Ventilation Strategies: Recruitment Maneuvers



Fig. 5.1
Types of recruitment maneuvers. Sigh maneuver (a), typical sustained inflation maneuver (b), slow stepwise maneuver based on step-by-step changes in PEEP (c)




5.3.1 Sigh


A RM consisting in interleaving uninterrupted mechanical ventilation with a certain number of breaths with a higher plateau pressure is referred to as ‘sigh’ RM and was the first technique proposed [24]. Such maneuver has been showed to improve oxygenation, lung compliance, and increase EELV, but these benefits are transient, and the sigh has to be performed frequently in order to maintain the patency of recruited lung units [27]. In animal models, patterns of sigh RMs with high frequency, e.g., 180/h as proposed in the first studies, were associated with elevated inflammation markers compared to ventilation without RMs and to CPAP maneuvers [28]. However, the clinical relevance of these aspects remains to be clarified. With the increased availability of more sophisticated ICU ventilators, sighs can be delivered with a predetermined frequency. Due to the lack of evidence concerning their safety and long-term efficacy, their routine cannot be recommended.


5.3.2 Sustained Inflation


Sustained inflation is the most described RM. It is performed increasing abruptly the airway pressure to a certain level and maintaining it constant for several seconds. A common sustained inflation maneuver consists in the application of a constant airway pressure of 40 cmH2O for 40s [29, 30]. These RMs can rapidly revert atelectasis and cause an improvement of oxygenation and lung function in the short term in clinical and experimental settings; however their role in achieving a prolonged gas exchange amelioration is less clear [20, 31, 32]. However, in a study comparing sustained inflation with other RMs, a more persistent effect was observed [33]. Since the pressure rise mediates both the effectiveness RM and VILI, caution is recommended in translating into clinical practice the results of small sampled observational studies, in which long-term safety is difficult to assess.


5.3.3 Slow Stepwise Maneuvers


Efforts were made by the researchers to find alternatives to the sustained inflation RM, possibly achieving a comparable efficacy with less risks in terms of hemodynamic impairment and barotrauma. Slow increases of plateau pressure, performed with stepwise adjustments of airway pressure and/or PEEP, were proposed with this aim in ICU patients [7] and during general anesthesia for surgery [34, 35]. Stepwise maneuvers could allow a better control of airway pressure increase compared to sustained inflation, resulting in a decreased risk for hyperinflation and hypotension. In several experimental studies, stepwise RMs resulted in a more prolonged benefit than conventional RMs [36], were associated with lower inflammatory markers [26], and reduced epithelial cell damage in mild ARDS [37]. Despite this, large trials are warranted to assess the safety and efficacy of RMs and advantages of specific techniques [38].



5.4 Evaluation of Recruitment Maneuver Effects


A single standard, repeatable and reliable mean to evaluate the effectiveness of a recruitment maneuver, is not universally accepted: it is still debated which is the best method to assess the efficacy of RMs. The assessment should be based on a thoughtful clinical judgment, relying on information derived from different monitoring techniques, and anatomical and functional evaluation should be integrated. The physiological parameters most commonly assessed include the PaO2/FiO2 ratio, pulmonary compliance, and the pressure-volume (P-V) curve. Imaging techniques can be extremely helpful and comprise computed tomography (CT), lung ultrasonography (LUS), and electric impedance tomography (EIT).


5.4.1 Blood Gas Analysis and PaO2/FiO2 Ratio


In the clinical practice, one of the most evaluated clinical outcome is the PaO2/FiO2 ratio, for its low cost, widespread availability, and ease of interpretation. Furthermore, it is intrinsically associated with the severity of ARDS [39] and is rapidly influenced by an effective RM. Transcutaneous SpO2 is a rough estimate of PaO2 and can be used to monitor in real time the modifications of gas exchange, and to verify that during the apnea phase of the RM, the oxygenation remains within a safety range. After the RM, a decrease of PaCO2 is usually observed consensually to the increase of the SpO2 and PaO2/FiO2 ratio, adding completeness to the overall evaluation. The major limit of blood gas analysis parameters is that they are strongly influenced by other variables. Before a RM, the baseline intrinsic recruitability of the lung and gas exchange are both related to the severity of ARDS [13, 40]: the PaO2/FiO2 ratio could show greater improvement in the most severe ARDS forms, compared to the mild ones [40]. Moreover, a transient improvement of gas exchange does not necessarily translate in improved outcome, as it might be achieved at the price of a higher risk for barotrauma and VILI. The ventilatory settings, including PEEP level and the cardiac output, can influence the PaO2/FiO2 ratio, leading to a misinterpretation of the evaluation of RMs [20]. Therefore, the PaO2/FiO2 ratio assessed by blood gas analysis and the SpO2 represent the two most widely available parameters used to evaluate RMs but, assessing the efficacy of the RM only on oxygenation, does not give a complete description of the recruitment and should be a part of an integrated evaluation.


5.4.2 Compliance and Pressure-Volume Curve


The lung (CL) is a measure of its ability to increase volume in response to an increase of the distending force, i.e., the transpulmonary pressure (Ptp), and is calculated as CL = ∆V / ∆Ptp. This parameter is the slope of the pressure-volume (P-V) curve. As already mentioned, Ptp is the mediator of the mechanical effects of a RM and reflects in magnitude the elastic recoil pressure of the lungs. Ptp can be estimated at the bedside as Ptp = Paw – Pes, where Paw is the airway pressure and Pes is the esophageal pressure, approximating the pleural pressure. Despite recommendations to increase its implementation in the clinical practice, the measurement of esophageal pressure is not often monitored in the ICU [41]. As a surrogate, most clinicians perform RMs relying on the respiratory system compliance (Crs), calculated as Crs = VT / (PPlateau – PEEP). Nowadays, all the ICU ventilators can calculate the Crs, helping to titrate the PEEP and to monitor the effectiveness of a RM. This approximation is often considered acceptable, but it must be stressed that in several cases such as in severe ARDS and morbid obesity [33], monitoring Crs can lead to a misinterpretation of the respiratory system mechanics and to an inappropriate setting of ventilatory parameters, including PEEP. Therefore, monitoring the transpulmonary pressure should be considered in patients with severe ARDS or in the obese [21, 33, 41]. The evaluation of a RM based on the ventilator P-V curve calculation relies on the assumption that the increase of the volume at a certain pressure is caused by the recruitment of non-aerated lung areas and has been demonstrated that this physiological property could be used to define the level of pulmonary recruitment because it tightly correlates with CT scan evaluation [42]. Likewise PaO2/FiO2, both CL and Crs tend to reflect the baseline severity of the patient’s lung condition, and, paradoxically, a greater improvement can often be seen in most severe patients, while this does not necessarily imply an improvement in outcome.

As the PaO2/FiO2 ratio assessment alone only reflects the transient improvement of gas exchange due to a RM, conversely a clinical evaluation limited to the observation of the P-V curve could only take into account changes in respiratory mechanics: a balance between the two should be achieved. Simple parameters such as the PaO2/FiO2 ratio, SpO2, and the Crs are easy-to-use tools, but they might not give a complete overview of the effects of RMs in all patients.


5.4.3 Computed Tomography


The quantitative analysis of lung CT scan allows to obtain useful information about the lung tissue aeration and, when performed at different pressure levels, can assess the potential recruitability. Historically, the concept of lung recruitability and PEEP titration in ARDS was investigated by means of CT analysis [13] that represents the most informative tool to assess lung aeration. CT can be considered as the gold standard to estimate lung recruitability, but it has several pitfalls that hamper its clinical application: the patients have to move to the ICU to the CT facility, the acquisition involves a high exposure to ionizing radiations, and the image post-processing is time-consuming. Several solutions are under investigation, including the possibility to assess visually images to avoid manual image segmentation [43], to extrapolate the information from a reduced number of CT slices [44], and to use low-dose protocols to reduce radiation exposure [45, 46]. While its role has been established to assess lung recruitability, CT cannot be used for the assessment of a single RM. Further studies are necessary to clarify whether RMs should be the included in the standard ventilatory approach to patients showing a high recruitability at the CT scan.

In conclusion, CT is fundamental for diagnosis, to evaluate the lung recruitability at the admission, to assess the best ventilatory settings, and to titrate the initial PEEP [47], but more versatile tools are warranted to assess reliably recruitability, possibly at the bedside [48, 49].


5.4.4 Lung Ultrasound


Lung ultrasound at the bedside is an increasingly popular technique in the ICU: it is an easy, cheap, repeatable, real-time, and noninvasive method to assess several lung conditions [50]. It has been also proposed as a tool to assess the efficacy of lung recruitment [19, 51]. The exam is performed scoring visually different pulmonary regions, in order to obtain a global score that correlates with the degree of lung aeration [51]. This application of lung ultrasound has not yet been completely standardized: its operator dependence raises concerns among some authors, and for this reason automated computer-based image analysis is under investigation [52, 53]. Moreover, lung ultrasound is not able so far to discriminate normal aeration from hyperaeration.


5.4.5 Electrical Impedance Tomography


Electrical impedance tomography (EIT) is a real-time imaging technique, which shows dynamically the lung aeration changes. It is based on the principle that changes in lung aeration modify the chest conductivity, and electrical signals recorded with electrodes placed on the skin are analyzed to produce a real-time lung aeration map. EIT is an emerging technique, but needs technical improvements to enhance resolution and clinical trials to clarify its role in decision making [54, 55].


Conclusions

While often useful as a rescue measure to overcome an acute gas exchange impairment in ARDS, there is still not univocal evidence that recruitment maneuvers can improve patient’s outcome. Alveolar recruitment can be achieved in many patients, but efforts must be made to balance between an acceptable oxygenation and the risk to deliver harmful pressures to the patient. Gas exchange, ventilator-derived parameters, esophageal pressure, and imaging techniques should be integrated to assess the efficacy and safety of recruitment maneuvers. After recruitment, a sufficient PEEP is mandatory to maintain the improvement. Stepwise slow recruitment maneuvers should be preferred to abrupt sustained inflation. Transient hypotension or desaturation are common during the procedure, but serious immediate adverse reactions are infrequent. However, long-term positive or negative effects are unknown.

At the moment, no evidence is available to support the use of recruitment maneuvers as a routine measure in all ARDS patients, but their indication should be tailored individually, as a part of a lung protective ventilatory strategy.


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Aug 26, 2017 | Posted by in Uncategorized | Comments Off on Ventilation Strategies: Recruitment Maneuvers
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