Circulatory failure generates hypoxia and leads to accumulation of reductive species in tissue and circulatory failure monitoring tools are needed but rare. Cardiopulmonary bypass (CPB) is known to promote brief circulatory failure during its initiation. In the present study, the Inventors demonstrate a correlation between whole blood redox potential and circulatory failure during (CPB). They made a prospective study with 17 patients eligible for cardiac surgery with cardiopulmonary bypass. They demonstrated a frank reduction of the whole blood redox potential during circulatory failure during the initiation of CPB. They also demonstrated that they were able to classify patients in 3 groups, one of them presenting an unfavorable post-operative outcome. Accordingly, the present invention relates to a method of diagnosing acute circulatory failure in a patient comprising determining the level of redox potential in a sample obtained from said patient, wherein the level of redox potential indicates whether the patient suffers or not from an acute circulatory failure.