The investigators designed this study to determine the predictive value for predicting fluid responsiveness of noninvasive evaluation of respiratory variation of peak velocity in brachial artery, in mechanically ventilated patients with acute circulatory failure.

Predicting the hemodynamic response to fluid administration (or fluid responsiveness) in critical ill patients is still a matter of concern, since fluid overload could worse the clinical situation of these patients. Parameters of fluid responsiveness usually require an invasive monitoring (like arterial pulse pressure variation).

We hypothesize that noninvasive evaluation of respiratory variation of peak velocity in brachial artery using Doppler ultrasound could provide a feasible estimation on fluid responsiveness in mechanically ventilated patients with acute circulatory failure.

Inclusion Criteria:

• Patients with controlled mechanical ventilation, equipped with an indwelling radial artery catheter and for whom the decision to give fluids will be taken because the presence of one or more clinical signs of acute circulatory failure:

  • systolic blood pressure <90 mmHg (or a decrease >50 mmHg in previously hypertensive patients)
  • the need of vasopressor drugs
  • oliguria (urine output <0.5 ml/kg/min for at least 2 h)
  • tachycardia
  • delayed capillary refilling
  • the presence of skin mottling

Exclusion Criteria:

• Contraindication for the volume administration: evidence of fluid overload and/or of hydrostatic pulmonary edema
• Patients with instable cardiac rhythm

Critical ill patients with controlled mechanical ventilation and acute circulatory failure.