Minimal Volume for a Fluid Challenge in Septic Patients

The administration of fluids intravenously is the first step in the stabilization of patients in septic shock. It is important to give the right dose of fluids, enough to improve the function of the cardiovascular system but not too much to drown the cells in water. In order to know that, the investigators use the \"fluid challenge\" which is the administration of a \"small amount\" of fluid in a short period of time in order to test the cardiovascular response to a dose of fluids. However, it is unclear how \"small\" this amount can be. The aim of this study is to find out the minimum volume required to perform an effective \"fluid challenge\". The investigators hypothesize that the changes in the mean pressure in the cardiovascular system (socalled mean systemic filling pressure or Pmsf) during the administration of small amounts of fluid can be used to detect the minimum volume required to effectively test the cardiovascular system with a fluid challenge. On the other hand, microcirculation may remain impaired despite the stabilization of the macrocirculation. The microcirculation can be much more dynamic than the macrocriculation, so it is essential to observe the changes of both over time. Patients admitted at the investigators\' intensive care unit (ICU) due to sepsis can be included in this study. The sample will be divided in 4 groups according to the volume of the fluid challenge: 2 ml/kg, 3 ml/kg, 4ml/Kg and 5 ml/kg. The investigators will measure the Pmsf non-invasively using a pneumatic tourniquet inflated during one minute. Invasive arterial blood pressure will be observed during the inflation of the cuff in the arm to determine the Pmsf. The least significant change of Pmsf using this method is 14%. The minimal volume will be that one that achieve that change in Pmsf.