The passive leg raising (PLR) test is a reversible preload challenge of around 300 mL of blood that can be repeated as frequently as required without infusing a drop of fluid. Two recent meta-analyses of many studies have confirmed the reliability of the PLR test to predict fluid responsiveness in patients with acute circulatory failure. Nevertheless, the effects of the PLR must be assessed by the direct measurement of cardiac output since changes in arterial pressure do not allow the assessment of the PLR hemodynamic effects with reliability. Moreover, cardiac output must be measured continuously and in real time.

In clinical practice, cardiac output measurement can be difficult for different reasons such as the unavailability of the echocardiography machine, or the absence of any continuous cardiac output monitoring technique at the time of acute circulatory failure occurrence.

Recently, it was shown that the decrease in pulse pressure variation (PPV) induced by the mini-fluid challenge (100 mL of colloid solution infused in 1 min) was able to predict fluid responsiveness with reliability in patients ventilated with low tidal volume (<8 mL/kg of ideal body weight).

Therefore, the investigators hypothesize that the changes in PPV induced by PLR test could be able to predict fluid responsiveness with reliability in mechanically ventilated patients with acute circulatory failure.

Inclusion Criteria:

• Mechanically ventilated patients

• Patients in whom the physician decides to perform a fluid challenge based on the presence of one of the following criteria:

  (1) systolic arterial pressure <90 mmHg, mean arterial pressure <65 mmHg, or the need for vasopressor infusion; (2) skin mottling; (3) urine output <0.5 mL/Kg/h for more than or equal to 2 hours; (4) lactate concentrations > 2 mmol/L

• Patients in whom the physician decides to perform a PLR test to predict fluid responsiveness

• Arterial catheter should be in place for invasive monitoring of arterial pressure and PPV

Non inclusion Criteria:

• Age <18 yrs
• Pregnancy
• Moribund patients
• Risk of fluid loading induced pulmonary edema
• Cardiac arrhythmias
• Contraindication to perform PLR (intracranial hypertension, pelvic fractures)

Exclusion Criteria :

• Changes in vasopressor and sedation agents dosages during the protocol
• Changes in ventilatory parameters during the protocol

Mechanically ventilated patients displaying acute circulatory failure signs in whom the physician decides to perform a fluid challenge and a PLR test to predict fluid responsiveness.