Key Question 1: What types of devices/techniques are currently used to assess cardiac output? Key Question 2: Does therapeutic management based on esophageal Doppler ultrasound-based cardiac output monitoring during surgery lead to improved patient outcomes (fewer complications and shorter hospital stay), compared to: a. Pulmonary artery catheter-based measurement of cardiac output via thermodilution? b. Catheter-based measurement of central venous pressure? c. Conventional clinical assessment (physical examination, fluid input and output measurements)? Key Question 3: Does therapeutic management based on esophageal Doppler ultrasound-based cardiac output monitoring during hospitalization lead to improved patient outcomes (fewer complications and shorter hospital stay), compared to: a. Pulmonary artery catheter-based measurement of cardiac output via thermodilution? b. Catheter-based measurement of central venous pressure? c. Conventional clinical assessment (physical examination, fluid input and output measurements)? Key Question 4: What complications, harms, and adverse events associated with esophageal Doppler ultrasound-based monitoring have been reported?