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Introduction: Cardiac surgery can be complicated by organ dysfunction resulting in increased hospital length of stay and mortality rates. Several studies have reported microvascular alterations after cardiac surgery but none has investigated the time course of these alterations. Near-infrared spectroscopy (NIRS) associated with a vascular occlusion test (VOT) may provide a reliable qualitative evaluation of the microcirculation. Hypothesis: To evaluate of the microvascular alterations occurring during cardiac surgery using NIRS. Methods: Seventeen patients undergoing elective cardiac surgery with cardiopulmonary bypass (CPB) were included. Measurements were performed the day before surgery, after anesthesia induction, immediately after surgery on admission to the ICU, and 24 hours after surgery. The NIRS probe was placed on the thenar eminence. After stabilization of the signal (StO2baseline), we performed a 3-min VOT.We calculated the decrease in the StO2 (descending slope) during VOTand the NirVO2 (the reverse descending slope multiplied by the average tissue hemoglobin index [THI]) during the first min of the VOT to estimate muscle oxygen consumption. We also calculated the ascending slope after deflation of the pneumatic cuff to assess reactive hyperemia as a marker of microvascular reactivity. Results: The mean age of the studied population was 64+12 years; the mean preoperative left ventricular ejection fraction was 56+8%. Anesthesia was standardized and corticosteroids were administered to all patients prior to CPB. The median duration of CPB was 113 (101-145)min. At ICU admission, 7 patients were receiving dobutamine (median dose 5 mcg/kg/min [3.9-5.7]) and 8 norepinephrine (0.04 mcg/kg/min [0.01- 0.035]). Baseline StO2values increased after induction (from 80+7.6 to 85.2+5.9, p=0.49) and decreased after CPB (to 77.5+8.7, p=0.001). The descending slope and the NirVO2 decreased after induction (p=0.03 and p=0.001), did not change during CPB, and had returned to baseline values by day 1 (p=0.068 and p=0.019).The ascending slope decreased after anesthesia induction (p=0.008) and remained stable thereafter. Conclusions: Cardiac surgery is associated with microvascular dysfunction. These abnormalities seem to be primarily related to the anesthesia, rather than the CPB, and had reversed by the first day after surgery.