Respiratory Variation of Carotid Doppler Peak Velocity (CDPV) for Non-invasive Prediction of Fluid Responsiveness

Inclusion Criteria: * Age 18-80 years * written informed consent * scheduled major abdominal surgery Exclusion Criteria: * Age \<18 or \>80 years * pregnancy * SIRS or sepsis * any kind of cardiac arrhythmia * known valve disease * known heart failure * any kind of known carotid stenosis * carotid doppler peak velocity \>182 cm/s before baseline measurement (expected stenosis) * missing indication for invasive arterial blood pressure monitoring (IBP) not related to the study * peripheral artery disease (PAD) * BMI \> 35 kg/m2 * intraabdominal hypertension * ASA-PSC of 4 * severe lung disease (e.g. COPD grade 3, fibrosis) * esophageal disease of any kind * participation in another clinical study