Volume expansion is the cornerstone of perioperative hemodynamic optimization. The main objective of volume expansion is to increase and maximize stroke volume. Hemodynamic changes have an impact on the autonomic nervous system. The analysis of heart rate variability allows an exploration of the autonomic nervous system and could therefore provide information on the effect of volume expansion. The Analgesia Nociception Index (ANI) is an analgesia monitor based on the concept of heart rate variability. By deviating from its original use, the investigators wish to evaluate the ability of ANI to identify a response to volume expansion.

Perioperative optimization is based on volume maximization using volume expansion. The main objective of volume expansion is to increase stroke volume. The Franck-Starling curve is schematically divided into two portions: a vertical portion which mean that an increase in preload secondary to volume expansion will induce an increase in stroke volume; and a flat portion where volume expansion will not induce an increase in stroke volume. Perioperative optimization is based on stroke volume maximization using volume expansion.

In case of hypovolemia, there is a compensatory sympathetic stimulation with parasympathetic withdrawal. Volume expansion will lead to an increase in venous return and therefore in cardiac output, which will result in a decrease in sympathetic tone. The autonomic nervous system is therefore directly affected by these load changes. The analysis of heart rate variability, which has been studied for several years in various setting, allows the assessment of autonomic nervous system, through monitoring of the electrocardiogram. Experimental and clinical studies report the ability of variations in heart rate variability to detect a situation of hypovolemia. In anaesthesia, heart rate variability is used to evaluate the nociception-antinociception balance. Indeed, any nociceptive stimulation leads to an increase in sympathetic tone. MetroDoloris has developed a non-invasive monitor that provides an index, the Analgesia Nociception Index (ANI), to evaluate the nociception-antinociception balance by analysing the autonomic nervous system.

In this study, the original use of ANI will be diverted from its initial use, to analyse the autonomic nervous system's response to volume expansion. Thus, the aim of the present study is to determine whether a change in Analgesia Nociception Index can track the stroke volume effects of a volume expansion using 250ml of crystalloid in the operating room.

The follow up will be restricted to the duration of surgical intervention. The last data will be collected 5 minutes after the end of volume expansion.

Inclusion Criteria:

• patient older than 18 years
• patient scheduled for neurosurgery in prone position
• patient equipped from an arterial catheter and stroke volume monitoring
• Patient for whom volume expansion is needed.

Exclusion Criteria:

• emergency surgery,
• cardiac dysfunction,
• arrythmia,
• beta-blockade therapy,
• pacemaker,
• intracranial hypertension,
• pregnancy,
• dysautonomia

Patient scheduled for neurosurgery