A group of engineers at Michigan State developed a novel waveform analysis technique ("Long Time Interval Analysis" [LTIA]) that attempts to estimate cardiac output non-invasively. Retrospective comparison of LTIA to invasive techniques (e.g. thermodilution) suggest acceptable agreement. Thus, a prospective trial of LTIA is warranted. This study compares LTIA to a validated measure of cardiac output - esophageal Doppler monitoring.

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