Introduction The microvascular dysfunction after cardiac surgery with cardiopulmonary bypass (CPB) and impaired tissue oxygen saturation (StO2) have been associated with adverse patient outcome.(1)Measuring StO2 using near-infrared spectroscopy (NIRS) with a reproducible vascular occlusion test (VOT) has shown to be a noninvasive method suitable for assesment of microcirculation(2). The aim of this study was to assess microcirculatory function with NIRS using the VOT during cardiac surgery with cardiopulmonary bypass in pediatric patients. Methods 63 pediatric cardiac patients undergoing cardiac surgery with CPB were prospectively investigated. For cardiac surgical patients, NIRS parameters during the VOT were assessed before anesthesia (T1), after anesthesia (T2), full flow CPB (T3), after CPB (T4) and sternum closure (T5).VOT is including baseline, ischemia, reperfusion and return to baseline. Results There was significant difference in occlusion slope (rate of oxygen consumption) between T1 and T5 (p = 0,001), T2 and T5 (p = 0,001), T4 and T5 (p = 0,001). Reperfusion slope (representing microcirculatory vasoactive response to hypoxia) was significantly different T1 and T2 (p = 0,004), T1 and T3 (p = 0,001), T1 and T4 (p = 0,006), T2 and T3 (p = 0,01), T2 and T5 (p = 0,03), T3 and T5 (p = 0,002), T4 and T5 (p = 0,04). Reperfusion time was significant differences T1 and T3 (p = 0,03), T2 and T3 (p = 0,0019, T2 and T4 (p = 0,004), T3 and T5 (p = 0,001), T4 and T5 (p = 0,003). Discussion Intraoperative monitoring of peripheral microvascular reactivity enables detection of alterations in microcirculatory function thereby may contribute to preventing impaired tissue perfusion(3). The combination of VOT with NIRS monitoring showed CPB induced microvascular dysfunction in pediatric cardiac surgery.