Title Monitoring of alveolar recruitment maneuvers using the Oxygen Reserve Index in the prevention of atelectasis formation in postoperative patients undergoing major surgery. Pilot,observational study. Background and goal of study While the monitoring of ventilatory mechanics has been done with detail and precision for many years, the monitoring of the oxygenation state has advanced remarkably in recent times. Currently, a parameter called Oxygen Reserve Index (ORI) is available, which is a relative indicator of arterial oxygen partial pressure (PaO2) in the range of 100-200 mmHg; Theoretically, when the PaO2 is in 100 mmHg the ORI value is 0 and grows progressively being 1 with PaO2 of 200 mmHg. This parameter seems very useful since it would allow a continuous and non-invasive detection of a PaO2 fall (from 200 mmHg to 100 mmHg) and therefore can warn of imminent hypoxemia. Pulmonary atelectasis occurs with hypoxemia, but these are masked when supplemental oxygen is given. In fact, with a FiO2 of 0.5 a PaO2 of 100 mmHg, it would indicate the presence of important alveolar shunt (atelectasis) and while the value of SpO2 would be 100%, the ORI value would be close to zero. Therefore, a decrease in ORI by maintaining the same FiO2, and based on the same principle of the SpO2-FiO2 curves, could detect at an early stage the formation of atelectasis before true hypoxemia occurs, without the need for arterial blood gas or More complex techniques such as measurement of transpulmonary pressure, Cdyn or volumetric capnography. Because of this, a clinical study is necessary to observe the behavior of the ORI during the ARM for an open lung strategy and to determine if the non-invasive parameter allows the detection of pulmonary opening and closing pressures using a FiO2 safety device ( Eg 0.5) where atelectasis may be masked by hyperoxia. The goal of the study is describe the changes produced during the different phases of an open lung strategy in the Oxygen Reserve Index.
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