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Restoring Alveolar Structure and Function in Parenchymal Lung Injury

Positive pressure ventilation (PPV) can be used to restore alveolar patency and function by physically forcing the alveolus open during inspiration (recruitment/dilation) and then preventing its recollapse (derecruitment) during expiration. Ideally, V/Q matching should be better and compliance improved.[5]
Proper setting of inspiratory and expiratory pressure and volume during PPV is critical to achieving these goals while at the same time avoiding ventilator-induced lung injury (VILI). Conceptually, the inspiratory pressure and volume must be of sufficient magnitude that alveolar units are opened but at the same time overdistention of alveolar units at end inspiration is avoided (the "volutrauma" component of VILI). The subsequent expiratory pressure and volume must also be of sufficient magnitude that significant derecruitment is avoided. This not only maintains V/Q relationships but also prevents a shear stress injury resulting from repetitive opening/closing of alveolar units (the "atelectrauma" component of VILI).[5,6]
In considering these goals, it is critical to remember that parenchymal lung injury is heterogeneous with regions of very diseased lung interspersed with (and often adjacent to) healthier units.[7] This means that ideal regional tidal volumes and expiratory pressures in one unit may be excessive in an adjacent unit and suboptimal in a third unit. Indeed, the art and science of establishing ventilator settings is creating a pattern that helps as many regions as possible, while minimizing injury in the remainder.
Recruitment Maneuvers: The Concept
Recruitment maneuvers (RMs) are positive pressure inflations above the set tidal volume during PPV with the goal of achieving maximal physiologic stretch in as many lung units as possible.[5,8-10] The concept behind RMs is that all alveolar units capable of being recruited are indeed opened. This inflation is often held for up to 1 minute for 2 reasons: (1) some alveolar units may require this amount of time to open; and (2) the surfactant monolayer has time to construct itself in these freshly opened alveoli. On the subsequent deflation, these recruited alveoli will maintain patency even though the applied transpulmonary pressure is lower than that required to initially open them. This phenomenon produces hysteresis and a shift leftward in the static PV relationship of the lung. In other words, closing pressures are lower than opening pressures and PEEP required to prevent derecruitment is less than PEEP required for recruitment.[8-10]
These concepts underscore the importance of linking RMs to establishing PEEP levels. RMs are the maneuver to initially open the alveoli; PEEP is the maneuver to maintain that patency. Discussion of one without consideration of the other makes little physiologic or clinical sense.
Recruitment Maneuvers: Techniques
A number of approaches to RMs have been reported in the literature.[10] Probably the most common is a sustained inflation to the conceptual maximal stretching pressure of the lung (eg, 30 to 45 cm H2O) for up to 1 minute. Alternative approaches involve elevations in PEEP that produce end inspiratory plateau pressures of 30 to 45 cm H2O for 1 or 2 minutes, and addition of 1 to 2 sigh breaths/minute with volumes/pressures approaching 30 to 45 cm H2O. Recruitment may also be facilitated by the prone position (cardiac weight removed from dorsal units) and spontaneous breaths (more uniform diaphragm movement).
The most important complication of RMs is a reduction in cardiac output/blood pressure as a consequence of the elevated intrathoracic pressures. Significant complications, however, seem uncommon, as only 6/408 RMs have had to be aborted in the ongoing Canadian high-frequency ventilation trial.
As noted above, the subsequent PEEP setting is critical. An inadequate PEEP setting will result in rapid loss of recruitment and return to baseline in gas exchange and mechanics. Should this occur, most would argue for a repeat RM with subsequent application of a higher PEEP. Indeed, this is the rationale behind one approach to PPV-PEEP settings: apply repeated RMs with different PEEP levels to establish the lowest PEEP capable of maintaining either acceptable gas exchange or lung compliance.[8-10]
Recruitment Maneuvers: Effectiveness
There are 2 fundamental ways to assess the effectiveness of an RM strategy: physiologic and clinical outcome. Unfortunately, considerably more physiologic data exist than outcome data.
The two most common physiologic assessments are gas exchange and lung mechanics. Numerous animal and human studies have confirmed that both of these parameters are often improved after RMs. Reasons for not improving these physiologic variables include: the very tenacious exudate of some pneumonias and other primary forms of ARDS, late-stage lung injury with considerable fibrosis, lungs that have already been appropriately recruited, and a suboptimal RM. The duration of effect depends heavily on the appropriateness of the subsequent PEEP setting as noted above.
Data on whether optimally recruited lungs as described above translate into better outcomes (eg, fewer ventilator days, better survival) are few. Animal studies examining the role of PEEP in parenchymal lung injury have universally shown that some PEEP is better than none in mitigating VILI.[6,9] However, these studies have not compared specific PEEP strategies that have been focused on gas exchange or lung mechanics to see if there is a superior approach in terms of survival. In the 2 human studies on protective ventilator strategies that limit end inspiratory stretch that show a survival benefit, one[11] used RMs and set PEEP according to a static PV curve to minimize derecruitment, and one conducted by the National Institutes of Health (NIH) ARDS Network[12] did not use RMs and set PEEP according to gas exchange and FiO2 needs. This gas exchange approach was further studied in a subsequent (as yet unpublished) NIH ARDS Network trial by comparing it to a more aggressive PEEP strategy (but no RMs). In this study, both PEEP strategies had comparable mortality. Finally, in an interesting analytical approach, Amato pooled the results of 4 clinical trials comparing protective lung ventilation (PLV) strategies to conventional mechanical ventilation. By controlling for ventilator settings and disease acuity, he was able to show that if the PLV strategy supplied more than 10 cm H2O of increased PEEP vs conventional, mortality was improved.
From all of the above, it would seem that, although theoretically attractive, RMs and PEEP optimized to gas exchange or mechanical function have really not been shown as yet to have a significant outcome benefit. Indeed, underscoring the notion that gas exchange improvement and outcome may NOT necessarily be linked is the observation from the original ARDS Network trial[12] that the high tidal volume group had a better PaO2/FiO2 but a worse outcome. Put another way, the price one pays for recruitment in one region may be counteracted by worse VILI in another region.