The goal of the study is to determine Eadyn as a functional measure of arterial load, in conjunction with other actual afterload indices, derived from both invasive arterial pressure tracing and non-invasive signals, such as arterial saturation, non-invasive cardiac output obtained by bio-reactance, and non-invasive stroke volume. A secondary aim is the correlation of the different non-invasive signals with the invasive arterial pressure tracing characteristics.

Fifteen patients, who provide written informed consent, will be included. All of them are scheduled for a major surgical intervention, in whom standard haemodynamic monitoring includes arterial pressure monitoring.

Non-invasive wired patches, connected to a Cheetah cardiac output monitor, will be installed, as well as traditional haemodynamic monitoring (ECG, SaO2, non-invasive blood pressure). A computer link between a Philips monitor (MP50 or MP70) is made, to obtain selected curves in a digital format for post-intervention processing. After awake insertion of an arterial catheter in the radial or brachial artery and induction of anaesthesia, patient becomes intubated and mechanically ventilated.

Before surgery, following data will be registered for 5 min.: ECG, invasive arterial pressure tracing, cardiac output (Cheetah monitor), SaO2 tracing, arterial pressure tracing and pulse pressure and stroke volume variation (Nexfin).

When SVV > 15%, the operation table will be put in Passive Leg Raising (PLR) to optimize the filling status of the patient (test of preload dependency). When positive haemodynamic reaction is present, the patients will obtain a rapid infusion bolus of 250 ml of crystalloids.

Inclusion Criteria:

• All patients scheduled for a major surgical intervention, in whom standard haemodynamic monitoring includes arterial pressure monitoring.

Exclusion Criteria:

• Minor surgery and absence of arterial pressure monitoring