Inclusion Criteria:  Age ≥ 18 years old Plan to undergo elective minimally invasive coronary artery bypass grafting (left anterior descending branch); Sign informed consent  Exclusion Criteria:  Left ventricular ejection fraction <40% Diabetes Renal insufficiency (serum creatinine>177umol/L) Liver insufficiency (AST, ALT>3 times) Peripheral vascular disease Carotid artery stenosis (>60%) VOT test contraindications (arm deformity, burns, arteriovenous shunt) Use glucocorticoids, vasoactive drugs, inotropic drugs or intra-aortic balloon counterpulsation (IABP) Allergic to colloidal fluids