L. Blanch, A. Villagra, and J. Lopez-Aguilar
Introduction
Mechanical ventilation is a supportive life-saving therapy in patients with acute lung injury (ALI) although it can inflict further lung damage to the lungs, indistin- guishable from the pulmonary alterations attributable to ALI, if it is not appropri- ately applied. Several studies also suggest that mechanical ventilation can cause inflammation of the lung capable of damaging distal organs and this response can be magnified if artificial ventilation results in high intrathoracic pressure, high lung volume leading to overdistension, and cyclic alveolar recruitment and dere- cruitment. Two randomized controlled studies focused on studying the effects of mechanical ventilation in patients with acute respiratory distress syndrome (ARDS) have shown that the use of high tidal volumes was associated with an increase in mortality [1, 2]. In both studies, the application of a tidal volume of 6 ml/kg and a moderate or high positive end-expiratory pressure (PEEP) level increased survival. Recruitment maneuvers have been proposed as adjuncts to lung protective ventilatory strategies to forestall atelectasis when small tidal vol- umes and low/moderate PEEP are used in ARDS. Recruitment maneuvers are based on the assumption that almost complete alveolar recruitment could be feasible during the early course of ARDS. In this context, recrutiment maneuvers could potentially recruit the lung parenchyma for gas exchange, and to prevent further lung damage to the re-aerated part of the lung.
Experimental Research on Recruitment Maneuvers
Recrutiment maneuvers restored oxygenation in surfactant-depletion models of ALI. Rismensberger et al. [3] have shown in saline-lavaged rabbit lungs that a single recrutiment maneuver resulted in better oxygenation without augmenting histologic injury at PEEP less than the lower inflection point (LIP) of the pres- sure/volume (P/V) curve, compared with the group with PEEP set above the LIP of the P/V curve without a recrutiment maneuver. Furthermore, these authors, using the same model, showed that a single sustained inflation to 30 cmH2O boosted the ventilatory cycle onto the deflation limb of the P/V curve. This strategy resulted in better oxygenation, static lung compliance and lung volumes. The different com- binations of tidal volume and PEEP, and their effects on lung recruitment have an
influence on the effects of recrutiment maneuvers on lung function. In saline-lav- aged rabbits, Bond et al. [3] showed that oxygenation improved after recrutiment maneuvers during volume controlled ventilation only during low tidal ventilation (7 ml/kg) and zero end-expiratory pressure (ZEEP) and, Van der Kloot et al. [5]
demonstrated no effect of recrutiment maneuvers when experimental animals with ALI were ventilated with a PEEP level higher than the LIP of the static P/V curve of the respiratory system. Interestingly, Takeuchi et al. [6] demonstrated that PEEP set at 2 cmH2O above the LIP of the P/V curve of the respiratory system was the most effective in maintaining gas exchange and minimizing lung injury compared to a PEEP level set at the maximum curvature of the deflation P/V curve. Whether recrutiment maneuvers are necessary to prevent alveolar collapse over time when optimal PEEP is used remains a matter of debate. Fujino et al. [7]
have shown that repetitive high-pressure recrutiment maneuvers were required to maximally recruit lung in a sheep model of ARDS and, the combination of other adjuncts to mechanical ventilation can be necessary to obtain a maximal response.
Cakar et al. [8] showed in an oleic acid model of ALI that recrutiment maneuvers had a greater effect in the prone than supine position and the oxygenation benefit of the recrutiment maneuvers was sustained in the prone position at lower PEEP compared with supine position.
Recruitment Maneuvers during Anesthesia
The rationale for using recrutiment maneuvers during anesthesia is to forestall atelectasis in dependent lung areas. During anesthesia, atelectatic lung may be re-expanded by a vital capacity maneuver, or by applying an inflation pressure of 40 cmH2O to the lungs. Re-expansion may have a sustained effect for at least 40 minutes if 40% oxygen in nitrogen is used. However, if pure oxygen is used after re-expansion, lung collapse reappears within a few minutes [9]. This time course of atelectasis suggests that gas composition and resorption phenomena play an important role in the recurrence of collapse in previously re-expanded atelectatic lung tissue.
Recruitment Maneuvers in Patients with ARDS
Lapinsky et al. [10] reported beneficial effects on oxygenation after 40 seconds of continuous PEEP of 40 cmH2O in 14 patients with ARDS. The recrutiment maneu- ver was considered safe although a sustained effect was achieved only if PEEP was increased after the recrutiment maneuver. Using a different technique to facilitate recruitment, Pelosi et al. [11] tested the effects of periodic increases in peak airway pressure (plateau pressure of 45 cmH2O) in patients with ARDS ventilated with PEEP (14.5 cmH2O) and observed significant improvements in oxygenation, in- trapulmonary shunt and lung mechanics. Similarly, Foti et al. [12] compared the effect of volume controlled ventilation at low PEEP with superimposition of peri- odic volume recruitment maneuvers with volume controlled ventilation at high PEEP in patients with ARDS. Sighs at low PEEP effectively improved oxygenation
and alveolar recruitment, but were relatively less effective than a continuous high PEEP level, thus suggesting that high PEEP better stabilizes alveoli, preventing the loss in lung volume.
Two observational studies have shown a modest and variable effect of the recrutiment maneuver on oxygenation when the ARDS patient is ventilated with a high level of PEEP. Richard et al. [13] demonstrated in patients with ARDS that increasing PEEP or performing a recrutiment maneuver appeared to be the two strategies to counteract low tidal volume and moderate PEEP-induced de-recruit- ment. When recrutiment maneuvers were performed in patients already ventilated with high PEEP, oxygenation modestly improved and had minimal effects on requirements for oxygenation support. Villagra et al. [14] studied the effect of a recrutiment maneuver that consisted of two minutes of pressure control ventilation (mean peak airway pressure 47 ± 4.5 cmH2O) and high end-expiratory pressure (mean PEEP 30 ± 4.9 cmH2O) in ARDS patients receiving a lung protective ap- proach strategy (tidal volume < 8 ml/kg and PEEP 3–4 cmH2O higher than the LIP on the P/V curve). Interestingly, recrutiment maneuvers did not improve oxygena- tion in the majority of patients regardless of the stage of ARDS, and venous admixture increased during recrutiment maneuver in non-responders. This dele- terious effect suggests that the recrutiment maneuver increased lung volume by overdistending the more compliant already-opened and aerated alveolar units, rather than recruiting collapsed units. Alveolar overdistension would favor capil- lary collapse in the healthy areas and diversion of blood flow into the collapsed areas. Furthermore, a negative correlation was found between recruited lung volume induced by PEEP before recrutiment maneuver and recrutiment maneuver -induced changes in oxygenation. These observational studies suggest that recru- timent maneuvers are not effective when the lungs have been near-optimally recruited by PEEP and tidal volume [13, 14].
Crotti et al. [15] have shown that the dynamics of recruitment and derecruitment in human ARDS of pulmonary origin are similar to those observed in oleic acid models of ALI. However, the potential for recruitment in human ARDS compared to highly recruitable animal models is rather limited. A recent study [16] found that recrutiment maneuvers transiently improved oxygenation only in patients with early ARDS who did not have impairment of chest wall mechanics and with a large potential for recruitment, as was indicated by low values of lung elastance. Clearly, further studies are needed to identify with simple and reliable technology those patients who will benefit from a recrutiment maneuver before recrutiment maneu- vers can be recommended as a routine practice to improve outcome in ARDS.
Recrutiment Maneuvers after Ventilator Disconnections and during Spontaneous Breathing
Patients with ARDS need periodic suctioning of secretions due to their inability to spontaneously clear airway secretions. Lu et al. [17] studied the physiologic effects of endotracheal suctioning in anesthetized sheep and found that endotracheal suctioning was associated with atelectasis, brochoconstriction, and a decrease in arterial oxygen saturation. The combination of hyperoxygenation before the re-
crutiment maneuver and a recrutiment maneuver after endotracheal suctioning entirely reversed the suctioning-induced increase in lung tissue resistance and atelectasis.
Diaphragmatic contraction during partial ventilatory support augments distri- bution of ventilation to dependent, poorly aerated but perfused lung regions.
Several recent reports have suggested that the addition of periodic hyperinflations during continuous positive airway pressure (CPAP) [18], the application of sighs during pressure support ventilation (19), and the use of airway pressure release ventilation improved lung function (20). The beneficial effects of periodic sighs and spontaneous breathing seems to favor the opening of previously collapsed lung and prevent lung collapse associated with low tidal volume and low PEEP levels used in patients who are recovering from an acute episode of respiratory failure.
Conclusion
Recrutiment maneuvers superimposed on mechanical ventilation have the poten- tial to recruit atelectatic lungs in the course of general anesthesia although the physiologic benefits are less evident in patients with ARDS. Patient selection, long term benefits, and safety of recrutiment maneuvers also remain to be determined.
Many different methods are recommended to achieve maximal lung recruitment but no evidence exits about which should be used. Finally, evidence for the bene- ficial effect of recrutiment maneuvers in ARDS is from non-randomized trials or from observational studies and, evidence from large, prospective and randomized clinical studies is still lacking. Therefore, the use of recrutiment maneuvers in ARDS is controversial even as rescue therapy to improve oxygenation in the most severe forms of human ARDS.
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