Central Venous Pressure Change With Volume Challenge in Patients With Hemodynamic Instability

Inclusion Criteria: * mechanically ventilated patients * acute circulatory failure defined by the need of vasopressive drugs (dopamine \>5µg/kg/per minute or norepinephrine) with signs of tissue hypoperfusion (e.g., altered mental state, mottled skin, urine output below 0.5 ml per kilogram of body weight per hour during at least 2 h). * admission serum lactate level more than 2.5mmol/L Exclusion Criteria: * severe hypoxemia defined as a ratio of arterial oxygen pressure to fraction of inspired oxygen \[PaO2/FiO2\] \< 100 mmHg