Bedside tools for assessing (de)recruitability

• Ensuring sufficient oxygenation and CO2 removal while avoiding further damage to the lung is one of the major challenges in treating ARDS patients.
• As the appropriate level of PEEP to be applied depends on the lung’s potential for recruitment, assessing the degree of recruitability can help guide the PEEP setting.
• One of two bedside methods for measuring recruitability based on measurements of respiratory mechanics is the recruitment-to-inflation ratio, which can be calculated from the monitoring parameters on the ventilator.
• The lower the R/I ratio, the lower the potential for recruitment.

Juggling these two balls is one of the major challenges in treating ARDS patients. In acute lung disease, the alveoli can be schematically divided into three groups: alveoli that are open and stable even at low pressure (as in a normal lung), collapsed alveoli that can be reopened and kept open by reasonably safe positive pressures (recruitable lung), and collapsed or consolidated alveoli that cannot be reopened (non-recruitable lung). Depending on the relative proportions of these groups at any given time, the lungs can be classified as close to mechanically normal, recruitable, or non-recruitable. In the case of a recruitable lung, delivering ventilation intended to recruit with the appropriate level of positive end-expiratory pressure (PEEP) is assumed to be protective. On the one hand, PEEP pushes the inspiratory pressures upwards, thus promoting the recruitment of collapsed alveoli and making the lung more likely to accept the tidal volume safely. On the other hand, an appropriate level of PEEP works as a mechanical stabilizer, opposing the cyclic alveolar de-recruitment during expiration and the resulting atelectrauma. The matter is different in the case of non-recruitable lungs or mechanically near-normal lungs: A high PEEP level would be both excessive and ineffective at the same time.

The simplest way to set PEEP in ARDS is to increase the setting while looking for a satisfactory improvement in oxygenation, but this approach is only sound in “responders” and may lead to overdistension - especially in those who don’t respond well to PEEP. Measuring recruitability could provide important additional information for finding the right PEEP.

But what is recruitability? Alveolar instability and the capability to respond to positive pressure are prerequisites for recruitability. It is clear that any lung with the potential for recruitment also has the potential for derecruitment, meaning that the unstable alveoli may also collapse again if the airway pressure decreases below a certain closing pressure. 

Bedside methods for assessing recruitability range from imaging by lung ultrasound or electric impedance tomography, through to approaches based on measurements of respiratory mechanics performed during specific maneuvers: the analysis of a static pressure-volume (PV) loop of the respiratory system and the recruitment-to-inflation ratio (R/I). Both methods require invasive ventilation and full patient relaxation.

The first method - the static PV loop – involves the use of a special tool on the ventilator to record volume against airway pressure during a slow, controlled inflation and deflation maneuver. A typical maneuver is set to explore the respiratory system during a slow constant pressure ramp of 2 cmH2O/s from 5 to 40 cmH2O, and back. A common alternative is to start and end at zero. In practice, these slow maneuvers explore two things in one go: the possible recruiting effects of slowly increasing pressure levels, then the possible de-recruitment associated with slowly decreasing ones. On a maneuver plot with Paw on the X-axis and Volume on the Y-axis, there is normally some hysteresis between the inflation and deflation limbs, which means that the deflation limb runs higher than the inflation limb. The further the two limbs move away from one another, the higher the degree of hysteresis. Interestingly, a larger hysteresis (i.e., a fatter loop) has been found to be associated with greater recruitment potential, while a smaller hysteresis (i.e., a thinner loop) indicates lower recruitment potential (Demory D, Arnal JM, Wysocki M, et al. Recruitability of the lung estimated by the pressure volume curve hysteresis in ARDS patients. Intensive Care Med. 2008;34(11):2019-2025. doi:10.1007/s00134-008-1167-81​, Chiumello D, Arnal JM, Umbrello M, et al. Hysteresis and Lung Recruitment in Acute Respiratory Distress Syndrome Patients: A CT Scan Study [published correction appears in Crit Care Med. 2022 Mar 1;50(3):e339]. Crit Care Med. 2020;48(10):1494-1502. doi:10.1097/CCM.00000000000045182​). The degree of hysteresis can be expressed as the normalized maximum distance (NMD), that is the ratio between the maximum distance between the two limbs and the maximum volume of the maneuver. A value for NMD of 0.41 corresponds to an intermediate level of recruitability and serves to distinguish between conditions with lower or higher recruitment potential (Chiumello D, Arnal JM, Umbrello M, et al. Hysteresis and Lung Recruitment in Acute Respiratory Distress Syndrome Patients: A CT Scan Study [published correction appears in Crit Care Med. 2022 Mar 1;50(3):e339]. Crit Care Med. 2020;48(10):1494-1502. doi:10.1097/CCM.00000000000045182​).

The second method – the recruitment-to-inflation ratio (R/I), otherwise known as “the simplified derecruitment technique” – was suggested by Chen et al. (Chen L, Del Sorbo L, Grieco DL, et al. Potential for Lung Recruitment Estimated by the Recruitment-to-Inflation Ratio in Acute Respiratory Distress Syndrome. A Clinical Trial. Am J Respir Crit Care Med. 2020;201(2):178-187. doi:10.1164/rccm.201902-0334OC3​, Chen L, Chen GQ, Shore K, et al. Implementing a bedside assessment of respiratory mechanics in patients with acute respiratory distress syndrome. Crit Care. 2017;21(1):84. Published 2017 Apr 4. doi:10.1186/s13054-017-1671-84​). It explores the possible derecruitment that takes place during conventional volume control ventilation (CMV) when PEEP is abruptly decreased by 10 cmH20 from a high level (between 15 and 18 cmH2O, maintained for at least 30 minutes) to a low level (between 5 and 8 cmH2O). The transition breath with inspiration starting from high PEEP and expiration ending at low PEEP results in a larger expiratory volume compared to the previous breaths. The additional expiratory volume consists of two completely different components. The former (denoted as deflated volume) depends on the deflation response of the alveoli that remain stable at the low PEEP level. The deflated volume explores the effect of pressure changes on inflation (I) and deflation of the baby lung, and is related to the baby lung’s size. The second component (denoted as de-recruited volume) depends on the de-recruitment of the unstable alveoli that immediately collapse and fully release their gas content when PEEP switches to the low level. The de-recruited volume explores the effect of pressure changes on recruitment (R) and de-recruitment. The R/I index corresponds to de-recruited volume divided by deflated volume. In practice, the index normalizes the observed de-recruitment by  the size of the baby lung and therefore expresses the degree of recruitability. 

Calculation of the R/I ratio is relatively easy, being based on data that can be read on the monitoring system of the ventilator: the expiratory volume at high PEEP, the expiratory volume during the PEEP-transition breath, and the respiratory system compliance at low PEEP. The lower the R/I ratio, the lower the potential for recruitment, with zero representing no recruitability. Intermediate values are around 0.5 (the median value of the population studied by Chen et al.), with values of more than 0.6 considered to represent high recruitability (Chen L, Del Sorbo L, Grieco DL, et al. Potential for Lung Recruitment Estimated by the Recruitment-to-Inflation Ratio in Acute Respiratory Distress Syndrome. A Clinical Trial. Am J Respir Crit Care Med. 2020;201(2):178-187. doi:10.1164/rccm.201902-0334OC3​). These results using the R/I ratio were compared with the hysteresis on static PV loops and a good agreement was found (Nakayama R, Bunya N, Katayama S, et al. Correlation between the hysteresis of the pressure-volume curve and the recruitment-to-inflation ratio in patients with coronavirus disease 2019. Ann Intensive Care. 2022;12(1):106. Published 2022 Nov 12. doi:10.1186/s13613-022-01081-x5​). 

Once either the hysteresis on a single static PV loop or the R/I ratio on a 10-cmH2O de-recruitment maneuver has identified a condition of high, intermediate, or low recruitability, what should be done with the PEEP setting in clinical practice? Low PEEP is usually the right choice for low recruitability. Even if high recruitability is commonly considered a good indication for high PEEP, it is important to note the recent word of caution about this concept (Gattinoni L, Collino F, Camporota L. Assessing lung recruitability: does it help with PEEP settings?. Intensive Care Med. 2024;50(5):749-751. doi:10.1007/s00134-024-07351-56​). And what about intermediate recruitability? Interestingly, the hysteresis evaluation on multiple static PV loops during a decremental PEEP trial has been suggested for a precise assessment of the most appropriate PEEP level for recruiting and mechanically stabilizing the unstable alveoli (Mojoli F, Pozzi M, Arisi E. Setting positive end-expiratory pressure: using the pressure-volume curve. Curr Opin Crit Care. 2024;30(1):35-42. doi:10.1097/MCC.00000000000011277​). Similarly, the R/I ratio approach has also been used for a granular assessment of de-recruitment during a decremental PEEP trial, but in this case the necessary technology is very demanding, because the measurement of the end-expiratory lung volume by nitrogen dilution is required (Grieco DL, Pintaudi G, Bongiovanni F, et al. Recruitment-to-inflation Ratio Assessed through Sequential End-expiratory Lung Volume Measurement in Acute Respiratory Distress Syndrome. Anesthesiology. 2023;139(6):801-814. doi:10.1097/ALN.00000000000047168​) 

The R/I ratio offers a means of assessing the degree of recruitability at the bedside using the standard monitoring parameters on Hamilton Medical ventilators. The other method involving the static PV loop can be applied using the P/V Tool, which is available on HAMILTON‑C3/C6 and HAMILTON‑G5/S1 ventilators (Available as an option on HAMILTON-G5 and HAMILTON-C3/C6 ventilatorsA​, Standard on the HAMILTON-S1B​). In this case, recruitability can be assessed directly from the pressure-volume curve in a single diagnostic maneuver. The P/V Tool can also be used to perform recruitment maneuvers or to find the optimal PEEP setting based on a granular assessment of the hysteresis made from multiple diagnostic maneuvers.