Assessing lung recruitability in ARDS

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Acute respiratory distress syndrome (ARDS) is characterized by lung collapse that decreases the size of the aerated lung (baby lung concept [1]), resulting in decreased respiratory system compliance and impaired oxygenation.

Take-away messages

  • Lung collapse in ARDS can be reversed in part by a recruitment strategy combining a recruitment maneuver and adequate PEEP setting to keep the lung aerated.
  • As ARDS patients have wide heterogeneity in their potential for recruitment, it is essential to first assess the amount of potentially recruitable lung for a given patient in order to align the treatment to the individual pathophysiology.
  • Two bedside methods for assessing recruitability based on pressure and volume measurements are Hysteresis of the low-flow pressure volume (PV) curve and Recruitment-to-Inflation (R/I) ratio
  • Used in conjunction with lung imaging, these bedside methods serve to strengthen the prediction of recruitment potential in early-onset ARDS patients.

Lung collapse is caused by inflammation in the alveoli due to the primary disease, but is also aggravated by fluid overload, prolonged supine positioning, controlled mechanical ventilation, obesity, and high FiO2. Part of the lung collapse can therefore be reversed using a recruitment strategy, which combines a recruitment maneuver and an adequate PEEP setting to keep the lung aerated [2]. However, the amount of potentially recruitable lung (which equates to the degree of so-called lung recruitability) varies widely in ARDS patients and is difficult to predict using the normal ARDS severity criteria, such as the PaO2/FiO2 ratio or static compliance [3]. Assessing lung recruitability is essential to determine the recruitment strategy for a given patient, in order to align the treatment to the individual pathophysiology. Recent data show that a recruitment strategy plus adequate PEEP is associated with good outcomes in patients with high potential for recruitment, but with very poor outcomes in patients with low potential for recruitment [4].

Methods for assessing recruitability are based on performing a trial to increase the aerated volume of the lung by increasing pressure and then measuring the change in volume, either by means of lung imaging or using the ventilator’s flow sensor.

Lung-imaging methods for assessing recruitability

CT scan is the gold standard method for assessing recruitability. A quantitative CT scan is performed at low and high pressure. Recruitment is calculated as the change in non-aerated lung tissue between the two conditions [5]. However, this method carries with it not only the risk associated with transporting the patient to the CT scan, but also of exposure to radiation. Calculations are complex and only semi-automatic, which makes the method difficult to use in daily clinical practice. Lung ultrasound is able to detect a re-aeration of the lung after recruitment and PEEP setting, but is not able to partition between the recruitment of collapsed alveoli and the inflation of poorly-aerated alveoli [6]. In addition, lung ultrasound does not provide a quantitative measurement of lung aeration and cannot assess overinflation [7]. Electrical impedance tomography (EIT) has the potential to detect recruitment of the dependent part of the lung, but does not provide an index of recruitability [8].

Bedside methods based on pressure and volume measurements to assess recruitability

A bedside method called Recruitment-to-Inflation (R/I) ratio was recently described [9]. An abrupt release of PEEP from 15 cmH2O down to 5 cmH2O increases the expired volume. The difference between this additional expired volume and the predicted volume increase multiplied by the difference in PEEP (compliance at low PEEP multiplied by PEEP change) estimates the volume recruited by PEEP. This recruited volume divided by the effective pressure change, taking into account the presence of complete airway closure, if any, calculates the compliance of the recruited lung. The R/I ratio is calculated by the compliance of the recruited lung divided by the compliance at low PEEP. A high R/I ratio means high potential for recruitment. In a study including 41 ARDS patients, the R/I ratio was inversely correlated to oxygenation at low PEEP; patients with a high R/I ratio had the lowest PaO2/FiO2 ratio. In addition, the R/I ratio was correlated to the change in SpO2 between low and high PEEP. In COVID-19 ARDS patients, the R/I ratio has great variability [10, 11] and is higher than in other types of ARDS [12].

Another method also suggested as a means of predicting lung recruitability in early-onset ARDS patients is hysteresis of the low-flow pressure volume curve, where hysteresis is calculated as the area enclosed by the pressure volume loop divided by the predicted body weight [13].

Combining the methods

None of the above-mentioned methods are perfect. Therefore, combining a bedside method with imaging serves to strengthen the prediction. When a recruitment strategy is performed, the effect on compliance, driving pressure, oxygenation, alveolar dead space, and hemodynamics is measured to confirm the effective recruitment.

The methods mentioned above have been studied in early-onset ARDS patients. Situations such as late-onset ARDS, single lung injury, patients on ECMO, and obese patients with dependent lung atelectasis have not yet been addressed in terms of predicting recruitability.


Because ARDS patients have wide heterogeneity in their potential for recruitment, assessing recruitability is essential to determine the recruitment strategy. A recruitment strategy that is not aligned with individual pathophysiology is associated with poor outcomes. Several bedside or imaging methods are available to assess recruitability.

HAMILTON-C3/C6* andHAMILTON-G5/S1* ventilators offer the P/V Tool as a standard or optional feature*, which allows you to assess lung recruitability using the R/I ratio or low-flow pressure-volume curve method.

* Not all ventilators or options available in all markets