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.

The lung (C_L) is a measure of its ability to increase volume in response to an increase of the distending force, i.e., the transpulmonary pressure (P_tp), and is calculated as C_L = ∆V / ∆P_tp. This parameter is the slope of the pressure-volume (P-V) curve. As already mentioned, P_tp is the mediator of the mechanical effects of a RM and reflects in magnitude the elastic recoil pressure of the lungs. P_tp can be estimated at the bedside as P_tp = P_aw – P_es, where P_aw is the airway pressure and P_es 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 (C_rs), calculated as C_rs = V_T / (P_Plateau – PEEP). Nowadays, all the ICU ventilators can calculate the C_rs, 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 C_rs 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 C_L and C_rs 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 C_rs are easy-to-use tools, but they might not give a complete overview of the effects of RMs in all patients.

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].

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.

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].