Proton Pump Inhibitor Prevent Dual Antiplatelet Agents Induced Gastrointestinal Ulcer After Coronary Artery Bypass Graft

Inclusion Criteria: Each patient should meet all of the inclusion criteria and none of the exclusion criteria for this study: 1. Provision of informed consent prior to any study specific procedures. 2. CABG surgery is performed for the first time. 3. Need 12-month DAPT (aspirin 100mg qd plus clopidogrel 75mg qd or aspirin 100mg qd plus ticagrelor 90mg bid depending on randomisation) immediately following CABG surgery according to the guideline. 4. Concomitant use of pantoprazole (40mg qd) for at least 1 moth following CABG surgery. 5. Willing to undergo EGD twice a year (at 6th and 12th month following CABG surgery, respectively). 6. Willing to undergo 13C-UBT prior to CABG surgery. Exclusion Criteria: Patients should not enter the study if any of the following exclusion criteria are fulfilled. 1. History of previous active peptic ulcer within 3 months prior to enrollment. 2. Planned use of PPI to treat gastric-acid associated diseases (e.g. gastroesophageal reflux disease, GERD) 3. Contraindications for aspirin, clopidogrel, ticagrelor and pantoprazole use (e.g. known allergy) 4. Anticipated concomitant oral or intravenous therapy with strong cytochrome P450 3A4 (CYP3A4) inhibitors or CYP3A4 substrates with narrow therapeutic indices, that cannot be stopped for the course of the study. * Strong inhibitors: ketoconazole, itraconazole, voriconazole, telithromycin, clarithromycin, nefazadone, ritonavir, saquinavir, nelfinavir, indinavir, atazanavir * CYP3A4 substrates with narrow therapeutic index: quinidine, simvastatin at doses \> 40mg daily or lovastatin at doses \> 40mg daily. 5. Need for chronic oral anticoagulant therapy or chronic low-molecular-weight heparin. 6. History of previous intracerebral bleeding at any time, GI bleeding within the past 3 months prior to enrollment, or major surgery within 30 days prior to enrollment. 7. Known moderate to severe liver disease (e.g. gastroesophageal varices, ascites and/ or clinical signs of coagulopathy) 8. Renal failure requiring dialysis. 9. Women of child-bearing potential who are not willing to use a medically accepted method of contraception that is considered reliable in the judgement of investigator OR women who have a positive pregnancy test at enrolment or randomisation OR women who are breasting-feeding. 10. Inability of patient to understand and/or comply with study procedures and/or follow up, in the opinion of the investigator, OR any conditions that, in the opinion of the investigator, many render the patient unable to complete the study. 11. Any condition which in the opinion of the investigator would make it unsafe or unsuitable for the patient to participate in this study (eg, long-term concomitant treatment with non-steroidal anti-inflammatory drugs \[NSAIDs\]), or any condition outside the atherothrombotic study area with a life expectancy of less than 2 years based on investigator's judgement. 12. Paticipation in another clinical study with an investigational product within 28 days prior to enrolment or previous randomization to an investigational product in another ongoing clinical study. Participation in any previous study with clopidogrel or tigagrelor.