Predicting arterial lactate and blood gas values with sufficient accuracy by simply analyzing central venous blood would be interesting in intensive care unit patients in whom the insertion of an arterial catheter or arterial punctures could be avoided. This prospective study aims at externally validating published mathematical models built to predict arterial values from central venous blood analysis.

Mathematical models for predicting arterial blood gas and arterial lactate have recently be published (PMID 27543529; see the citations section). The aim of the study is to externally validate these models in a multicenter (11 centres in France) cohort of intensive care unit patients suffering from acute circulatory failure. For the peru pose of the study, patients will undergo concomitent central venous/arterial blood sampling from 1 to 4 times during their intensive cafe unit stay. Clinical characteristics of the patients and central venous blood gas and lactate values will be used to predicted arterial values according to previously published formula. Predicted and actual arterial values of each variable of interest (pH, PaCO2, SaO2, lactate) will then be compared.

Inclusion Criteria:

• age>18 years
• patient admitted in a intensive care unit
• patient already carrying an indwelling arterial catheter and a superior vena cava central catheter
• patient presenting an acute circulatory failure defined by the presence of at least two of the following items: a) Persistent hypotension (systolic arterial blood pressure < 90 mm Hg or mean arterial pressure < 65 mm Hg) or need for continuous intravenous vasopressor treatment (at any dose). b) Presence of at least one of the following signs of hypoperfusionfusion :Change in mental status ; Skin mottling ; Oliguria (Urine output <0.5 ml/kg body weight for at least one hour); Arterial lactate > 2 mmol/l or peripheral venous lactate > 3.2 mmol/l; ScvO2 <70%.
• Routine medical management requires central venous blood sampling for ScvO2 measurement and arterial blood sampling for gazometry.

Exclusion Criteria:

• Opposition to participation in the study
• Impossibility to take a concomitant arterial and venous gazometric sample.
• Patient treated with ECMO (extracorporeal membrane,oxygrnation), intermittent dialysis or continuous dialysis during sampling
• Patient hospitalized without consent and/or deprived of liberty by court decision.

Studied patients will be adult patients admitted to intensive car unit and presenting signs of acute circulatory failure. Patients should carry arterial and central venous catheters for their medical needs and not exclusively for the purpose of the study.