Acute circulatory failure is frequent, affecting up to one-third of patients admitted to intensive care units (ICU). Monitoring hemodynamics and cardiac function is therefore a major concern. Analysis of respiratory diameter variations of the superior vena cava (SVC) is easily obtained with transesophageal echocardiography (TEE) and is helpful to assess fluid responsiveness.

Transthoracic echocardiography (TTE) exploration of the SVC is not used in routine. Recently, micro-convex ultrasound transducers have been marketed and these may be of use for non-invasive SVC flow examination. However, analysis of diameter variations of the SVC with TTE does not seem to be possible since the approach from the supraclavicular fossa does not allow for a good visualization of the SVC walls.

It was recently demonstrated in a short pilot study that TTE examination of the SVC flow with a micro-convex ultrasound transducer (GE 8C-RS) seems both easy to learn and to use (feasibility = 84.9%), and is reproducible in most ventilated ICU patients with an intraclass correlation coefficient for the systolic fraction of the superior vena cava flow of 0.90 (95% confidence interval [0.86-0.93]).

The hypothesis is that cardio-respiratory interactions in intubated-ventilated patients are responsible of SVC flow variations and that the analysis of the SVC flow respiratory variations could be a new predictive tool of fluid responsiveness.

Inclusion Criteria:

• Adult patients (≥ 18 years old)
• Admission in ICU after tracheal intubation or tracheal intubation during the ICU stay
• Volume-controlled ventilation with a tidal volume of 8 mL/kg
• Patient or family agreement for the inclusion

Exclusion Criteria:

• Persistence of spontaneous breathing
• Cardiac arrhythmia
• Severe Acute Respiratory Distress Syndrome, defined as PaO2/FIO2 ratio < 100
• Acute right ventricular failure defined by S'VD < 10 cm or Tricuspid Annular Plane Systolic Excursion (TAPSE) < 10 mm measured with Transthoracic Echocardiography (TTE)