Intraoperative fluid management is key component of care for patients undergoing surgery. Hypovolemia and hypervolemia both associate with increased morbidity, length of stay in the intensive care unit and mortality. Thus, maintaining adequate intravascular volume yet avoiding fluid overload is crucial to achieve optimal outcomes. Goal-directed fluid therapy based on arterial pressure waveform analysis is widely used for intraoperative fluid management and have been shown to improve surgical outcomes compared with conventional clinical assessment in several studies. However, dynamic indices of arterial pressure waveform analysis such as pulse pressure variation (PPV) and stroke volume variation (SVV) are altered by certain situations including elevated intra-abdominal pressure and Trendelenburg position. Intravascular fluid status might thus be misinterpreted. Carbon dioxide pneumoperitoneum with increased intra-abdominal pressure and Trendelenburg position are commonly seen in laparoscopic surgeries including colorectal, gynecological, and genitourinary procedures. Understanding how dynamic indices change in these clinical situations are essential for achieving appropriate intraoperative fluid management. This study focus on identifying the effects of different levels of intra-abdominal pressure and angles of Trendelenburg position on dynamic indices of arterial pressure waveform.

In this prospective observational study, the investigators will enroll 100 patients undergoing laparoscopic surgery for medical reason. Anesthetic management and surgery will be performed as usual clinical practice.

The investigators will record the digital data exported from standard monitoring instruments, including electrocardiography, photo-plethysmography , blood pressure, neurological system information (Bispectral index and Density spectral array ), the respiratory gas monitoring (gas analyzer and respiratory waveform) and dynamic indices of arterial pressure waveform analysis (cardiac index, stroke volume variation, pulse pressure variation, hypotension prediction index... etc.) from HemoSphere advanced monitoring platform with Acumen Hypotension Prediction Index Software (Edwards Lifesciences) every 20 seconds.

After anesthesia induction as routine clinical practice, slow IV fluid infusion will be maintained. The patient will be placed in Trendelnburg position with different angles ranging from 0-25 degree. CO2 pneumoperitoneum will be created by surgeons for laparoscopic surgery. Changes in levels of intra-abdominal pressure and angles of Trendelenburg position will be recorded. The recording is ended after emergence when surgery ends. All physiological data and demographic data will be stored in digital media after being de-linked from personal identification.

Data analysis and Statistics will be particularly performed to explore the effects of levels of intra-abdominal pressure and angles of Trendelenburg position on dynamic indices of arterial pressure waveform. Methods including signal processing, modeling, classification will be used.

Inclusion Criteria:

• Patients aged between 20 and 80
• Scheduled for laparoscopic surgery
• American Society of Anesthesiologists (ASA) physical status I to III.

Exclusion Criteria:

• Neurologic or behavioral disorders
• American Society of Anesthesiologists (ASA) physical status ≥ IV
• History of arrhythmia
• Drug abuse or alcoholism
• Resting room air SpO2 < 90%.

The investigators plan to enroll patients who will laparoscopic surgery under general anesthesia. These patients will be screened for eligibility.