We are grateful for the interest of Glossop and Esquinas in our study.1 In our study, we found that in patients undergoing open abdominal surgery and ventilated with protective tidal volumes, high positive end-expiratory pressure (PEEP) combined with recruitment manoeuvres did not influence postoperative spirometry function tests compared with low PEEP without recruitment manoeuvres.2
After open abdominal surgery, postoperative pulmonary complications (PPC) are frequent and associated with increased morbidity and mortality.3 Our study results from a preplanned, single-centre substudy of the Protective Ventilation, high versus low PEEP during general anesthesia for open abdominal surgery (PROVHILO) trial ‘High versus low PEEP during general anaesthesia for open abdominal surgery’,4 where we showed that a strategy with high PEEP and recruitment manoeuvres did not protect against PPC in this important patient population. Those findings suggested that mechanical ventilation should be performed with low stress and strain of aerated lung structures, which in turn is associated with low driving pressures, as well as with reduced opening and closing of alveolar units. This ventilatory strategy is conceivable in the presence of low degrees of atelectasis, leading us to the concept of ‘permissive atelectasis’ during surgery.5
In this current substudy,2 postoperative pulmonary functional tests were performed in a limited number of patients in a single centre. However, our analysis showed no differences or clear trends in favour of a clinically relevant effect of intraoperative high or low PEEP on postoperative pulmonary functional tests. We can exclude that different results might have been reached even with a larger population of patients. Smaller but statistically significant differences would be of minor clinical impact. Additionally, postoperative analgesia techniques were standardised ‘a priori’ by protocol, and were absolutely comparable between high and low PEEP groups. Consequently, we believe that the mode of postoperative analgesia used, should not have introduced any error into the findings.
We agree with Glossop and Esquinas that in patients with intermediate to high risk of developing PPC, the application of postoperative noninvasive respiratory support might attenuate and potentially reverse the restrictive pulmonary syndrome, which frequently occurs following major abdominal surgery. However, the prophylactic use of noninvasive respiratory support to prevent PPC in these patients has not been clearly established.6,7
In line with what has been suggested by Glossop and Esquinas, we should develop well validated and easy to use preoperative predictive scoring systems to identify those patients at risk, define guidelines for intraoperative ventilation strategies, optimise postoperative analgesia and consider prophylactic use of noninvasive respiratory support to improve outcomes in surgical patients undergoing high-risk surgery. To reach these aims, low tidal volume is now almost generally accepted during intraoperative ventilation.5
Glossop and Esquinas proposed a level of PEEP between 6 and 8-cm H2O with recruitment manoeuvres during surgery to prevent PPC. However, the PROVHILO study showed no differences in PPC between an intraoperative level of PEEP less than or equal to 2 and 12-cm H2O.4 Subsequent individual data meta-analysis including a large number of patients, showed even an increased risk of complications in the postoperative period when levels of PEEP higher than 5-cm H2O were used.5 Also, why should an ‘intermediate’ PEEP of 6 to 8-cm H2O as compared with a ‘high’ PEEP of 12-cm H2O be better to avoid atelectasis or pleural effusions?
In general, there is no evidence from randomised controlled trials in favour of an ‘intermediate’ PEEP, whereas meta-analyses suggest that even ‘intermediate’ levels of PEEP are associated with PPC. More importantly, driving pressure has recently been proposed as a key parameter to optimise mechanical ventilation and to potentially prevent.5,8 Driving pressure is determined by the ratio between tidal volume and the compliance of the respiratory system. As the compliance of the respiratory system is related to the aerated lung volume, the driving pressure represents an easy measure to use so as to set mechanical ventilation according to the global strain, that is, the ratio between tidal volume and lung volume. In turn, strain has been found to be strictly associated with ventilator-associated lung injury.
We believe that an intraoperative mechanical ventilation strategy, including low tidal volume, low inspiratory plateau pressure, low driving pressure and low PEEP level might minimise lung injury and reduce PPC. Importantly, optimisation of ventilatory variables in specific categories of patients and surgical procedures that were not considered in these previous studies, for example, morbidly obese patients, laparoscopy, cardiothoracic surgery, neurosurgery and emergency, deserves further investigation. The application of noninvasive respiratory support early in the postoperative period seems attractive to prevent PPC, but no definitive data are available in support of its use.
In conclusion, we believe that the role of intraoperative protective mechanical ventilation on postoperative outcome, even for short periods of ventilation, has been clearly emphasised. Overall, ‘less is more’, but further studies are needed to optimise intraoperative mechanical ventilation in specific patients or specific types of surgery.
Acknowledgements related to this article
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