The studied population will be all patients receiving methylene blue for refractory vasoplegic shock (VS) after Cardiopulmonary Bypass (CPB). Refractory VS is defined as follow: a dose of norepinephrine > 0.5µg/kg/min to obtain a mean arterial pressure of 65-75 mmHg with a normal or increase cardiac (> 2 L.min-1.m-2). Patients will be included in the investigator's 20-bed adult cardiothoracic intensive care unit (ICU) in a tertiary teaching hospital (Hopital Cardiologique Louis Pradel, Hospices Civils de Lyon).

Each of the variable related to macro- and microcirculatory will be evaluated before and one hour after start of MB infusion.

Macrocirculatory parameters will include :

• Mean, systolic, diastolic arterial, heart rate (HR)
• Cardiac output (CO), sequential vascular response (SVR) and stroke volume (obtained with transthoracic echocardiography or transpulmonary thermodilution catheter if available).
• A passive leg raising (PLR) test will be performed before and one hour after MB infusion. Patients will be classified as fluid responder if they had a CO of at least 10% after PLR.

Microcirculatory parameters will include :

• Cutaneous and gingival refill time
• Tissue oxygen saturation combined with vascular occlusion test (VOT)
• Video microscopy measurements of the sublingual microcirculation
• Metabolic variables: Arterial blood gases including arterial lactate and central venous blood gases.

The resaturation speed during reperfusion is defined as maximum StO2 - minimum StO2 /time of reperfusion. Tissue oxygen saturation (StO2) will be measured by near-infrared reflectance spectroscopy (NIRS) optode applied to the medial surface of the left or right forearm, 5 cm below the elbow. The sensor will be connected to the four-wavelength O3 Regional Oximetry device (Masimo Incorporation). All StO2 values will be recorded continuously and read every second. Data will be recorded online, transferred to a laptop with a specific software designed by Masimo (Masimo Instrument Configuration Tool (MICT) Version 1.0.4.9), and stored for further analysis. After completion of a baseline set of measurements for each patient, a vascular occlusion test (VOT) will be then perform using a manual pneumatic cuff inflator (Spengler SAS, Antony, France) positioned at the upper extremity of the ipsilateral upper limb.

Proportion of Perfused Vessel (PPV), Total Vessel Density (TVD), Perfused Vessel Density (PVD), Microcirculatory Flow Index (MFI) and heterogeneity index will be reported. Video microscopy measurements will be collected before and one hour after the beginning of MB infusion.