Intraoperative optimization of cardiac output using goal directed fluid challenge is associated with better outcome. Dynamic preload dependence indicators are well validated during surgery in supine position. Positioning from supine to sitting position under anesthesia is responsible of several hemodynamic changes. Predictability and cut-off values of dynamic preload dependence indicators during surgery in sitting position are unknown. The objective of this prospective study is to evaluate ability of three dynamic preload dependence indicators, i.e. PPV (Pulse Pressure Variation), SVV (Stroke Volume Variation) and PVI (Plethysmographic Variability Index), during neurosurgery in sitting position.

The primary objective is to determine PPV, SVV and PVI thresholds for which patients respond to fluid challenge in sitting position.

The secondary objectives are to determine variations of PPV, SVV and PVI measures before and after positioning in sitting position, and to compare these values to each other.

Inclusion Criteria:

• neurosurgery in sitting position
• age > 18

Exclusion Criteria:

• arrhythmias
• severe heart failure
• severe lung disease
• contraindications to esophageal Doppler monitoring probe insertion (i.e. esophageal weakness)
• renal failure