Pulse wave Transit time (PWTT) is a parameter calculated from ECG and pulseoximeter. It is supposed to help assessing the preload Status and guide intraoperative fluid therapy. This Project aims to validate the benefit of PWTT assessment in an observational clinical fluid study. In case of hypovolemia, patients will receive a fluid Bolus and hemodynamic data from clinical Standard Monitoring, esophageal doppler and PWTT will be recorded before and after the fluid Bolus. Receiver operating characteristic (ROC) curve will be used to determine the validity of PWTT as indicator for fluid responsiveness and its cut off value.

PWTT is determined by measuring the beginning and the end of pulse wave duration and can be calculated as the time interval from the ECG R-wave peak to the rise point of the pulse oximeter wave.

it consists of two components: pre ejection period (PEP) and arterial pulse wave transit time (a-PWTT). PEP is defined as the time from the ECG R wave to the rise point of the aortic root pressure wave. a-PWTT is defined as the time from the rise point of the aortic pressure wave to the rise point of the pulse oximeter wave. a-PWTT is the component which is directly related to the velocity of the pulse wave. However noninvasively, we can measure only PWTT, which also includes PEP. In general, PEP change over short periods of time is negligible in most cases, so we can assume that PWTT corresponds to

a-PWTT. In studies using several studies using animals and healthy volunteers, PWTT showed good correlation with stroke volume or systolic blood pressure. Also in experimental and clinical setting, it is shown that PEP changes indicate change in preloads.

It is not known at this moment, which of the following factors to be considered in processing raw data to acquire accurate PWTT value on predicting fluid responsiveness.

1. The beginning of pulse wave can be assessed by the appearance of either Q wave, which represents the initial phase of depolarisation going through the interventricular septum or R wave, which represents the ventricular depolarisation in ECG.
2. The end of pulse wave duration can be assessed by peripheral plethysmography mostly from a finger tip but also from an ear lobe.
3. The pulse wave time can be simply measured as it is, but can also be adjusted by heart rate using Bazett-Formula.
4. Not only the simple PWTT but also ventilatory induced fluctuation of PWTT (ΔPWTT) may be used for predicting fluid responsiveness.

Inclusion Criteria:

• Adults 18 - 80 years old
• elective open abdominal surgery
• clinical indication for direct arterial blood pressure Monitoring
• supine Position intraoperatively

Exclusion Criteria:

• under 18 years, over 80 years old
• pregnant
• no informed consent
• systemic inflammatory response syndrome (SIRS) or Sepsis
• Severe cardiac, coronary or vascular disease
• Arrythmia
• Cardiac valves anomaly
• BMI > 35
• American Society of Anesthesiologists (ASA) physical status classification system: 4
• Lung disease that excludes Ventilation with a tidal volume >8ml/kg
• esophageal pathology
• planned prone position surgery

Patients who are scheduled for elective open abdominal surgery