, if the patient is still hypoxemic, the following adjustments can be made:
Reduce T Low by 0.05-0.1 second if end-expiratory flow rate is <50% of the peak expiratory flow rate or if the release volume is >8 mL/kg of ideal body weight. This will provide a higher auto-PEEP and thus better oxygenation.
Increase P High by 1-2 cm H2O but you need to remain within the limitation of protective lung strategy with a P High ≤30 cm H2O (or ≤35 cm H2O in morbid obese patients).
Increase T High by 0.5-1 second.
Last resort: Increase P Low by 1.2 cm H2O.
What if the patient has hypocapnia?
If the patient is hypocapnic, you need to decrease minute ventilation by decreasing the release volume frequency or the volume of the releases. This can be done by the following interventions:
Decrease P High if able from oxygenation standpoint.
Increase T High.
Decrease T Low.
REFERENCES
Zhou Y, Jin X, Lv Y, Wang P, Yang Y, Liang G, Wang B, Kang Y. Early application of airway pressure release ventilation may reduce the duration of mechanical ventilation in acute respiratory distress syndrome. Intensive Care Med. 2017 Nov;43(11):1648-1659. doi: 10.1007/s00134-017-4912-z. Epub 2017 Sep 22. PMID: 28936695; PMCID: PMC5633625.
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