Microvascular Reactivity as a Predictor of Major Adverse Events in Patients With On-pump Cardiac Surgery: A Prospective Observational Study

The VOT was performed five times in each patient, before the induction of general anesthesia (baseline, T0), 30 min after the induction of general anesthesia (T1), 30 min after applying CPB (T2), 10 min after injection of protamine (T3), and after sternal closure (T4). Before induction of anesthesia, an NIRS sensor (INVOS® 5100C Cerebral/Somatic Oximeter; Medtronic, Minneapolis, MN, USA) was placed on the thenar eminence and an automated tourniquet (A.T.S® 3000 Automatic Tourniquet System; Zimmer Inc., Warsaw, IL, USA) was placed around the upper arm. The arterial catheter was placed in the contralateral radial artery and the baseline blood pressure was measured. When the baseline tissue oxygen saturation (StO2) was stabilized, the automatic tourniquet was inflated to 50 mmHg over the patient's baseline systolic blood pressure and maintained for 5 min. After the 5-min ischemic period, the tourniquet rapidly deflated to 0 mmHg. StO2 data were continuously recorded during the VOT procedure. Baseline StO2, minimum StO2 during the 5-min inflation of the tourniquet, and maximum StO2 during deflation of the tourniquet were obtained. The occlusion slope and recovery slope were calculated based on the measured StO2 data. The occlusion slope, which is related to oxygen extraction, was defined as the slope of the StO2 descent to the lowest value. The recovery slope, which is related to microvascular reactivity, was calculated from the deflation of the tourniquet until the recovery of StO2 to the highest value.