The Total Cavopulmonary Connection (TCPC), a variant of the Fontan operation used for palliative cardiovascular repair of patients with single ventricle physiology, creates a passive system of blood flow into the pulmonary circulation for which energy efficiency may be critical to long term outcome. Clinical studies have shown that reverse flow in the TCPC is an indication of poor clinical status in these patients. Using numerical simulations, we demonstrate that reverse flow leads to increased energy losses in compliant vessels. Such an effect can potentially set off a series of spiraling negative events with decreased ventricular function leading to reverse flow, which causes decreased energy efficiency, which in turn leads to worsening function, and so forth, thereby suggesting one cause of progressive heart failure in this patient group.