Influence of Inspiratory Pause on Ventilatory Efficiency in Robotic Surgery. A Prospective Paired Study.

The investigators aim to determine if the modification of the end inspiratory pause (EIP) during mechanical ventilation adds benefit when applied to patients undergoing robotic surgery and who are ventilated under an individualized open lung approach (iOLA) strategy. The EIP is an adjustable parameter of volume controlled ventilation modes usually set as a percentage of the total inspiratory time. It represents the phase comprised between the moment in which the volume programmed in the ventilator has already been administered (which marks the end of the inspiratory flow), and the opening of the expiratory valve (which marks the beginning of expiration). The investigators will study whether modifications of the EIP produce variations in the "quantity" of the lung that participates in gas exchange (respiratory volume). To do so, the investigators will sequentially apply different EIP to participants (paired study). The investigators´ hypothesis is that increasing the EIP up to a level, may diminish the lung volume that does not participate in breathing (the physiological dead space- VDphys), thereby increasing the respiratory volume. To note: the VDphys includes the "conduction" volume, that represented by trachea, bronchi, et cetera, which is in charge of driving the "air" towards the respiratory zones, and the alveolar dead space (those zones of the respiratory volume that due to different reasons do not directly participate in gas exchange: alveoli ventilated but not perfused, areas of overdistension, etc. The investigators will measure dead volumes by mean of specific non-invasive monitoring (volumetric capnography) coupled to the anesthesia workstation, and the mechanics of lung and the distribution of the gas within it by means of electric impedance tomography, a non-invasive technique showing continuous images of patient's lung. The estimation of the respiratory volume will help the investigators to more precisely adjust the amount of oxygen and anesthetic gases that must be administered in function of patients´ gases consumption, a calculated parameter that is function of the respiratory volume and that will also be tested during the study. The investigators will also accurately measure patient oxygenation by means of arterial blood samples extracted from a radial artery catheter. Apart from sequential modifications in the EIP, the ventilation strategy applied to patients will be that used in the investigators´ usual practice (described below).