Data collection Microcirculation will be studied with side-stream dark field (SDF) imaging (Microscan; MicroVision Medical, Amsterdam, Netherlands) at three times; Immediately before starting CBP (T0) , 30 min after initiation of bypass (T1), and 30 min after weaning from bypass (T2) Side stream dark field imaging is equipped with sterile caps to avoid contamination. Briefly, after gentle cleansing by isotonic-saline-drenched gauze, avoiding pressure artefacts, 5 steady images of at least 20 seconds each will be obtained and stored under a random number. Offline blind analysis of each video will be done by two investigators. A previously validated semiquantitative score will be used . It distinguishes between no flow (0), intermittent flow (1), sluggish flow (2), and continuous flow (3). A value is assigned to each individual vessel. The overall score, called the microvascular flow index (MFI), is the average of the individual values. For each patient, the values from 5 videos will be averaged. In addition, vascular density will be quantified as the number of vessels per millimeters squared. To determine heterogeneity of perfusion, the flow heterogeneity index will be calculated as the highest MFI minus the lowest MFI divided by the mean MFI. Finally, the percent of perfused vessels and the total and capillary perfused vascular densities will be calculated. The percent of perfused vessels will be calculated as the number of vessels with flows 2 and 3 divided by the total number of vessels and multiplied by 100. The perfused vascular density will be calculated as the number of vessels multiplied by the fraction of perfused vessels. These quantifications of flow were made per group of vessel diameter: small (capillaries), 10 to 20 μm; medium, 21 to 50 μm; and large, 51 to 100 μm Sample size (number of participants included) 68 patients (34 in each group) Power analysis was performed using Student's t-test for independent samples with MFI as the primary outcome. A previous study demonstrated that MFI during cardiopulmonary bypass was approximately 2.6 + 1. Sample size was calculated to detect a 25% difference in MFI between the two groups with a power of 0.8 and an alpha error of 0.5. A minimum of 62 patients (31 in each group) would be necessary, and this was increased to 68 (34 in each group) to compensate for dropouts.