Patients age ≥ 16 years, from both genders, admitted to the HCPA ICU, receiving invasive mechanical ventilation, with hemodynamic instability defined as need for norepinephrine infusion and/or intravascular fluid administration to maintain systolic arterial blood pressure > 90 mm Hg; with arterial line in place (radial or femoral) and pulmonary arterial catheter in place.