Long Time Interval Analysis for Non Invasive Measurement of Cardiac Output

Inclusion Criteria: * Adult (18 years or older) * Undergoing hepatobiliary surgery (open or laparoscopic) requiring general anesthesia as part of the patient's clinical care * Attending anesthesiologist plans to place an intra-arterial catheter for routine care * Attending anesthesiologist believes that non-invasive cardiac output monitoring would benefit the patient Exclusion Criteria: * Minors (17 years or younger) * Parturient (pregnant women) * Cognitively impaired * Unable to sign informed consent form * Prisoner * Prior esophageal surgery * Moderate to severe aortic regurgitation * Mechanical cardiac support (e.g., intra-aortic balloon pump) * Severe, persistent arrhythmias