Background: It is still difficult to predict the outcome in patients requiring Fontan Revisions and in those who have evidence of congestive hepatopathy and probable cirrhosis requiring major cardiac surgery including heart transplant. Over the years, many prognostic indices have been derived from laboratory results of blood tests, clinical and physiological variables (or some combination thereof), from liver imaging to liver histology, which has issues of sampling error, medical risks and technical difficulty. None of these have proved entirely satisfactory. Predicting morbidity or survival is particularly important when deciding about Fontan revisions versus the need for heart transplantation. What is needed here is a truly reliable test of liver function that can help predict outcome, on the basis of a single measurement within few days of a planned revision. For this purpose, it is desirable that the chosen tests of liver performance be safe, non-invasive, easy to perform, have a rapid turnaround for results, and be readily repeatable.