The recruitment-to-inflation (R/I) ratio is a recent tool that has been developed to evaluate the potential for lung recruitment in patients with acute respiratory distress syndrome (ARDS) [1]. It is calculated as the ratio between the compliance of the recruited lung following the application of a high positive end expiratory pressure (PEEP) to that of the respiratory system measured at lower PEEP. This parameter can be easily measured at bedside with any intensive care unit (ICU) ventilator [1]. Identifying ARDS patients with high potential for lung recruitment is important to help choosing ventilatory settings, particularly the PEEP level [1]. In the landmark study by Chen et al. [1], a cut-off value of R/I at 0.5, i.e. the median value of the cohort, was proposed to define patients with low (R/I ≤ 0.5) or high (R/I > 0.5) potential for lung recruitment. Since then, the R/I ratio has been used for phenotyping ARDS [2] or to assess the effects of interventions (e.g. prone positioning [3], lung recruitment maneuvers [4]) according to the potential for lung recruitment. Before using this very promising tool at a large scale in trials, it is important to verify the accuracy and the consistency of its values across the ICU ventilators. Indeed, errors in measures of both volumes and pressures are common with most modern ICU ventilators, even after careful calibration, and often excess 10% [5]. In the present bench study, we aimed to assess accuracy of modern ICU ventilators for measuring R/I ratio set at 0.0, 0.5 and 1.0 on a lung model simulator.

We used an ASL-5000 lung model (Ingmar Medical, Pittsburgh, PA) to simulate PEEP-induced recruited lung volume (Vrec) by modifying the compliance of the lung model at high PEEP, in order to obtain R/I ratios equal to 0.0, 0.5 or 1.0. At low PEEP, the compliance of the test lung was set at 40 ml/cmH2O (i.e., a common value in ARDS). At high PEEP, the compliance was either unchanged (40 ml/cmH2O) or increased to 60 or 80 ml/cmH2O to obtain the abovementioned R/I ratios. Thus, for a 10 cmH2O difference between low and high PEEP, the expected Vrec was 0, 200 and 400 ml, respectively.

Five modern ICU ventilators were assessed: Carestation (General Electric, Fairfield, CO), Servo I (Maquet, Solna, Sweden), Hamilton C5 (Hamilton, Rhäzuns, Switzerland), Infinity C500 (Dräger, Lübeck, Germany) and Evita XL (Dräger, Lübeck, Germany). Ventilators were fully checked and calibrated according to the manufacturers’ specifications. The Y-piece of the double-limb ventilator tubing was directly connected to the ASL-5000. Ventilators were set in volume control mode with tidal volume (VT) 400 ml, inspiratory flow 60 l/min, respiratory rate 20 breaths/min, and FIO2 21%. The low and high PEEP were set at 5 and 15 cmH2O, respectively. R/I ratios were calculated from the data measured by the pressure transducers and flowmeters of the respirators, as previously described [1]. They included plateau pressure, PEEP total, expired VT, and end-expiratory lung volume change when PEEP was abruptly decreased from 15 to 5 cmH2O on a single breath (Table 1). Measures of parameters needed to calculate R/I ratio were performed twice with each ventilator. The differences between the measured and the theoretical R/I ratios were calculated for each ventilator.

Table 1 Measures of R/I ratio with 5 ventilators according to theoretical R/I ratio equal to 0.0, 0.5 and 1.0
Full size table

As shown in Table 1, R/I ratios were overestimated by 4/5 of the ventilators for theoretical R/I of 0.0 or 0.5 and underestimated by 3/5 of the ventilators for theoretical R/I of 1.0 (Table 1). For the theoretical R/I = 0.5 (i.e. the value commonly used in the literature to discriminate recruiters and non-recruiters), the error in the measured R/I was > 0.05 (> 10%) with 4/5 ventilators and > 0.1 (> 20%) with 3/5 ventilators (Table 1). For this condition, the highest overestimation of R/I was + 0.17 (+ 34%), the highest underestimation of R/I was − 0.24 (− 48%) and the highest error in Vrec was + 90 ml (+ 45%). The highest difference in R/I between 2 ventilators was 0.4.

The present study highlighted that clinically relevant underestimations or overestimations of the R/I ratio and/or of the Vrec are common when measured with modern ICU ventilators. Several clinical studies used a single cut-off value of R/I to discriminate groups of patients with low or high potential for lung recruitment [1,2,3,4]. However, our results showing large errors in the measurements of the true values of R/I, despite highly standardized bench conditions, suggest that using a single cut-off R/I value to individualize treatments of a given patient may be inappropriate and could even lead to opposite therapeutic strategies.

Clinicians should be aware of a range of values around a given cut-off R/I value (i.e. a grey zone) for which no conclusion may be drawn concerning potential for lung recruitment, especially if they use different models of ventilators in the same ICU. These insights need to be taken into account for interpreting and/or designing future studies on R/I ratios.