In a recent study (Vignon 2017), respiratory variations of the diameter of superior vena cava had a greater diagnostic accuracy than pulse pressure variations and inferior vena cava respiratory variations in 540 medical and surgical ICU patients. But this indicator has not been investigated yet in cardiac surgery. The investigator aim to study respiratory variations of superior vena cava for predicting fluid responsiveness after cardiac surgery.

In post-operative cardiac surgery patients with hemodynamic failure, the investigator will measure respiratory vena caval variations with TEE (Trans oesophageal echocardiography). Then, the investigator will evaluate fluid responsiveness after a fluid challenge (Trendelenburg).

A ROC curve will be constructed in order to assess the optimal sensitivity and specificity of this parameter.

Inclusion Criteria:

• Hemodynamic instability with hypotension,
• Low cardiac output or norepinephrine requirements in the first 6 post-operative hours.

Exclusion Criteria:

• TEE (Trans oesophageal echocardiography) contra-indication
• Trendelenburg contra-indication

patients with hemodynamic instability and hypotension