Contrary to "immobility" and, to a lesser extent, to the "hypnosis/unconsciousness" component of general anaesthesia, the monitoring of "analgesia" remains largely elusive, evaluated mainly through poorly sensitive and potentially undesirable changes in patients' vital signs. This has led the industry to pursue the development of various devices and indices based on other physiological parameters such as heart rate variability (HRV), electroencephalogram (EEG), skin conductance, to name only a few. To the best of the knowledge, none of these parameters on its own has shown sufficient capacity in detecting different degrees of pain/analgesia balance to gain wide clinical use.

The purpose of this prospective observational study is to evaluate the response of a single-parameter index (the Analgesia Nociception Index [ANI]) and a multi-parameter index (the Nociception Level [NoL] Index) when patients under combined general anaesthesia/epidural anaesthesia for laparotomies are subjected to a standardized painful stimulus (a tetanic stimulation over the ulnar nerve at 70 mA, 100 Hz for 30 seconds) at different doses of remifentanil infusion at steady state (0,005 mcg/kg/min; 0,05 mcg/kg/min; 0,1 mcg/kg/min; 0,15 mcg/kg/min). With the painful stimulus held constant but the analgesia provided gradually increased, it is expected to characterize the response of these two indices to different levels of nociception/anti-nociception balance.

Patients scheduled for elective abdominal surgery by laparotomy (general surgery or gynaecological surgery) will receive a thoracic epidural (around T9-T10/T10-T11) and general anesthesia. Apart from standard monitoring, the ANI (derived from the EKG) and the NoL index (obtained via a finger probe) will be recorded. The epidural catheter will be injected with 6-10 ml of lidocaine 2% with epinephrine 5 mcg/ml to produce anesthetic level of neuraxial blocade and then be perfused with the same solution at 4-8 ml/hour. After incision and confirmation of adequate blocade with the epidural local anesthetic, the patient will receive a series of painful electric stimulation with a standard nerve stimulator applied over the ulnar nerve in tetanic mode for 30 seconds at 100 Hz and 70mA. Each stimulation will occur at a different rate of infusion of IV remifentanil at steady state (0,005 mcg/kg/min; 0,05 mcg/kg/min; 0,1 mcg/kg/min; 0,15 mcg/kg/min). A stimulation at 0,005 mcg/kg/min will be done before and after epidural loading with local anesthetics to look for altered variations in analysed parameters. The variation and the kinetic of the variations after stimulations will be observed for heart rate, mean blood pressure, BIS, ANI and NoL index.

Inclusion Criteria:

• ASA status I, II or III
• Elective abdominal surgery with median laparotomy under general anaesthesia and epidural analgesia.

Exclusion Criteria:

• Coronary artery disease
• Serious cardiac arrhythmias (including atrial fibrillation)
• Patient refusal
• History of substance abuse
• Chronic use of psychotropic and/or opioid drugs
• Use of drugs that act on the autonomic nervous system (including β-blockers)
• History of psychiatric diseases or psychological problems
• Contraindications to epidural analgesia
• Allergy to remifentanil
• Unexpected difficult airway requesting excessive, possibly painful airway manipulations.
• Dural puncture during epidural catheter installation
• Failure of epidural analgesia
• Surgical unexpected complications requiring strong haemodynamic support (transfusions, vasopressors, inotropes)

Patients scheduled for elective abdominal surgery by laparotomy (general surgery or gynaecological surgery) who will receive a thoracic epidural (around T9-T10/T10-T11) and general anesthesia.