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.