Volume expansion is the cornerstone of perioperative hemodynamic optimization. The main objective of volume expansion is to increase and to maximize stroke volume. Despite national and international recommendations, stroke volume monitoring is clearly not widely adopted. This is mostly due to the cost and the invasiveness of the devices allowing stroke volume monitoring. End tidal carbon dioxide is monitored in all patients undergoing general anaesthesia and is totally non-invasive. A strong relationship between stroke volume and end tidal carbon dioxide have been already demonstrated. The aim of the present study is to determine if change in End Tidal Carbon Dioxide can assess the stroke volume effects of a volume expansion of 250ml of crystalloid in the operating room

The main objective of volume expansion is to increase stroke volume. 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 which mean that a same increase in preload will not induce an increase in stroke volume. Perioperative optimization is based on stroke volume maximization using volume expansion. Despite national and international recommendations, stroke volume monitoring is clearly not widely adopted. This is mostly due to the cost and the invasiveness of the devices allowing stroke volume monitoring. End tidal carbon dioxide is monitored in all patients undergoing general anaesthesia and is totally non-invasive. Several studies evaluated the relationship between stroke volume and end tidal carbon dioxide and other studies evaluated the ability of changes in end tidal carbon dioxide to assess changes in stroke volume induced by volume expansion with discordant results. These studies included small sample size and were heterogeneous (intensive care unit and nor operating room, volume expansion or passive leg raising, crystalloid or colloid, 500ml and not 250 ml ….). Nowadays, it is not clear if changes in end tidal carbon dioxide can be considered as a surrogate of changes in stroke volume during a volume expansion in operating room patients. Thus, the aim of the present study is to determine if change in End Tidal Carbon Dioxide can assess the stroke volume effects of a volume expansion of 250ml of crystalloid in the operating room.

The follow up will be restricted to the duration of volume expansion. The last data will be collected just after the end of volume expansion.

Inclusion Criteria:

• Patient older than 18 years
• Scheduled for neurosurgery in prone position
• Equipped from an arterial catheter and stroke volume monitoring

Exclusion Criteria:

Patient with cardiac dysfunction and/or lung disease

Patient older than 18 years scheduled for neurosurgery in prone position, equipped from an arterial catheter and stroke volume monitoring in Bordeaux University Hospital