Background: Predicting preload responsiveness by using dynamic indicators before administering fluids to critically ill patients is nowadays routinely performed at the bedside. Unlike other dynamic indicators of preload responsiveness that require cardiac output monitoring, pulse pressure variation (PPV) can be simply obtained via an arterial catheter . However, PPV is not reliable in mechanically ventilated patients with spontaneous breathing activity. We hypothesized that an increase in PPV after a tidal volume (TV) challenge (TVC) or a decrease in PPV during passive leg raising (PLR) will predict preload responsiveness in such cases.

Objective: to examine if the change in PPV during PLR and after a TVC can predict preload responsiveness in patients with mechanical ventilation and persistent spontaneous breathing

Prospective non interventional study conducted in two intensive care units. Patients under mechanical ventilation with spontaneous cycles, for whom the physician in charge decided to test preload responsiveness were included. We collected demographic and clinical information, the use of mechanical ventilation and its parameters, the use of vasopressors and their dosage and blood lactate.

Firstly, transthoracic echocardiography (TTE) was performed to measure the velocity time integral (VTI) of the left ventricular outflow tract .

A PLR maneuvre is performed with TTE probe in place and a new assessement of VTI is measured, in addition to the PPV and other hemodynamic parmeters (Diastolic, mean and systolic arterial pressure, heart rate, CVP). Delta VTI is the difference between VTI during PLR and VTI at baseline divided by VTI at baseline.

Patients were considered as preload responsive when delta VTI was ≥10%. Secondly, a TVC was performed by increasing the TV by 2mL/kg predicted body weight from its baseline value . PPV was recorded before and after the TVC in addition to the other hemodynamic parmeters listed above. Further more, respiratory parameters are collected: ventilator setting, plateau pressure, upper airway pressure, before and during the TVC.

Inclusion Criteria:

• adult critically ill patients if the clinicians in charge decided to test preload responsiveness because of mean arterial pressure (MAP) < 65 mmHg (measured by an arterial catheter) or other symptoms of acute circulatory failure (e.g. oliguria, skin mottling, tachycardia, hyperlactatemia)

Exclusion Criteria:

• Arrythmia
• no spontaneous breathing
• difficult condition for tansthoracic echocardiography

Patients under mechanical ventilation with spontaneous cycles, for whom the physician in charge decided to test preload responsiveness were included with because of mean arterial pressure (MAP) < 65 mmHg (measured by an arterial catheter) or other symptoms of acute circulatory failure (e.g. oliguria, skin mottling, tachycardia, hyperlactatemia)