Recruitment-to-inflation ratio to assess response to PEEP during laparoscopic surgery: A physiologic study

Abstract

Study objective: During laparoscopic surgery, the role of PEEP to improve outcome is controversial. Mechanistically, PEEP benefits depend on the extent of alveolar recruitment, which prevents ventilator-induced lung injury by reducing lung dynamic strain. The hypotheses of this study were that pneumoperitoneum-induced aeration loss and PEEP-induced recruitment are inter-individually variable, and that the recruitment-to-inflation ratio (R/I) can identify patients who benefit from PEEP in terms of strain reduction.

Design: Sequential study.

Setting: Operating room.

Patients: Seventeen ASA I-III patients receiving robot-assisted prostatectomy during Trendelenburg pneumoperitoneum.

Interventions and measurements: Patients underwent end-expiratory lung volume (EELV) and respiratory/lung/chest wall mechanics (esophageal manometry and inspiratory/expiratory occlusions) assessment at PEEP = 0 cmH₂O before and after pneumoperitoneum, at PEEP = 4 and 12 cmH₂O during pneumoperitoneum. Pneumoperitoneum-induced derecruitment and PEEP-induced recruitment were assessed through a simplified method based on multiple pressure-volume curve. Dynamic and static strain changes were evaluated. R/I between 12 and 4 cmH₂O was assessed from EELV. Inter-individual variability was rated with the ratio of standard deviation to mean (CoV).

Main results: Pneumoperitoneum reduced EELV by (median [IqR]) 410 mL [80-770] (p < 0.001) and increased dynamic strain by 0.04 [0.01-0.07] (p < 0.001), with high inter-individual variability (CoV = 70% and 88%, respectively). Compared to PEEP = 4 cmH₂O, PEEP = 12 cmH₂O yielded variable amount of recruitment (139 mL [96-366] CoV = 101%), causing different extent of dynamic strain reduction (median decrease 0.02 [0.01-0.04], p = 0.002; CoV = 86%) and static strain increases (median increase 0.05 [0.04-0.07], p = 0.01, CoV = 33%). R/I (1.73 [0.58-3.35]) estimated the decrease in dynamic strain (p ≤0.001, r = -0.90) and the increase in static strain (p = 0.009, r = -0.73) induced by PEEP, while PEEP-induced changes in respiratory and lung mechanics did not.

Conclusions: Trendelenburg pneumoperitoneum yields variable derecruitment: PEEP capability to revert these phenomena varies significantly among individuals. High R/I identifies patients in whom higher PEEP mostly reduces dynamic strain with limited static strain increases, potentially allowing individualized settings.

Keywords: Alveolar recruitment; Atelectasis; Laparoscopic surgery; PEEP; Recruitment-to-inflation ratio; Respiratory mechanics.