Pulse CONtour and BIoimpedance for Measurements of Stroke Volume During Changes in CARdiac Preload

Fluid therapy guided by stroke volume response to repeated fluid challenges is used for hemodynamic optimization during general anesthesia. Two types of devices that measure stroke volume non-invasively are primarily available for intermediate-risk surgery under general anesthesia. They are based on two different techniques, (1) pulse contour analysis, and (2) transthoracic bioimpedance. They have never been compared with each other. Therefore, it is not known (1) whether they are interchangeable for the measurement of stroke volume changes (usually assessed as the "concordance" of two techniques), and (2) whether one is better than the other.

The aim is to study the concordance of these two non-invasive hemodynamic devices. it will use both of them in patients undergoing intermediate risk surgery. They will be tested during modifications of cardiac preload induced by Trendelenburg and anti-Trendelenburg positioning, as well as during intraoperative fluid challenges, vasopressor boluses and alveolar recruitment maneuvers.

The main objective is to assess the concordance of two non-invasive devices using the technology described above : the Clearsight (Edwards Lifesciences, pulse contour analysis), and the Niccomo (Medis, bioimpedance). Both will be used in each patient included in the study. They will be tested during modifications of cardiac preload induced by sequences of position changes (see below) including Trendelenburg and anti-Trendelenburg positioning, performed systematically before and after the procedure, as well as during fluid challenges, vasopressor boluses and alveolar recruitment maneuvers (performed during the procedure according to the needs of the anesthesiologist in charge of the patient). Both monitors will also be compared with an invasive monitor of stroke volume (Vigileo/Flotrac, Edwards Lifesciences), and the respiratory pulse pressure variation will also be measured during the sequences of position changes and will be used as a gold standard for fluid responsiveness.

Eligibility Criteria:
Inclusion Criteria:

* Eligible and suitable for intermediate risk surgery, validated by anaesthesia consultation.

There is no definitive definition of intermediate risk. By reference to the literature, surgeries longer than 2 hours (excluding "low risk") and less than 3% postoperative mortality (excluding high-risk surgery). It includes the majority of non-serious visceral (abdominal and gynecological) surgery, plastic or cervical-facial surgery for neoplastic removal or long-term reconstruction, or long-term spinal surgery or neurosurgery of the base of the skull.

- For which the anesthesiologist in charge has planned an invasive monitoring (radial catheter) of arterial blood pressure.

Exclusion Criteria:

* Urgent surgery
* Intracranial hypertension
* Orthostatic hypotension
* Extreme anthropometry: weight \>150 kg and 30 kg, size 120 cm and \>230 cm
* Related to NICCOMO monitoring:

  * Rhythmic anomaly: Atrial fibrillation, tachycardia with heart rate \>200 beats per minute.
  * Aortic disease: Severe sclerosis
  * Valve dysfunction: severe aortic regurgitation,
  * Patient with aortic prosthesis or pacemaker using impedance detection.
  * Occurrence of tachycardia \> 200 bpm, occurrence of hypertension with mean arterial pressure \>130 mm Hg
  * Simultaneous use of electrical cauterization systems during surgical procedures
  * Septic shock
* Related to CLEARSIGHT monitoring

  * Patient with prior distal perfusion disorders
  * Lower extremity obliterans arteritis stage IV