Study of Changes on Mean Systemic Filling Pressure (Pmsf) and Microcirculation After a Fluid Challenge in Septic Patients: Looking for the Minimal Volume

PURPOSE AND DESIGN The main objective of this study is to find out the minimum volume of fluid required to perform an effective "fluid challenge" in septic patients. An effective fluid challenge would be one that is able to increase the mean pressure in the cardiovascular system when there is no blood circulation or socalled the mean systemic filling pressure (Pmsf). It is similar to the pressure into a pneumatic tyre, which is only related to the volume of air and the elastic properties of the tyre wall. In order to do this, the investigators will observe the changes in mean systemic filling pressure after small amounts of fluids so that the investigators will be able to realise how much volume is required to increase that pressure.

To assure that the change in Pmsf is a real change, the investigators have conducted a pilot study in order to determine the precision of the Pmsf measurement using the Pmsf-arm technique. The least significant change is about 14 % from baseline measurement. The Pmsf-arm technique is a process very similar to that the investigators use to measure non-invasive blood pressure. The investigators will measure the arterial-venous equilibrium pressure in the arm after a rapid and brief interruption of blood circulation. In order to do this, a totally noninvasive automatic tourniquet is attached to the arm where the arterial line has been inserted and the tourniquet is inflated during 60 seconds. This process will be done at baseline and after the fluid challenge. The second objective is observe the microcirculation during an affective fluid challenge and correlate the changes observed with the changes in the cardiac function and the Pmsf.

The third objective is to observe the changes generated by the vasoactive therapy in the macro and microcirculation according to the cardiac response and the type of vasoactive therapy used.

In the investigators' clinical practice, the investigators have observed that in some cases the infusion of 250 ml of fluid has no impact at all in the cardiovascular function. The investigators believe that in those cases the investigators may have not given enough fluid to increase the Pmsf and test the cardiovascular response.

RECRUITMENT The sample will be recruited by convenience. It is important to understand the difference between fluid resuscitation and fluid challenge. The fluid resuscitation requires normally large amount of volume in a medical emergency in order to achieve cardiovascular stability. Once the patient is minimally stable, a fluid challenge is normally carried out in order to find out if the patient may benefit from more fluids. As the performance of a fluid challenge is part of the normal clinical practice once the patient has been admitted to ICU, there is minimal extra risk from participation in this study.