Thermodilution Cardiac Output: A Concept Over 250 Years in the Making

The need to quantify blood flow through the heart has led to the development of different techniques for its measurement. The 3 main approaches are the Fick method, dye dilution, and thermodilution techniques. The latter 2 are based on the use of indicators that indirectly quantify blood flow. These have slowly been developed over centuries, from the concept of measuring blood flow, to a technique, and its clinical utility. Thermodilution is the most popular dilution method used for measuring cardiac output (CO) in the clinical setting. The information obtained during this procedure is relevant in the process of clinical decision making in patients with critical illness, valvular heart disease, and congestive heart failure. The technique increased in popularity in the early 1970's after Swan and Ganz invented the pulmonary artery catheter that simplified thermodilution enough to utilize it as a bedside procedure. This was only possible with simple yet clever engineering methods that are not commonly known in the medical community. Despite these advancements, the concept of measuring CO by dilution techniques is one where its optimal use in the clinical setting continues to be an area of investigation. The thermodilution concept and the mechanism of measuring CO will be discussed in the following review.