Venous return is driven by the pressure gradient between mean systemic pressure and right atrial pressure. However, mean systemic pressure is more a physiological concept than an available clinical measurement. Indeed, the mean systemic pressure is the one that would be measured anywhere within the vascular system after cardiac arrest. Recent advances in hemodynamic monitoring have made MSP available to the clinician through beat-by-beat cardiac output measurements during respiratory pauses under mechanical ventilation. In this way, it is possible to calculate MSP, the pressure gradient and the venous return resistances. The investigators aim is to explain the hemodynamic variations and the decrease in cardiac output observed during prone position. The investigators hypothesis is that venous return resistances are increased during prone position probably following an increased intra-abdominal pressure.

Inclusion Criteria:

• Prone position neurosurgery
• High-risk neurosurgery

Exclusion Criteria:

• Emergency surgery
• Pregnancy

Patient scheduled for neurosurgery in the prone position