The primary goal of this study was to assess the ability of PI changes induced by a Vt challenge or fluid challenge test to detect preload responsiveness in critically ill adult patients on mechanical ventilation with low-tidal volume.

The secondary aims were (i) to compare the changes in PI with VTI during a Vt challenge test (ii) to compare the changes in VTI and PI during a volume expansion challenge test, (iii) to compare the Vt-induced changes in PI to changes induced by fluid challenge test, (iv) to compare the Vt-induced changes in VTI to changes induced by fluid challenge test.

Inclusion Criteria:

Patients of both genders aged 18 years or older Invasive mechanical ventilation in assist controlled mode with a Vt of 6 mL/kg PBW, Patients with circulatory failure Decision by the clinicians in charge to assess preload responsiveness through a Vt Challenge and a Fluid Challenge test.

Patients in sinus rhythm

Exclusion Criteria:

Open chest Pulmonary hypertension Poor lung compliance (Crs <30 ml/cmH2O) Obesity (BMI≥35) Pregnancy Intraabdominal hypertension Prone position Atrial fibrillation

The study will be conducted on 75 mechanically ventilated patients with circulatory failure.