Influence of Inspiratory Pause on Ventilatory Efficiency and Tidal Volume Distribution in Patients Undergoing Robotic Prostate Surgery and Ventilated With an Individualized Open Lung Approach. A Prospective Paired Study.

Introduction

Mechanical ventilation (MV) during general anesthesia entails some adverse effects that cannot be completely avoided. In recent years, pulmonary protective ventilation (PPV) strategies, based on the use of a low tidal volume (TV) with the intention of reducing the biological trauma associated to MV, are gaining prominence in surgical and intensive care patients. A tailored open-lung approach (tOLA) has recently emerged as one of these PPV strategies. It combines the use of alveolar recruitment manoeuvers (ARM), aiming to "open" collapsed areas of the lung, with the application of individualised positive end expiratory pressure (PEEP) with the objective of avoiding the re-collapse of those areas during expiration. The rationale supporting OLA strategies assumes that achieving more homogeneous alveolar ventilation by preventing atelectasis, while avoiding alveolar overdistension, leads to a more efficient gas distribution and exchange. This allows less traumatic ventilation and reduces the inflammatory response. The use of a tOLA has shown promising results in terms of improving oxygenation and reducing the risk of severe postoperative pulmonary complications.

Along with the above, prolonging the end inspiratory pause (EIP), while maintaining an adequate expiratory time, has shown benefits in terms of improving effective alveolar ventilation and enhancing gas exchange. The investigators recently demonstrated the effects of increasing the EIP when ventilating patients with a tOLA strategy. In that work, the investigators examined the effects of two EIP (10 and 30% of the inspiratory time) on the respiratory mechanics of patients undergoing major surgery. The investigators studied the effect of EIP on driving pressure (Pdriv), plateau pressure (Pplat), compliance of the respiratory system (Crs) and PEEP. The investigators also evaluated the dynamic distribution of TV during the ventilatory cycle using electrical impedance tomography (EIT) and studied the effect of EIP on gas exchange by means of arterial gasometry. In that study, the benefits of a longer EIP were seen with both standard PPV (TV of 7 mL of predicted body weight (PBW) and a PEEP of 5 cmH2O), and with a tOLA. The tOLA strategy was associated with a significant increase in PEEP, Pplat, arterial pressure of oxygen (PaO2), and Crs, with a significant decrease in Pdriv and arterial pressure of carbon dioxide (PaCO2), and with a more homogeneous gas distribution compared to standard PPV. On the other hand, the use of a longer EIP showed a significantly lower PEEP, Pdriv and mean airway pressure (Pmean) together with a higher Crs among patients ventilated under a tOLA strategy. However, it was not an objective of the addressed work to study the effects of the EIP modification on ventilatory efficiency, measured as the relation between VDphys and TV (VDphys/TV), something that the investigators will address in this work.

The investigators hypothesize that increasing the EIP in the context of a tOLA may improve alveolar ventilation by reducing the VDphys/TV ratio; in other words, by reducing the dead volume. To test this hypothesis, the investigators will study patients submitted to abdominal robotic surgery. The investigators´ intention is to verify if the potential benefits associated with the combined use of a tOLA and a longer EIP are observed in this type of surgery, where the gas insufflated into the abdomen together with the surgical position (forced trendelenburg) are known to jeopardize ventilation conditions.

Methods