Measuring cardiac output (CO) is an essential part of cardiovascular monitoring for patients who are critically ill, have undergone surgery, or have cardiovascular disease. Changes in CO are used to identify a change in the hemodynamic status or to ascertain the need for or efficacy of treatment in critically ill patients and perioperatively in patients at high risk for morbidity and mortality. The longterm standard for monitoring CO is pulmonary artery catheter (PAC) thermodilution; however, this technique has been increasingly criticized because it is invasive and it is not clear whether the risks outweigh the benefits. Alternative, less invasive technologies have been introduced into clinical practice.