• area under ROC curve of pulse pressure variation before volume expansion and their positive and negative predictive values; responders are patients increasing mean arterial pressure [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  

• area under ROC curve of pulse plethysmographic variation before volume expansion and their positive and negative predictive values; responders are patients increasing mean arterial pressure [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of pulse pressure variation before volume expansion, positive and negative predictive values; responders are patients showing any clinical improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of pulse plethysmographic variation before volume expansion, positive and negative predictive values; responders are patients showing any clinical improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of central venous pressure before volume expansion and their positive and negative predictive values; responders are patients increasing mean arterial pressure [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of central venous pressure before volume expansion, positive and negative predictive values; responders are patients showing any clinical improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of central venous saturation before volume expansion, positive and negative predictive values; responders are patients showing any clinical improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of central-arterial PCO2 pressure difference before volume expansion, positive and negative predictive values; responders are patients showing any clinical improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of arterial lactate before volume expansion, positive and negative predictive values; responders are patients showing any clinical improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of fractional excretion of sodium before volume expansion, positive and negative predictive values; responders are patients showing any clinical improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• area under ROC curve of sodium/potassium urinary ratio before volume expansion, positive and negative predictive values; responders are patients showing any clinical improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• comparison between the number of patients with criteria for pulse pressure variation calculation and the number of patients with other predictive variables [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• estimation of variables independently associated to arterial pressure increase [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  
• estimation of variables independently associated to clinically relevant improvement [ Time Frame: one hour after the end of fluid administration ] [ Designated as safety issue: No ]
  

Severe hypotension is a common life-threatening conditions in critically ill patients. Discriminating between the need for volume expansion or inotropic/vasoactive support is a main clinical goal. Pulse pressure variation are an accurate index of fluid responsiveness, but it needs controlled mechanical ventilation with a tidal volume of at least 8 ml/kg and absence of arrhythmias. This could limit its clinical applicability. Moreover pulse pressure variation accurately predicted cardiac output increment. However the cardiac output increase is not a clinically relevant target in absence of low cardiac output. The objectives of the present protocol are: 1) to calculate the accuracy of cardiovascular and renal variables to predict either arterial pressure increase or clinically relevant improvement after fluid administration and 2) to verify how often dynamic indices could be applied in clinical practice. Arterial pressure increase is defined by mean arterial pressure above 65 mmHg or an increment greater than 20 % respect to basal value. Clinically relevant improvement is defined by restoring adequate values of arterial pressure or cardiac index or diuresis or central venous saturation if they are inadequate before fluid administration. These adequate values are 65 mmHg for mean arterial pressure, 2.5 l.min-1.m-2 for cardiac index, 0.5 ml.kg-1.h-1 for the diuresis, 70 % for central venous saturation.