Respiratory effects of lung recruitment maneuvers depend on the recruitment-to-inflation ratio in patients with COVID-19-related acute respiratory distress syndrome

Abstract

Background: In the context of acute respiratory distress syndrome (ARDS), the response to lung recruitment maneuvers (LRMs) varies considerably from one patient to another and so is difficult to predict. The aim of the study was to determine whether or not the recruitment-to-inflation (R/I) ratio could differentiate between patients according to the change in lung mechanics during the LRM.

Methods: We evaluated the changes in gas exchange and respiratory mechanics induced by a stepwise LRM at a constant driving pressure of 15 cmH₂O during pressure-controlled ventilation. We assessed lung recruitability by measuring the R/I ratio. Patients were dichotomized with regard to the median R/I ratio.

Results: We included 30 patients with moderate-to-severe ARDS and a median [interquartile range] R/I ratio of 0.62 [0.42-0.83]. After the LRM, patients with high recruitability (R/I ratio ≥ 0.62) presented an improvement in the P_aO₂/F_iO₂ ratio, due to significant increase in respiratory system compliance (33 [27-42] vs. 42 [35-60] mL/cmH₂O; p < 0.001). In low recruitability patients (R/I < 0.62), the increase in P_aO₂/F_iO₂ ratio was associated with a significant decrease in pulse pressure as a surrogate of cardiac output (70 [55-85] vs. 50 [51-67] mmHg; p = 0.01) but not with a significant change in respiratory system compliance (33 [24-47] vs. 35 [25-47] mL/cmH₂O; p = 0.74).

Conclusion: After the LRM, patients with high recruitability presented a significant increase in respiratory system compliance (indicating a gain in ventilated area), while those with low recruitability presented a decrease in pulse pressure suggesting a drop in cardiac output and therefore in intrapulmonary shunt.

Keywords: Acute respiratory distress syndrome; Mechanical ventilation; Recruitability; Recruitment maneuver; Respiratory mechanics.

Fig. 1

Effects of the LRM on respiratory system compliance, as a function of the patients’ lung recruitability. A pre- and post-LRM C_rs in patients with low lung recruitability (red circles) or high lung recruitability (blue squares). B The change in C_rs induced by the LRM in patients with low lung recruitability (red circles) or high lung recruitability (blue squares). Low and high lung recruitability was defined, respectively, as an R/I ratio below or above the median value for the cohort (0.62). C_rs: respiratory system compliance, ns: nonsignificant, R/I: recruitment-to-inflation ratio

Fig. 2

Effects of the LRM on respiratory mechanics and gas exchanges, as a function of the patients’ lung recruitability. Effects of LRM on P_aO₂/F_iO₂ and ventilator ratio (A, B), end-expiratory and end-inspiratory transpulmonary pressures (C, D), and pulse pressure and heart rate (E, F). Patients with low lung recruitability (red circles) and high lung recruitability (blue squares) were defined by an R/I below or above the median value for the cohort (0.62). HR: heart rate, P_aO₂/F_iO₂: partial pressure of oxygen to inspired oxygen fraction, P_L,EE: transpulmonary pressure at end-expiration, P_L,EI: transpulmonary pressure at end-inspiration, PP: pulse pressure, R/I: recruitment-to-inflation ratio, VR: ventilator ratio