Airway pressure release ventilation: An alternative mode of mechanical ventilation in acute respiratory distress syndrome
ABSTRACTAcute respiratory distress syndrome (ARDS) results in collapse of alveoli and therefore poor oxygenation. In this article, we review airway pressure release ventilation (APRV), a mode of mechanical ventilation that may be useful when, owing to ARDS, areas of the lungs are collapsed and need to be reinflated (“recruited”), avoiding cyclic alveolar collapse and reopening.
KEY POINTS
- The advantages and disadvantages of APRV are related to its two components: high mean airway pressure and spontaneous ventilation.
- Several studies show APRV to have physiologic benefits and to improve some measures of clinical outcome, such as oxygenation, use of sedation, hemodynamics, and respiratory mechanics.
- No study has reported that fewer patients die if they receive APRV compared with conventional protective ventilation.
- APRV is a promising mode, and further research is needed to strengthen support for its more widespread use.
In the early stages of acute respiratory distress syndrome (ARDS), multiple areas of the lung collapse, most often in the dependent regions. A factor involved in this process is the loss of functional surfactant, creating a condition in which alveolar units are unstable and prone to collapse due to unopposed surface tension. This situation, similar to that in premature infants, results in a reduced volume of aerated lung, intrapulmonary shunting, and, therefore, poor oxygenation.
The treatment of this alveolar collapse is lung reinflation (or “recruitment,” a term first used by Lachmann).1 Gattinoni et al2 showed that the percentage of recruitable lung could range from a negligible fraction to 50% or more.
There are various means of reopening injured lungs and keeping them open. The choice of recruitment maneuver is based on the individual patient and the ventilatory mode.3
In this article, we review airway pressure release ventilation (APRV), a mode of mechanical ventilation that may be useful in situations in which, due to ARDS, the lungs need to be recruited and held open. APRV was developed as a lung-protective mode, allowing recruitment while minimizing ventilator-induced lung injury.
BASIC PRINCIPLES OF PROTECTIVE VENTILATION
This curve has two inflection points between which its slope is steep, indicating greater compliance or elasticity. Below the lower inflection point, the alveoli may collapse; above the upper inflection point, the lung loses its elastic properties and the alveoli are overdistended. To protect the lungs, the challenge in mechanical ventilation is to keep the lungs between these two points throughout the respiratory cycle.
Avoiding lung collapse by using PEEP
During mechanical ventilation, the pressure in the lungs is lowest, and thus the alveoli are most prone to collapse, at the end of expiration.
We want to prevent the alveoli from collapsing with each expiration and reopening with each inspiration, as this cycle of opening and closing damages them (causing atelectrauma, ie, cyclical atelectasis).4 Preventing it prevents the release of inflammatory mediators and the perpetuation of lung injury (biotrauma).5
The solution is to apply positive end-expiratory pressure (PEEP), taking into account the value of the lower inflection point when setting the PEEP level.
Villar et al6 compared outcomes in an intervention group that received a PEEP level 2 cm H2O above the lower inflection point plus low tidal volumes, and in a control group that received higher tidal volumes and low PEEP (5 cm H2O). The study was stopped early, after significantly more patients had died in the control group than in the intervention group (53% vs 32%, P = .04).
Avoiding overdistention by keeping the tidal volume low
Tidal volumes that exceed the upper inflection point overstretch the lung and induce volutrauma, which can manifest as pneumothorax or pneumomediastinum, or both—the lungs rupture like a balloon. Also, overdistention produces liberation of inflammatory mediators in the blood (biotrauma). High tidal volumes should therefore be avoided or limited as much as possible.
The ARDS Network,7 in a multicenter, randomized, controlled trial, showed that fewer patients die if they receive mechanical ventilation with low tidal volumes rather than higher, “conventional” tidal volumes. Patients were randomized to receive either a tidal volume of 6 mL/kg and a plateau pressure lower than 30 cm H2O or a tidal volume of 12 mL/kg and a plateau pressure lower than 50 cm H2O. They were followed for 180 days or until discharged home, breathing without assistance. A total of 861 patients were enrolled. The mortality rate was significantly lower in the low tidal volume group than in the group with conventional tidal volumes, 31% vs 40%.
Lower tidal volumes were also associated with faster attenuation of the inflammatory response.8
Amato et al9 randomized 58 patients to receive mechanical ventilation with tidal volumes of either 6 mL/kg or 12 mL/kg. The PEEP level was maintained above the lower inflection point. At 28 days, 62% of the patients in the intervention group were still alive, compared with only 29% in the control group. However, many concerns were expressed over the high mortality rate in the control group.
Based on these studies, the use of low tidal volumes with appropriate levels of PEEP to ensure lung recruitment is the current standard of care in mechanical ventilation of patients with ARDS.10
