Individual PEEP in Obesity: Comment

We read with great interest the article published by Li et al.,1 which addresses a topic still under discussion in the literature: the individualization of positive end-expiratory pressure (PEEP) in mechanically ventilated patients during surgery. In this study, the authors compared the development of atelectasis in patients with obesity undergoing laparoscopic bariatric surgery using two mechanical ventilation strategies. The first ventilatory strategy included a titrated PEEP, whereas the second used a fixed PEEP of 8 cm H2O (both used low and comparable tidal volumes). The main finding obtained by the authors was a 3.7% difference in the development of atelectasis in the first postoperative hour in favor of the titrated PEEP strategy.Although the results are interesting, we would like to raise the following points:First, the difference in the percentages of patients with atelectasis was small and had little clinical relevance. This finding could be related to the fact that both groups underwent a recruitment maneuver after anesthesia induction, introducing bias. It would have been more interesting to know the results when comparing a strategy without recruitment maneuvers and fixed PEEP versus “open lung” and individualized PEEP. Comparisons between high and low fixed PEEP strategies have not shown any difference in the literature.2,3Second, the percentage of atelectatic lung parenchyma was 13.1% and 9.5% in the fixed and individualized PEEP groups, respectively, which was superior to those obtained in other studies related to PEEP individualization in anesthetized patients.4 If we add the poorly aerated parenchyma to the nonaerated lung compartment, the percentage of not–well aerated lung increases up to 41.9% and 39.4%, respectively. Although both groups received a recruitment maneuver before starting pneumoperitoneum, the respiratory mechanical conditions changed once the pneumoperitoneum was established. An increase in the intraabdominal pressure is transmitted to the thorax, increasing the probability of alveolar collapse and the need for higher PEEP.5 The addition of 2 cm H2O to individualized PEEP, as performed in this protocol, was not sufficient to avoid alveolar collapse. To prevent this phenomenon, two approaches can be used: (1) a recruitment maneuver followed by PEEP titration with the pneumoperitoneum in place, which is perfectly feasible if coordinated with the surgical team; or (2) obtaining the airway opening pressure by using a low-flow maneuver at a low respiratory rate under low PEEP.6Third, the use of dynamic compliance in the choice of individualized PEEP is striking. Most studies have used quasi-static compliance for PEEP titration with the aim of minimizing the resistive component, as shown in the equation of movement of the respiratory system: P = Flow × resistance + tidal volume × elastance + PEEP.7 With the intention of being pragmatic, concepts of physiology, such as the individualization of resistive and elastic components that generate pressure, can be lost. In addition, a prolonged expiratory pause is not necessary to obtain reliable alveolar pressure; 0.4 s is enough, which makes the maneuver feasible in the operating room setting. Under zero-flow conditions, we obtain the plateau pressure and calculate the static compliance using the following formula: Vt/(Plateau pressure – PEEP). Driving pressure (Plateau – PEEP) is the parameter best correlated in the literature with ventilator-induced lung injury and is commonly used for PEEP titration in most articles in different clinical scenarios.8,9Finally, it is interesting to highlight the wide variability that the authors observed in individualized PEEP, independent of the body mass index. This can be clearly illustrated in two extreme cases with body mass index of 35 and close to 55, where both patients received the same individualized PEEP. This finding supports the concept that there are no demographic or anthropometric parameters that allow the use of fixed PEEP in a specific group of patients. Consequently, it is pertinent to continue investigating the individualization of ventilatory parameters to reduce the incidence of postoperative pulmonary complications.The authors declare no competing interests.