Aprv

Other approaches to open-lung ventilation: Airway pressure release ventilation
Nader M. Habashi, MD, FACP, FCCP

Objective: To review the use of airway pressure release ventilation (APRV) in the treatment of acute lung injury/acute respiratory distress syndrome. Data Source: Published animal studies, human studies, and review articles of APRV. Data Summary: APRV has been successfully used inneonatal, pediatric, and adult forms of respiratory failure. Experimental and clinical use of APRV has been shown to facilitate spontaneous breathing and is associated with decreased peak airway pressures and improved oxygenation/ventilation when compared with conventional ventilation. Additionally, improvements in hemodynamic parameters, splanchnic perfusion, and reduced sedation/ neuromuscularblocker requirements have been reported.

Conclusion: APRV may offer potential clinical advantages for ventilator management of acute lung injury/acute respiratory distress syndrome and may be considered as an alternative “open lung approach” to mechanical ventilation. Whether APRV reduces mortality or increases ventilator-free days compared with a conventional volume-cycled “lung protective”strategy will require future randomized, controlled trials. (Crit Care Med 2005; 33[Suppl.]:S228 –S240) KEY WORDS: airway pressure release ventilation; airway pressure release ventilation; spontaneous breathing; lung protective strategies; acute lung injury; acute respiratory distress syndrome

irway pressure release ventilation (APRV) was initially described by Stock and Downs (1, 2) as continuouspositive airway pressure (CPAP) with an intermittent pressure release phase. Conceptually, APRV applies a continuous airway pressure (Phigh) identical to CPAP to maintain adequate lung volume and promote alveolar recruitment. However, APRV adds a time-cycled release phase to a lower set pressure (Plow). In addition, spontaneous breathing can be integrated and is independent of the ventilator cycle(Fig. 1). CPAP breathing mimics the gas distribution of spontaneous breaths as opposed to mechanically controlled, assisted, or supported breaths, which produce less physiological distribution (3– 6). Mechanical breaths shift ventilation to nondependent lung regions as the passive respiratory system accommodates the displacement of gas in to the lungs. However, spontaneous breathing during APRVresults in a more dependent gas distribution when the active respiratory system draws gas into the lung as pressure changes and

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From the Multi-trauma ICU, R Adams Cowley Shock Trauma Center, Baltimore, MD. Copyright © 2005 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins DOI: 10.1097/01.CCM.0000155920.11893.37

flow follow a similar time course (7–9). As a result,by allowing patients to spontaneously breathe during APRV, dependent lung regions may be preferentially recruited without the need to raise applied airway pressure. APRV has been used in neonatal, pediatric, and adult forms of respiratory failure (1– 4, 6, 10 –22). Clinical studies using APRV are summarized in Table 1 (1– 4, 12, 18, 23–28). In patients with decreased functional residual capacity(FRC), elastic work of breathing (WOB) is effectively reduced with the application of CPAP. As FRC is restored, inspiration begins from a more favorable pressure/volume relationship, facilitating spontaneous ventilation and improving oxygenation (29). However, in acute lung injury/acute respiratory distress syndrome (ALI/ ARDS), the surface area available for gas exchange is significantly reduced.Despite optimal lung volume, CPAP mandates that unaided spontaneous breathing manage the entire metabolic load or CO2 production. However, CPAP alone may be inadequate to accomplish necessary CO2 removal without producing excessive WOB. In contrast to CPAP, APRV interrupts airway pressure briefly to supplement spontaneous minute ventilation. During APRV, ventilation is augmented

by releasing...