The intraoperative assessment of analgesia remains problematic and no analgesia monitor is used in routine practice.

The intraoperative assessment of analgesia remains problematic and no analgesia monitor is used in routine practice. Currently, this assessment is based on hemodynamic changes, although hypertension or tachycardia is neither sensitive nor specific. An overdose in opioids exposes to many side effects ranging from respiratory depression to phenomena of post-operative hyperalgesia. Conversely, an under-dosage in opioids is at the origin of a nociceptive stress whose neurovegetative and hormonal response can aggravate post-operative morbidity or promote the occurrence of implicit memorization.

Nociceptive influx can not be detected directly. However, its ascent to the somesthetic area of the cerebral cortex leads to physiological reactions that can be evaluated. In particular, there are numerous anatomical and functional interactions between the autonomic nervous system and the nociceptive tract in the brain stem. These interactions are mainly at the level of the gray periqueducal substance, the rostral ventro-lateral bulbous region, the solitary tract nucleus and the hypothalamus. The nociceptive influx thus leads to changes in the activity of the ANS. The study of changes in the ANS would therefore be a method of assessing the intensity of the nociceptive influx.

This objective method can be used in the non-communicating or unconscious patient to evaluate the pain and to adapt the analgesic treatment to the best.

Obese patients who receive surgical management often have comorbidities such as restrictive syndrome, or obstructive sleep apnea syndrome. They thus represent a target of choice in perioperative analgesic optimization in order to avoid respiratory complications that may be related to over or under assay of morphine such as atelectasis, alveolar hypoventilation.

It is hypothesized that this device is more effective for the intraoperative evaluation of analgesia than the usual method based on hemodynamic changes in intraoperatively anesthetized patients for bariatric surgery.

Inclusion Criteria:

• Major Patient Age ≥ 18 years
• Adult patients candidate for planned surgery with BMI> 30.
• Affiliation to social security.
• Patients who were clearly informed and signed consent.

Exclusion Criteria:

• Patient with a heart rhythm disorder.
• Respiratory rate less than 9 cycles / min.
• Irregular spontaneous ventilation.
• Perioperative use of a drug acting on the sinus node (eg atropine, etc.)
• History of neuropathic pain.
• Patient under morphine until the day before surgery.
• Patients under guardianship or curatorship or deprived of liberty