Effects of patient's inspiratory effort, as assessed by esophageal pressure, on Pulse Pressure Variation assessment of fluid responsiveness in patients undergoing mechanical ventilation.

Patients will be monitored with a monitor for hemodynamic assessment (Most Care, Vytech Health, Padova, Italy), which uses a Pressure Recording Analytical Method (PRAM). PRAM is a method designed for arterial pressure-derived continuous Cardiac Output and it is the only methodology that does not need any starting calibration, central venous catheterization, or any adjustments based on experimental data. As a consequence, PRAM needs only an arterial line (radial, brachial, femoral) for working. Furthermore, we will position a catheter able to measure the esophageal pressure to properly assess the respiratory effort during the study protocol. The catheter will be connected to a dedicated pressure transducer and a recording system. In order to ascertain its correct positioning, an end-expiratory occlusion test will be performed. This test is used in spontaneously breathing patients, to validate the correct position of esophageal balloon; it is performed during an end-expiratory occlusion maneuver with the simultaneous measurement of the changes in airway and esophageal pressure during an inspiratory effort (i.e., no air flow) (Baydur’s occlusion test). The position of the esophageal catheter is considered acceptable when the ratio between the changes in two pressures is close to unity. In healthy subjects with or without anesthesia, when the esophageal catheter was correctly positioned, it has been found that the difference between the changes in esophageal and airway pressure was lower than 15 %. When the patient has been monitored with Most Care device, baseline data will be acquired for 2 minutes during Pressure Support Ventilation (PSV) without modification of the clinical settings. Afterwards, the patients will undergo to the following different ventilator settings lasting 5 minutes each according to computer generated random sequence: 1) PSV with a 50% increment of the baseline pressure support; 2) PSV with a 50% reduction of the baseline pressure support; 3) Assist/Controlled Ventilation with a tidal volume equal to 6 ml/kg of ideal body weight; 4) Assist/Controlled Ventilation with a tidal volume equal to 8 ml/kg of ideal body weight; 5) Assist/Controlled Ventilation with a tidal volume equal to 10 ml/kg of ideal body weight. Between each single trial of incremental pressures, a return to baseline conditions will be guarantee for 15 minutes, to avoid carry-over effects. In the end, after all the trials mentioned above, a single fluid challenge of 500 ml of normal saline will be infused in 5 minutes and the hemodynamic response will be evaluated to define if the patient is fluid responsive or not (i.e. if the cardiac index incremented of 15% at least, or not). All the conditions will be tested in one session and the entire study protocol will be conducted by one of the physicians involved in the research.