The aim of the study is to assess the accuracy of a new paradigm in measuring the level of nociception during lumbar surgery. The paradigm is based the measurement of the baroreflex in response to noxious stimuli in anesthetized patients.

The study aims at exploring the changes in the baroreflex induced by 3 noxious stimulation in anesthetized patients: tracheal intubation, tetanic stimulus, and surgical incision. Patients are randomly assigned to one of 2 groups: remifentanil-2 or remifentanil-4, corresponding to the target effect site concentration of this drug delivered through a TCI system. Other drugs included propofol and cisatracurium.

The paradigm exploring the baroreflex collects information continuously recorded of the RR intervals and the changes in systolic arterial pressure, before and during noxious stimuli. The hypothesis is that this paradigm explores nociception as the group with smaller doses in remifentanil will have more cardiovascular reactions in response to noxious stimuli.

Inclusion Criteria:

• ASA classification I-II
• undergoing lumbar surgery for discal hernia
• body mass index <30 kg/m2

Exclusion Criteria:

• ASA classification III-IV
• cardiac history that precludes RR recording : atrial fibrillation, arterial hypertension, active cardiac pacing, chronic treatment with beta-blockers, alpha-blockers, calcium-blockers and angiotensin receptors inhibitors
• diabetes
• regular intake of cocaine, alcohol
• pregnancy