Compliance and dead space fraction indicate an optimal level of positive end-expiratory pressure after recruitment in anesthetized patients

BACKGROUND: Optimal positive end-expiratory pressure (PEEP) can be defined as the PEEP that prevents recollapse after a recruitment maneuver, avoids over-distension, and, consequently, leads to optimal lung mechanics at minimal dead space ventilation. In this study, we analyzed the effects of PEEP and recruitment on functional residual capacity (FRC), compliance, arterial oxygen partial pressure (Pao(2)) and dead space fraction, and we determined the most suitable variables indicating optimal PEEP. METHODS: We studied 20 anesthetized patients with healthy lungs undergoing faciomaxillary surgery. After a stepwise increase of PEEP/inspiratory pressures (0/10, 5/15, 10/20, 15/25 cm H(2)O, each level lasting for 20 min) using a pressure-controlled ventilation mode, a recruitment maneuver (at 20/45 cm H(2)O for a maximum of 20 min) was performed, followed by a stepwise pressure reduction (15/25, 10/20, 5/15, 0/10 cm H(2)O, with 20 min at each level). At each pressure level, FRC, compliance, Pao(2), and dead space fraction were measured. RESULTS: When comparing the values before and after recruitment at identical PEEP levels, all variables showed significant changes at 10/20 cm H(2)O; compliance was also significantly higher at the pressure step 15/25 cm H(2)O. In addition, FRC values showed significant differences at 5/15 cm H(2)O and 15/25 cm H(2)O. CONCLUSIONS: All variables showed the positive effects of PEEP in conjunction with a recruitment maneuver. Optimal PEEP was 10 cm. H(2)O because at this pressure level the highest compliance value in conjunction with the lowest dead space fraction indicated a maximum amount of effectively expanded alveoli. FRC and Pao(2) were insensitive to alveolar over-distension.