◆ ‘Recruitment manoeuvre’ stands for a process of intentional transient increase of transpulmonary pressure above values used during regular mechanical ventilation, whose main objective is to open unstable distal airways and/or airless alveoli.
◆ The effectiveness of recruitment manoeuvres is probably related to the nature, phase, and/or extent of the lung injury, as well as the recruitment technique itself.
◆ Recruitment manoeuvres can be performed in several different ways. Today, the most relevant recruitment manoeuvres in terms of clinical applicability in the intensive care unit (ICU) are sustained inflation manoeuvres, high pressure-controlled ventilation, incremental PEEP, and intermittent sighs.
◆ Anaesthesiologists commonly perform bag squeezing as a recruitment manoeuvre, as it is simple to perform during surgery. This procedure closely resembles the sustained inflation technique.
◆ Ventilation with variable tidal volumes, may be a simple and interesting alternative for lung recruitment in the ICU, but clinical evidence has to be demonstrated yet.
Protective mechanical ventilation (MV) represents one of the most important interventions in patients suffering from the acute respiratory distress syndrome (ARDS). Albeit MV is not curative, it is useful for reducing the work of breathing and maintaining an adequate gas exchange.
Lung recruitment manoeuvres (RMs) have been suggested as a means of homogenizing the lung structure and distribution of the mechanical stress across the lungs. Such effects of RMs can be achieved provided enough pressure is applied for sufficient time at the airways, and maintained if adequate levels of positive end expiratory pressure (PEEP) are used. When RMs effectively open atelectatic tissue, shear stress and cyclic collapse/reopening are reduced. Also, if an increased end-expiratory lung volume is achieved after RMs, the applied tidal volume (VT) will be distributed across a larger lung surface, resulting in reduced regional dynamic stress and strain.
Results from clinical trials have suggested that RMs are useful to revert life-threatening hypoxaemia and derecruitment resulting from suctioning procedures of the airways, as well as disconnection from the mechanical ventilator. Nevertheless, no study has shown that RMs reduce mortality or the duration of mechanical ventilation in patients with ARDS. In contrast, there is increasing clinical evidence that low VT in combination with RMs and adequate levels of PEEP are useful for reducing the incidence of post-operative pulmonary complications in patients undergoing open abdominal surgery. However, those beneficial effects cannot be ascribed solely to RMs themselves.
This chapter aims at:
◆ Defining RMs and describing host-dependent factors that may influence their performance.
◆ Describing established RMs and their functional and biological impact.
◆ Presenting experimental data on new promising forms of RMs that may become relevant in clinical practice.
Definition of recruitment manoeuvre
RM stands for a process of intentional transient increase of transpulmonary pressure (PL) above values used during regular mechanical ventilation, whose main objective is to open unstable distal airways and/or airless alveoli. RM is also often termed alveolar recruitment manoeuvre, since opening of collapsed alveolar units is believed to represent the major effect of a RM. It must be kept in mind, however, that the existence of alveoli closure, and obviously also its potential reversal by RMs, in ARDS has been questioned. It has been shown in dogs with experimental ARDS induced by intravenous administration of oleic acid, that the alveoli of dependent zones were not collapsed, but rather filled with exudate. Whereas the end expiratory lung volume did not change after RMs and higher PEEP, the lung function improved importantly. Thus, it is possible that RMs exert part of their effects through redistribution of intra-alveolar fluid across lung units.
While non-supported spontaneous breathing, patient position, and higher levels of PEEP alone can increase the regional PL and result in opening of lung units, they cannot be termed RMs in a strict sense.
Factors influencing the efficacy of recruitment manoeuvres
The lung response to RMs is determined mainly by host-dependent factors—cause and severity of lung injury and position of the lungs with respect to the gravity gradient.
Origin and severity of lung injury
ARDS is the result of different insults to the lungs. There are two major types of ARDS, namely primary (also known as direct or pulmonary) and secondary (also known as indirect or extra-pulmonary) ARDS. The structure that is primarily damaged in pulmonary ARDS is the alveolar epithelium, and its typical hallmark is filling of the intra-alveolar space by oedema, fibrin, and neutrophilic aggregates. In extrapulmonary ARDS, distal organs release pro-inflammatory mediators into the bloodstream. Those mediators initiate the damage of the pulmonary capillary endothelium, leading to microvessel congestion and interstitial oedema with comparatively less flooding of intra-alveolar space.
There is a considerable body of evidence showing that the type of lung injury in ARDS importantly influences the performance of RMs. In an investigation on three models of ARDS in dogs, namely saline lung lavage (surfactant depletion), intravenous oleic acid administration and pneumonia, RMs were particularly effective in improving oxygenation and increasing the end-expiratory lung volume after surfactant depletion. In contrast, animals with pneumonia showed almost no improvement in lung function following RMs and high PEEP. Riva et al. compared the effects of a RM in models of pulmonary and extrapulmonary ARDS induced by intratracheal and intraperitoneal instillation of Escherichia coli lipopolysaccharide with similar transpulmonary pressures. They found that the RM was more effective for opening collapsed alveoli in extrapulmonary compared with pulmonary ARDS, improving lung mechanics and oxygenation with limited damage to alveolar epithelium. In contrast, Grasso et al. reported that RMs combined with high PEEP levels can lead to hyperinflation due to inhomogeneities in the lung parenchyma, independent from the origin of the injury insult (pulmonary or extrapulmonary).
In patients suffering from ARDS, Gattinoni and colleagues found that RMs are more efficient in severe, compared with non-severe ARDS. Using whole lung computed tomography, those authors showed that the reduction in non-aerated and poorly aerated lung tissue was more pronounced in those patients showing the most severe gas exchange and lung tissue abnormalities.
Prone positioning may not only contribute to the success of RMs, but can be considered itself to be a RM. During prone position, the transpulmonary pressure in dorsal lung areas increases, opening alveoli and improving gas exchange. Some authors reported that in healthy, as well as lung-injured animals, mechanical ventilation leading to lung over distension and cyclic collapse/reopening is associated with less extensive histological change in dorsal regions when animals are in prone, as compared with supine position. It is still unclear if body position affects the distribution of lung injury, but the development of ventilator-induced lung injury (VILI) due to excessively high VT seems to be delayed during prone compared with supine position.
In 2013, Guerin et al. demonstrated in a multicentre trial that the early application of prone-positioning sessions of at least 16 hours significantly decreased 90-day mortality in patients with severe ARDS.
Types of recruitment manoeuvre
Intensive care unit setting
RMs can be performed in several different ways. Today, the most relevant RMs in terms of clinical applicability are sustained inflation (SI) manoeuvres, high pressure-controlled ventilation, incremental PEEP, and intermittent sighs. However, the best way to perform RMs has not been established and, in fact, it may vary according to specific circumstances.
The SI is by far the most frequently used RM. Commonly, it is performed by application of a continuous pressure of ≈40 cmH2O for up to 60 seconds at the airways , as illustrated in Fig. 120.1. The SI is effective in decreasing lung atelectasis, improving lung functional variables of oxygenation and respiratory mechanics, and also counteracting endotracheal suctioning-induced alveolar derecruitment . However, the efficacy of SI has been questioned and other studies showed that SI may be ineffective , short-lived, associated with circulatory impairment , increased risk of baro-/volutrauma, reduced net alveolar fluid clearance, and even worsened oxygenation .
Due to those side effects, researchers developed other types of RMs, among which the most important are:
◆ PCV applied with PEEP above the upper inflection point of the pressure–volume curve of the respiratory system, followed by change to VCV with stepwise decrease of PEEP and inspiratory plateau pressure below the upper inflection point (Fig. 120.3) .
Among those manoeuvres, the MRS is probably the one with the highest clinical impact. This manoeuvre was described in detail by Borges et al. , who treated 26 patients with sequential increments in PEEP at constant driving pressure until PaO2 + PaCO2 ≥ 400 mmHg. During the MRS, PEEP values of 45 cmH2O and inspiratory peak pressures as high as 60 cmH2O can be achieved. The major side effects of the MRS are transient haemodynamic depression and hypercapnia, major clinical consequences, e.g. pneumothorax, have not been observed. Recently, de Matos et al.  enhanced MRS by combining Borges’ recruitment phase with a PEEP titration phase consisting of decremental PEEP steps from 25 to 10 cmH2O. They concluded that MRS successfully recruited non-aerated lung areas for extended periods of time. These authors also did not observe barotrauma, nor significant clinical complications despite peak airway pressures of up to 60 cmH2O during the MRS, mean titrated PEEP of 25 cmH2O, and resulting inspiratory plateau pressures as high as 40 cmH2O in the first day following the MRS.
Operation room setting
There is a robust physiological rationale supporting the use of RMs for reversing lung collapse during general anaesthesia. Loss of lung aeration in dorsal lung zones is well documented in general anaesthesia, and invariably related to the development of atelectasis .
Anaesthesiologists more commonly perform RMs by squeezing the anaesthesia bag, in a procedure that closely resembles SI. Bag squeezing is relatively easy and simple to perform, but keeping the airway pressure, while fresh gas flows from the anaesthesia machine is tricky, even using pressure-release valves. Furthermore, when switching from manual to controlled ventilation the pressure in the anaesthesia circuit may drop to zero, leading to derecruitment. Certainly, SI can be applied by using continuous airway pressure (CPAP), but this mode is not present in most anaesthesia ventilators. Thus, during general anaesthesia, RMs are more easily conducted under tidal ventilation, mainly during volume controlled ventilation (VCV).
Fig. 120.8 illustrates two strategies for recruiting the lungs in the VCV mode. In the first variant, the PEEP is increased stepwise until the desired PEEP and inspiratory plateau pressure levels are achieved. Following that, the PEEP is reduced in steps of 2–5 cmH2O to determine which PEEP value is associated with the lowest static elastance of the respiratory system (decremental PEEP trial). In the second variant, the PEEP is increased to 10–15 cmH2O and maintained constant thereafter. The respiratory rate is reduced to 6–10/min, while VT is increased stepwise by 2–4 mL/kg until an inspiratory plateau pressure of 30–40 cmH2O is achieved. Finally, an adequate and protective VT is set at the ventilator. This manoeuvre has been used successfully in a multicentre randomized clinical trial on the intra-operative use of PEEP . Furthermore, using a similar RM and higher PEEP levels in patients undergoing open abdominal surgery, Severgnini et al. reported an improved lung function as long as 5 days after surgery, compared with a ventilation strategy without RMs and low PEEP.
Shall the anaesthesia ventilator not be able to provide higher PEEP values, the VT or the driving pressure can be titrated to achieve the desired opening pressure target during a few breaths. The appropriate target must take into account the elastance of the chest wall, which will reduce the effective transpulmonary pressure. In patients with non-injured lungs and normal chest wall elastance, airway pressures of 40 cmH2O are enough to recruit the lungs (14), but morbidly obese patients may require up to 60 cmH2O (15).
Impact of recruitment manoeuvres on ventilator-induced lung injury
While much is known about the impact of RMs on lung mechanics and gas exchange, only a few studies addressed their effects on VILI. Steimback et al.  evaluated the effects of frequency and inspiratory plateau pressure (Pplat) during RMs on lung and distal organs in rats with acute lung injury (ALI) induced by paraquat. They observed that, although a RM with standard sigh (180 sighs/hour and Pplat = 40 cmH2O) improved oxygenation, and decreased PaCO2, lung elastance, as well as alveolar collapse, it resulted in hyperinflation, ultrastructural changes in alveolar capillary membrane, and increased lung and kidney epithelial cell apoptosis. On the other hand, the reduction of the sigh frequency from 180 to 10 sighs/hour at the same Pplat (40 cmH2O) diminished lung elastance and improved oxygenation, with a marked decrease in alveolar hyperinflation, and apoptosis in lung and kidney epithelial cells. The association of this sigh frequency with a lower Pplat of 20 cmH2O worsened lung elastance, histology and oxygenation and increased PaCO2. We speculate that there is a sigh frequency threshold beyond which the intrinsic reparative properties of the lung epithelium are overwhelmed. Although the optimal sigh frequency may be different in healthy and ALI animals/patients, our results suggest that RMs with high frequency or low plateau pressure should be avoided. Theoretically, a RM using gradual inflation of the lungs might yield a more homogeneous distribution of pressure throughout the lung parenchyma, avoiding repeated manoeuvres and reducing lung stretch while allowing effective gas exchange.
Riva et al.  compared the effects of SI using rapid high recruitment pressure at 40 cmH2O for 40 s with the so-called RAMP manoeuvre, a slow increase in airway pressure up to 40 cmH2O during 40 seconds in paraquat-induced ALI. They reported that the RAMP manoeuvre improved lung mechanics with less alveolar stress. Among other RMs proposed as alternatives to SI, RAMP may differ according to the time of application and mean airway pressure.
Variable mechanical ventilation as recruitment manoeuvre
Variable mechanical ventilation is characterized by breath-by-breath changes in airway pressures and VT. Variable mechanical ventilation has been shown to improve oxygenation, respiratory mechanics, and reduce diffuse alveolar damage in experimental models of ARDS . Among the possible mechanisms of variable mechanical ventilation, lung recruitment is likely the most important one.
The use of variable VT during volume controlled ventilation significantly improved lung function in experimental models of atelectasis . In patients during surgery for repair of abdominal aorta aneurysms, variable mechanical ventilation improved arterial oxygenation and compliance of the respiratory system, as compared with non-variable mechanical ventilation .
Experimental evidence suggests that variable mechanical ventilation is superior to conventional RMs regarding the efficiency of opening and keeping lung units open. Variable ventilation improved recruitment in both normal and injured lungs in mice . Fig. 120.9 illustrates the effects of variable mechanical ventilation on lung aeration in a model of experimental ARDS.
Despite the large evidence regarding the potential of variable ventilation to promote lung recruitment, this mechanism is less probable during assisted ventilation. In experimental ALI, it has been shown that variable pressure support ventilation improved oxygenation , but this effect was mainly related to lower mean airway pressures and redistribution of pulmonary blood flow towards better ventilated lung zones.
The use of RMs in patients suffering from ARDS, as well as in patients undergoing surgery under general anaesthesia, remains controversial. The effectiveness of such manoeuvres is likely related to the nature, phase, and/or extent of the lung injury, as well as the recruitment technique itself. The SI represents the most commonly used RM, but it may lead to a paradoxical impairment of oxygenation due to redistribution of pulmonary blood flow and also to circulatory depression. With the purpose of reducing such side-effects, several RMs have been suggested. In order to be effective, RMs must take different aspects into account, namely the level of the recruiting pressure, the time, as well as the pattern/frequency with which this pressure is applied to achieve recruitment. Besides the SI, a couple of RMs that seem particularly interesting for use in clinical practice on the ICU are:
◆ Incrementally increased driving pressure at a fixed level of PEEP.
◆ PCV applied with PEEP above the upper inflection point of the pressure-volume curve of the respiratory system, followed by change to VCV with stepwise decrease of PEEP and inspiratory plateau pressure below the upper inflection point.
◆ Prolonged lower pressure recruitment manoeuvre with PEEP elevation up to 15 cmH2O and end-inspiratory pauses for 7 seconds twice per minute during a 15-minute period.
◆ Intermittent sighs to reach a specific plateau pressure in volume or pressure control mode.
◆ Long slow increase in inspiratory pressure up to 40 cmH2O.
◆ ‘Maximum recruitment strategy’.
In the operation room, the RM that can be performed with all anaesthesia ventilators consists of a PEEP increase to 10–15 cmH2O, a reduction of the respiratory rate to 6–10/min, and a stepwise increase of VT by 2–4 mL/kg until an inspiratory plateau pressure of 30–40 cmH2O is achieved. The use of variable ventilation, i.e. application of random variable tidal volumes, may also prove a simple and interesting alternative for lung recruitment in the clinical scenario, mainly on the ICU. Clinical evidence suggests that the use of RMs is useful as a rescue strategy for hypoxaemia, but studies on the impact of RMs on morbidity and mortality in intensive care, as well as surgical patients are warranted.
We thank Marcelo Gama de Abreu (Department of Anesthesiology and Intensive Care Therapy, University Hospital Carl Gustav Carus, Dresden, Germany) who helped draft the manuscript.
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