The objective of the present research is a combination of a one-sided test of non-inferiority and a one-sided test of superiority. A stepped approach will be used to evaluate these hypotheses:

1. a less invasive intervention (i.e., no indwelling arterial catheter insertion until felt absolutely needed, according to consensual and predefined safety criteria) is non inferior to usual care (i.e., systematic indwelling arterial catheter insertion in the early hours of shock) in terms of mortality at day 28 (non-inferiority margin of 5%).
2. a less invasive intervention is not only non-inferior but also superior to usual care in terms of mortality.

Multi-centre, pragmatic, randomised, controlled, open, two-parallel group, non-inferiority clinical trial.

Inclusion Criteria:

• Age ≥ 18 years the day of inclusion

• Existence of an acute circulatory failure defined by the presence of the following items 1 and 2:

  1. Persisting hypotension (systolic blood pressure less than 90 mmHg or mean arterial blood pressure less than 65 mmHg) for more than 15 min at intensive care unit admission or within the following 24 hours, OR requirement of continuous intravenous vasopressor treatment (i.e. any dose of norepinephrine / epinephrine)
  2. Presence at least one of the following signs of hypoperfusion: alteration of mental status; skin mottling; oliguria defined as a urine output < 0.5 mL/kg body weight for at least one hour; arterial lactate > 2 mmol/L; peripheral venous lactate > 3.2 mmol/L; ScvO2 <70%

• Free express oral and informed consent of the patient or a proxy in case of impossibility for the patient to consent; emergency inclusion possible when legal representatives and patient's family are not available

• French health insurance holder

Exclusion Criteria:

• Acute circulatory failure, as defined by items 1 and 2 in inclusion criteria list (cf. supra) present for more than 24 hours
• Non invasive blood pressure (NIBP) device fails to display a blood pressure value, or cuff placement impossible
• Patient for whom an Extra-Corporeal Membrane Oxygenation (ECMO) therapy (either veno-arterial or venous-venous) is already in place or is to be initiated within the next 6 hours
• Patient treated with vasopressor doses of more than 2.5 μg/kg/min of norepinephrine tartrate plus epinephrine for at least 2 hours (i.e., for instance, more than 8 mg of norepinephrine tartrate in 50 mL at the rate of 66 mL/hour for a patient weighing 70 kg) (please note that in fact this dosage corresponds to 1.25 μg/kg/min of norepinephrine base)
• Severe traumatic brain injury (i.e., traumatic brain injury with a Glasgow coma scale score of less than 9 before sedation)
• Patient previously included in the trial
• Body mass index (BMI) above 40 kg/m2
• Pregnancy
• Brain death
• Moribund patient
• Patient known, at time of inclusion, as being under guardianship, authorship or curators