Acute circulatory failure reduces oxygen delivery below cellular requirements, potentially leading to organ failure. Intravenous fluids are generally administered with the aim of increasing cardiac output and restore organ perfusion. Nevertheless, only 50% of patients increase their cardiac output, while in the remainder not only does fluid loading provide no benefit but it also leads to volume overload (peripheral and pulmonary edema).

There are two types of resuscitation fluids, colloids and crystalloids. Given their oncotic pressure, colloids should remain in the intravascular space, while crystalloids distribute into the whole extracellular compartment, potentially increasing the risk of tissue edema. Surprisingly, only few studies directly compared albumin and crystalloids in terms of their overload-related side effects.

Electrical impedance tomography (EIT) is a noninvasive, radiation-free, lung imaging modality, which shows lung impedance as determined by small electrical currents. An increase in intrapulmonary gas volume increases impedance, while an increase in blood or fluid volume, lowers it. EIT has a high temporal resolution, allowing to assess ventilation and perfusion in real-time. Preliminary data suggest its value to assess the variations of intrathoracic fluid in patients with pulmonary edema.

The aim of the present single-blind, randomized, controlled study is to compare the effect of a fluid challenge with albumin vs. crystalloids on EIT-derived lung impedance in a group of 56 critically ill patients with acute circulatory failure. Our hypothesis is that fluid challenge with albumin leads to a lesser decrease in lung impedance, that is a lesser extravasation of fluids into the lungs.

Hemodynamic and respiratory variables, blood samples, cardiac ultrasound and EIT measurements will be recorded before the fluid challenge, and repeated at the end of fluid infusion, 20 and 60 minutes after. Factorial Analysis of variance for repeated measures will be used to assess the effects of fluid loading

Inclusion Criteria:

• Patients >18 years with acute circulatory failure of any cause, receiving mechanical ventilation using volume-assisted control mode, without any spontaneous breathing activity, scheduled for a fluid bolus by their treating physician will be enrolled. Patients will only be included in the study once.

Acute circulatory failure will be defined as the presence of a systolic blood pressure (SBP) ≤90 mmHg or mean arterial pressure (MAP) ≤70 mmHg or requiring vasopressors to maintain SBP >90 mmHg or MAP >70 mmHg, along with one or more of the following: 1) urinary flow ≤0.5 mL/kg/h for ≥2 hours, 2) heart rate ≥100 beats per minute, 3) presence of skin mottling, 4) blood lactate concentration ≥2 mmol/L, 5) oxygen saturation in the central venous blood <65%.

Exclusion Criteria:

• Patients having right ventricular dysfunction, anuria, pregnancy, presence of pneumothorax or lung emphysema, previous history of severe chronic obstructive pulmonary disease (GOLD III-IV) or contraindications to the use of EIT (e.g., presence of pacemaker or automatic implantable cardioverter-defibrillator) and impossibility to place the EIT belt in the right position (e.g., presence of surgical wounds dressing) will be excluded