Respiratory System Mechanics during Low Versus High Positive End-Expiratory Pressure in Open Abdominal Surgery: A Substudy of PROVHILO Randomized Controlled Trial

BACKGROUND: In the 2014 PROtective Ventilation using HIgh versus LOw positive end-expiratory pressure (PROVHILO) trial, intraoperative low tidal volume ventilation with high positive end-expiratory pressure (PEEP = 12 cm H₂O) and lung recruitment maneuvers did not decrease postoperative pulmonary complications when compared to low PEEP (0-2 cm H₂O) approach without recruitment breaths. However, effects of intraoperative PEEP on lung compliance remain poorly understood. We hypothesized that higher PEEP leads to a dominance of intratidal overdistension, whereas lower PEEP results in intratidal recruitment/derecruitment (R/D). To test our hypothesis, we used the volume-dependent elastance index %E₂, a respiratory parameter that allows for noninvasive and radiation-free assessment of dominant overdistension and intratidal R/D. We compared the incidence of intratidal R/D, linear expansion, and overdistension by means of %E₂ in a subset of the PROVHILO cohort. METHODS: In 36 patients from 2 participating centers of the PROVHILO trial, we calculated respiratory system elastance (E), resistance (R), and %E₂, a surrogate parameter for intratidal overdistension (%E₂ > 30%) and R/D (%E₂ < 0%). To test the main hypothesis, we compared the incidence of intratidal overdistension (primary end point) and R/D in higher and lower PEEP groups, as measured by %E₂. RESULTS: E was increased in the lower compared to higher PEEP group (18.6 [16.22] vs 13.4 [11.0.17.0] cm H₂O·L^-1; P <.01). %E₂ was reduced in the lower PEEP group compared to higher PEEP (-15.4 [-28.0.6.5] vs 6.2 [-0.8.14.0] %; P <.05). Intratidal R/D was increased in the lower PEEP group (61% vs 22%; P =.037). The incidence of intratidal overdistension did not differ significantly between groups (6%). CONCLUSIONS: During mechanical ventilation with protective tidal volumes in patients undergoing open abdominal surgery, lung recruitment followed by PEEP of 12 cm H₂O decreased the incidence of intratidal R/D and did not worsen overdistension, when compared to PEEP ≤2 cm H₂O.