The aim of this study is to assess whether changes in the plethysmography variability index, during a tidal volume challenge, can reliably detect simultaneous changes in arterial blood pressure pulsatility, in patients hospitalized in intensive care unit. If results will be positive, this will allow the test to be performed even in the absence of an invasive arterial catheter.

Among patients in a state of shock one of the first treatments is volume expansion, consisting in a fast intravenous injection of crystalloid or colloid solutions in order to increase cardiac preload and consequently cardiac output. However, this augmentation of the cardiac output happens only in half of the patients.To avoid ineffective fluid administration several tests have been developed.

One of these tests is called the "tidal volume challenge" (TVC). It consists of transiently increasing for 1 minute the volume of air inhaled with each breath, in mechanically ventilated patients, and then evaluating whether this change impacts the pulsatility of the arterial blood pressure, detected by an arterial catheter. This test seems reliable, but it requires an arterial catheter to be in place.

It has been suggested in previous studies that pulsatility of blood pressure can be estimated by the pulsatility of the plethysmography signal ("plethysmography variability index"), obtained from the oxygen saturation signal that's measured in all intensive care patients. The advantage is that this measurement only requires a sensor placed at the end of a finger, on the earlobe or on the forehead.

The aim of the study is to assess whether changes in the plethysmography variability index (PVI) during a TVC can reliably detect simultaneous changes in blood pressure pulsatility. This study will include patients hospitalized in intensive care unit, mechanically ventilated, in whom physicians have decided to perform a TVC as common practice. Changes, occurred during this test, in arterial blood pressure pulsatility and in plethysmography pulsatility index will be measured and compared. If results will show that changes in PVI during TVC are reliable for measuring blood pressure pulsatility changes during this test, this will allow the test to be used even in the absence of an arterial catheter.

Inclusion Criteria:

• patients hospitalized in intensive care unit
• physicians' decision to perform a tidal volume challenge
• hemodynamic monitoring with a calibrated pulse contour analysis device (PICCO2) in place
• monitoring of oxygen saturation with Masimo SET
• mechanical ventilation with tidal volume of 6 ml/kg on ideal body weight

Exclusion Criteria:

• thoracic drainage
• patient under judicial protection

Patients hospitalized in intensive care unit, mechanically ventilated with a tidal volume of 6 ml/kg on ideal body weight, with oxygen saturation monitoring with Masimo SET and cardiac output monitoring with PICCO2 system in place.